Studies and reports in hydrology 39 Recent titles in this series:
20. Hydrological maps. Cosdition Unesco- WMO. 21: World catalogue of very large floods/Répertoire mondial des très fortes crues. 22. Floodflow computation. Methods compiled from world experience. 23. Water quality surveys. 24. Effects of urbanization and industrialization on the hydrological regime and on water quaiity. Proceedings of the Amsterdam Symposium. October 1977/Effets de l’urbanisation et de l’industrialisation sur le régime hydrologique et sur la qualité de l’eau. Actes du Colloque d’Amsterdam. Octobre 1977. Co-edition IAHS-Unesco - Coédition AISH-Unesco. 25. World water balance and water resources of the earth. (English edition). 26. Impact of urbanization and industrialization on water resources planning and management. 27. SociMconomic aspects of urban hydrology. 28. Casebook of methods of computation of quantitative changes in the hydrological regime of river basins due to human activities. 29. Surface water and ground-water interaction. 30. Aquifer contamination and protection. 31. Methods of computation of the water balance of large lakes and reservoirs. Vol. I Methodology Vol. II Case studies (in preparation) 32. Application of results from representative and experimental basins. 33. Groundwater in hard rocks. 34. Groundwater Models. Vol. I Concepts, problems and methods of analysis with examples of their application. 35. Sedimentation Problems in River Basins. 36. Methods of computation of low stream flow. 37. Roceedings of the Leningrad Symposium on specific aspects of hydrological computations for water projects (Russian). 38. Methods of hydrological computations for water projects. 39. Hydrological aspects of drought. 40. Guidebook to studies of land subsidence due to groundwater withdrawal. 41. Guide to the hydrology of carbonate rods.
Quadriiinguai publication: Engüsh-French-Spanish-Russian.
For details of the complete series please see the list printed at the end of this work. Hydrological aspects of drought
A contribution to the International Hydrological Progr amme
Prepared by a joint Unesco/WMO panel M.A. Beran and J.A. Rodier, rapporteurs
Unesco - WMO The designations employed and the presentation of material throughout this publication do not imply the expression of any opinion whatsoever on the part of the publishers Concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.
Published in 1985 by the United Nations Educational, Scientific and Cultural Organization 7, place de Fontenoy, 75700 Paris, and the World Meteorological Organization, 41, avenue Giuseppe-Motta, Geneva Printed by: Presses Universitaires de France, Vendôme ISBN 92-3-102288-1 O Unesco/WMO 1985 Printed in France Preface
Although the total amount of water on earth is generally assumed to have remained virtually constant, the rapid growth of population, together with the extension of irrigated agriculture and industrial development, are stressing the quantity and quality aspects of the natural system. Because of the increasing problems, man has begun to realize that he can no longer follow a “use and discard” philosophy - either with water resources or any other natural resources. As a result, the need for a consistent policy of rational management of water resources has become evident. Rational water management, however, should be founded upon a thorough understanding of water availability and movement. Thus, as a contribution to the solution of the world’s water problems, Unesco, in 1965, began the first world-wide programme of studies of the hydrological cycle - the International Hydrological Decade (IHD). The research programme was complemented by a major effort in the field of hydrological education and training. The activities undertaken during the Decade proved to be of great interest and value to Member States. By the end of that period, a majority of Unesco’s Member States had formed IHD National Committees to carry out relevant national activities and to participate in regional and international co-operation within the IHD programme. The knowledge of the world’s water resources had substantially improved. Hydrology became widely recognized as an independent professional option and facilities for the training of hydrologists had been developed. Conscious of the need to expand upon the efforts initiated during the Intemational Hydrological Decade and, following the recommendations of Member States, Unesco, in 1975, launched a new long-term intergovernmental programme, the International Hydrological Programme (IHP), to follow the Decade. Although the IHP is basically a scientific and educational programme, Unesco has been aware from the beginning of a need to direct its activities toward the practical solutions of the world’s very real water resources problems. Accordingly, and in line with the recommendations of the 1977 United Nations Water Conference, the objectives of the International Hydrological Programme have been gradually expanded in order to cover not only hydrological processes considered. in interrelationship with the environment and human activities, but also the scientific aspects of multi- purpose utilization and conservation of water resources to meet the needs of economic and social development. Thus, while maintaining IHP’s scientific concept, the objectives have shifted perceptibly towards a multidisciplinary approach to the assessment, planning, and rational management of water resources. As part of Unesco’s contribution to the objectives of the IHP, two publication series are issued: “Studies and Reports in Hydrology” and “Technical Papers in Hydrology.” In addition to these publications, and in order to ex- pedite exchange of information in the areas in which it is most needed, works of a preliminary nature are issued in the form of Technical Documents. The purpose of the continuing series “Studies and Reports in Hydrology” to which this volume belongs, is to pre- sent data collected and the main results of hydrological studies, as well as to provide information on hydrological research techniques. The proceedings of symposia are also sometimes included. It is hoped that these volumes will furnish material of both practical and theoretical interest to water resources scientists and also to those involved in water resources assessments and the planning for rational water resources management. Contents
FOREWORD
1. INTRODUCTION ...... 1
1.1 General ...... 1.2 Hydrological drought ...... 1.2.1 Various aspects of drought ...... 1.2.2 The definition of drought ...... 1.2.3 Various aspects of hydrological droughts ......
2 . CHARACTERISTICS OF HYDROLOGICAL DROUGHT ...... 5
2.1 General ...... 5 2.2 The persistence phenomenon ...... 5 2.2.1 Introduction and summary ...... 5 2.2.2 Sources of persistence ...... 5 2.2.3 The evidence for and against interannual persistence ...... 6 2.2.4 Can the conflicting evidence be reconciled? ...... 7 2.2.4.1 Can processes with low pl display long runs? ...... -7 2.2.4.2 Evidence for persistence being more evident after extreme years ...... 8 2.2.4.3 Do drier and more northerly Sahel stations display more Persistence? .... 9 2.2.4.4 Do station groups display more persistence than individuals? ...... 9 Within year persistence 2.2.5 ...... 10 2.2.5.1 Hydrological subsystem ...... 10 2.2.5.2 Atmosphere and groundlair interface systems ...... 10 Conclusions about persistence 2.2.6 ...... 11 Space and time heterogeneity 2.3 ...... 11 Introduction 2.3.1 ...... 11 Spatial heterogeneity 2.3.2 ...... 12 2.3.2.1 The cause of spatial variability ...... 12 spatial extent of drought 2.3.2.2 ...... 12 Mapping drought intensity by means of rainfall data 2.3.2.3 ...... 13 2.3.2.4 Problems in mapping hydrological variables ...... 15 Statistical treatment of spatial variability 2.3.2.5 ...... 16 Temporal heterogeneity 2.3.3 ...... 16 'General features of temporal heterogeneity 2.3.3.1 ...... 16 2.3.3.2 Case studies of temporal variability ...... 17 Runoff variability 2.3.3.3 ...... 17 Conclusion 2.3.4 ...... 17 Droughts and climatic changes 2.4 ...... 19 2.4.1 General ...... 19 2.4.2 Drought occurrence since 1850 ...... 19 &e droughts periodic? 2.4.3 ...... 20 2.4.4 Droughts and climate in motion ...... 21 Anthropogenic climate change 2.4.5 ...... 22 Climate history 2.4.6 ...... 22 2.4.7 Climate change and drought incidence ...... 23 References to Chapter 2 2.5 ...... 23 3 . FACTORS RESPONSIBLE FOR THE DROUGHTS ...... 27
3.1 Particular aspects of the dynamics of the air masses ...... 27 3.1.1 Introduction ...... ; ...... 27 3.1.2 Some physical factors associated with mid-latitude dr ought ...... 27 3.1.2.1 Subsidence and its causes ...... 27 3.1.2.2 The role of high pressure cells ...... 33 3.1.2.3 Teleconnections between pressure anomalies ...... 33 3.1.2.4 Self generating mechanisms ...... 35 3.1.2.5 The role of sea surface temperature ...... 35 3.1.2.6 Long waves in the westerlies ...... 35 3.1.2.7 External factors responsible for drought ...... 43 3.1.3 Tropical droughts ...... 44 3.1.3.1 West Africa ...... 44 3.1.3.2 Drought in India ...... 48 3.1.3.3 Drought in South America ...... 51 3.1.3.4 Australian drought ...... 52 3.1.3.5 Drought in South Africa ...... 52 3.2 Influence of man ...... 52 3.2.1 Introduction ...... 52 3.2.2 Anthropogenic effects on climate ...... 52 3.2.2.1 Individual mechanisms ...... 52 3.2.2.2 Drought effect ...... 55 3.2.2.3 Local effects on the climate ...... 55 3.3 Possibilities of forecasting drought ...... 55 3.3.1 Meteorological methods of forecasting ...... 55 3.3.1.1 Analogue methods ...... 55 3.3.1.2 Linear regression methods ...... 56 3.3.1.3 Teleconnections ...... 56 3.3.1.4 Statistical and kinematic methods ...... 56 3.3.1.5 Contingency tables ...... 57 3.3.1.6 Use of air-sea interactions ...... 57 3.3.1.7 Statistical time series forecasts ...... 58 3.3.1.8 Extrapolation in time using cyclicities ...... 58 3.3.2 Hydrological methods of forecasting drought ...... 58 3.3.2.1 Recession based methods ...... 58 3.3.2.2 Regression methods ...... 59 3.3.2.3 Cycles in annual streamflow ...... 59 3.3.3 Review of accuracy ...... 59 3.4 References to Chapter 3 ...... 60
4 . METHODOLOGY FOR THE STUDY OF DROUGHT AND EXCEPTIONAL LOW RIVER FLOWS .... 65
4.1 Choice of indices for depth of discharge and precipitation ...... 65 4.2 Use of historical and geomorphological information ...... 66 4.2.1 Historical documents and folk memory ...... 66 4.2.2 Geomorphological and other palaeoenvironmental indicators ...... 66 4.2.3 Droughts of the immediate past from indirect evidence ...... 67 4.3 Analysis of precipitation depth and other climatic variables ...... 67 4.3.1 General ...... 67 4.3.2 Annual rainfall ...... 68 4.3.2.1 Pluviosity ...... 68 4.3.2.2 Percentiles ...... 68 4.3.2.3 Distribution fitting ...... 68 4.3.2.4 Run length and run sum ...... 70 4.3.3 Analysis of rainfall in other durations ...... 70 4.3.4 Spatial description of drought rainfall ...... 71 4.3.5 More complex climate based indicators ...... 71 4.4 Analysis of discharge ...... 72 4.4.1 Special purpose measurements during low flow periods ...... 72 4.4.2 Analysis of discharges of rivers with one defined rainy and one defined dry season ...... 72 4.4.3 Analysis of discharge for rivers without well defined rainy or dry seasons . 73 4.5 Fitting statistical distributions to drought data ...... 74 4.5.1 Choice of distributions ...... 74 4.5.2 Methods of fitting distributions ...... 75 4.6 Possibilities of drought prediction by correlation with geological and other characteristics of basins ...... 77 4.6.1 General ...... 77 4.6.2 The UK scheme for estimating low flow characteristics of ungauged catchments ...... 77 4.6.3 Relationship with catchment characteristics ...... 77 4.6.4 Internal duration and frequency linkages ...... 79 4.6.5 Low flow estimation for ungauged catchments in the Sahel zone ...... 79 4.6.6 Concluding remarks ...... 80 4.7 References to Chapter 4 ...... 82
5 . THE RECENT DROUGHT IN TROPICAL AREAS ..... 85
5.1 Introduction ...... 85 5.2 General character of this drought ...... 85 5.2.1 Regional coverage ...... 85 5.2.2 Characteristics of tropical droughts generally and of the recent drought . . 87 5.3 Available data ...... 87 5.4 Analysis of the data ...... 88 5.4.1 Numerical indices ...... 88 5.4.2 Some details of the analysis ...... 88 5.5 Results ...... 89 5.5.1 Average annual discharges during the drought ...... 89 5.5.1.1 Presentation of results ...... 89 5.5.1.2 Discussion of results ...... 89 5.5.1.3 Year by year account of the drought ...... 89 5.5.1.4 Comparisons outside the tropical zqne ...... 92 5.5.1.5 Summarising statement ...... 92 5.5.2 Maximum and minimum yearly discharges during the drought ...... 93 5.5.2.1 Maximum yearly discharge ...... 93 5.5.2.2 Annual minimum discharges ...... 94 5.6 Comparison with earlier droughts ...... 95 5.6.1 AfricanSahel ...... ’...... 95 5.6.2 India ...... 96 5.6.3 Central America and Mexico ...... 96 5.6.4 South America ...... 96 5.6.5 Tropical South Africa ...... 98 5.6.6 Australia and Oceania ...... 98 5.6.7 More extensive historical researches ...... 98 5.7 Second period of the recent drought in tropical areas ...... 100 5.8 Conclusion ...... 101 5.9 References to Chapter 5 ...... 115
6 . TEMPERATE ZONE DROUGHT ...... 117
6.1 Introduction ...... 117 6.1.1 Region31 coverage ...... 117 6.1.2 Characteristics of temperate zone drought ...... 117 6.1.3 Contrast between Sahel and temperate zone drought ...... 119 6.1.3.1 Climatic contrasts ...... 119 6.1.3.2 Vegetative and societal contrasts ...... 119 6.1.3.3 Hydrological contrasts ...... 120 6.2 Droughts of the recent past ...... 121 6.2.1 Western Europe ...... 121 6.2.2 Past droughts in North America ...... 122 6.2.2.1 General ...... 122 6.2.2.2 West maritime region ...... 122 6.2.2.3 Northern prairies ...... 123 6.2.2.4 The northeast region ...... 126 6.3 The 1972 drought in the European territory of the USSR ...... 126 6.4 Details of drought in western Europe during the 1970s ...... 127 6.4.1 General ...... 127 6.4.2 The 1976 drought in Belgium and its consequences ...... 127 6.4.3 The 1971-74 and 1976 droughts in Czechoslovakia ...... 128 6.4.4 The drought in France from December 1975 to July 1976 ...... 129 6.4.5 The 1976 drought in the Federal Republic of Germany (Bavaria) ...... 130 6.4.6 The 1976 drought in the Netherlands ...... 131 6.4.7 Drought in the 1970s in the United Kingdom ...... 131 6.5 Droughts in the temperate zone of the United States during the 1970s .... 134 6.5.1 Areas affected during the 1976-77 drought ...... 134 6.5.2 Chronology of the drought in western USA ...... 134 6.5.3 Chronology of the drought in the northern prairie region ...... 136 6.6 References to Chapter 6 ...... 137
7 . PROSPECTS FOR THE LIMITATION OF THE CONSEQUENCES OF HYDROLOGICAL DROUGHT . . 141
7.1 General ...... 141 7.2 Surface water management ...... 141 7.3 Groundwater management ...... 142 7.3.1 The need to avoid overexploitation ...... 142 7.3.2 Groundwater source development ...... 143 7.3.3 Possibilities for augmenting aquifer recharge and yield ...... 143 7.3.4 Groundwater quality problems ...... 143 7.3.5 General concluding remarks ...... 144 7.4 Reduction of evaporation ...... 144 7.5 Artificial enhancement of precipitation ...... 144 7.6 Land management. logistical and social measures for mitigating drought consequences ...... 145 7.7 References to Chapter 7 ...... 145
8 . RECOMMENDATIONS ...... 147
8.1 General recommendations for research ...... 147 8.1.1 Introduction ...... 147 8.1.2 Research into drought indices ...... 147 8.1.3 Droughts in time and space ...... 147 8.1.4 Drought mechanisms ...... 148 8.1.5 Drought surveys ...... 148 8.1.6 Drought consequences ...... 148 8.2 Suggestions for international co-operation ...... 149 8.3 Reference to Chapter 8 ...... 149 Foreword
A series of studies on droughts were undertaken during the International Hydrological Decade launched by Unesco in 1964. From 1968 to 1973, during the severe drought which affected the Sahel and other tropical regions, the interested governments and international Organizations did their best to amelio- rate the direct consequences of this catastrophe, to study the conditions and causes of the drought, and to recommend measures which could, in the future, mitigate the effects of such droughts. As far as the assessment of rainfall and runoff is concerned, the WMO Executive Committee's Panel of Experts for the International Hydrological Decade requested that a report be prepared on this subject by the WMO rapporteur on low flows and related aspects of droughts. This report, written in co-operation with the International Association of Hydrological Sciences, was pre- sented by that Association to the International conference on the Results of the International Hydrological Decade and on Future Programmes in Hydrology convened jointly by Unesco and WMO at Unesco Headquarters in Paris (2-13 September 1974). The Intergovernmental Council of the International Hydrological Programme of Unesco decided at its first session (Paris, 9-17 April 1975) to appoint a rapporteur with the task of preparing a state-of-the-art report on hydrological aspects of droughts (IHP project 3.5). The rapporteur was to work in close co-operation with WMO and IAHS, and take into account the work done by other international organizations concerning droughts (in particular FAO and UNEP) and to make proposals for future action. At its fifth session in Ottawa (July 19761, the Commission for Hydrology of WMO also appointed a rapporteur to study and apply methods of indexing continental scale droughts and possibly extending this to encompass the 1975-1976 European drought. During its second session, the Intergovernmental Council of the IHP (20-27 June 1977) decided to recommend the establishment of a joint Unesco/WMO panel to prepare a state-of-the art report on hydrological aspects of droughts. The joint panel composed of the Unesco and WMO rapporteurs, Dr. J.A. Rodier (France) and Mr. M.A. Beran (United Kingdom), met in Paris on 8 November 1977 and revised the draft outline which had been presented by Dr. Rodier to the IHP Intergovernmental Council at its June 1977 session and distributed the task between Unesco and WMO for the preparation of the report. The following experts have also contributed to the preparation of this report : Dr. L. Dorize (France) for sections 2.4, 3.1 and 3.2; Professor J. Flohn (Federal Republic of Germany) for section 2.3; Professor J. Namias (USA) for sections 3.1 and 3.3; Mr. L. Serra (France) for sections 1.2 and 4.1.
M.A. Beran J.A. Rodier 1. Introduction
1.1 GENERAL
Drought is generally viewed as a sustained and regionally extensive occurrence of below average natural water availability, either in the form of precipitation, river runoff or groundwater. Drought should not be confused with aridity which applies to those persistently dry regions where, even in normal circumstances, water is in short supply. The consequences of droughts are felt most keenly in areas which are in any case arid. However it is manifested, drought adversely affects the economy by reducing, or even eliminating, agricultural production, herds of cattle, energy generation, and domestic and industrial water supply. Developing countries are particularly prone to these adverse effects on two counts: if directly affected by a drought the difficult economic situation in the developing country hinders its ability to take swift action to reduce the disastrous consequences; and if drought strikes a cereal supplying developed country the supply of the commodity to the developing country is reduced and its price is increased. Drought may be so severe that famine may ensue, and in some cases the situation may be such that, despite international cooperation, it could cause the death of millions of human beings. As has been implied, the essential feature of drought is that it is tied to the idea of a deficit in the supply of moisture for some specific purpose. Abnormal low flows in rivers are, of course, generally experienced during any drought but their study is rather different. Low flow studies will be concerned with the statistical treatment, and the understanding of the physical development, of flows at a point along a river in the shoyt term. By contrast the study of drought concerns the description of rainfall, river flow, soil moisture and groundwater over a season, year or several years and also of the spatial extent of the phenomenon. Of the numerous aspects of drought, the present report concentrates on the single aspect of hydrological drought, i.e. the deficit in the runoff of rivers, with some attention to the deficit in precipitation and the deficit in groundwater.
1.2 HYDROLOGICAL DROUGHT
1.2.1 Various aspects of drought Drought by definition consists of a sustained period of deficit perhaps lasting a few months or even many years. Conditions within a drought may vary considerably in space and time in accordance with the spatio-temporal irregularity of the rainfall distribution and with the heterogeneity of,the hydrological response of the river basins that are affected. The character of drought may be different for the different climatological and hydrological regimes that are found in the world. As explained below it also differs very much according to the use to which the water is put. An example of this variation with water use is found in the more arid part of the Sahel where a shortage of rain depth and duration during the rainy season need not much affect the pasture so long as germination and growth is permitted, however grain production may be very much reduced. So such a drought may have a more marked effect on the cereal grower than on the pastoralist. In tropical areas if the yearly distributions of precipitation is such that there are a small number of runoff-producing heavy storms at the end of the rainy season in addition to a number of slight rainfalls throughout that season, there may be a deficit in annual rainfall but the total yearly runoff may be normal. Thus although there is no hydrological drought in this circumstance nevertheless crop yield will be low. A water deficit during the critical period for agriculture is considered as a drought by
1 farmers, but if the preceding winter and spring had been very humid reservoirs may well be full and, as far as hydro-electric plant managers are concerned, there is no drought. In years when the total rainfall is normal but of low intensity, and especially when associated with high winds, the recharge to aquifers will be inadequate even if river runoff is normal. There will then be drought for the users of the aquifer. The most severe droughts, such as the recent case in the Sahel, suffer on all counts: low rainfall, low river flow and storages, and depleted aquifers. Of the sorts of drought which have been mentioned above those which concern agriculture are discussed in technical note No. 138 "Drought and agriculture" produced by a WMO working group. As stated above hydrological drought is considered to be a deficit of runoff below normal conditions, or else a depletion of aquifer levels even though, through over-exploitation, the water supply furnished by the aquifers may remain the same as before the drought. 1.2.2 The definition of drought
It is tempting to search for simple statements such as are found in dictionaries which encapsulate the idea of a drought. This is hardly possible because hydrological observations that are made - rain depth or river flow relative to their average - are far from constant in time and are not qynchronous one with another. The drought of one year or season does not equally and simultaneously affect all the points of the globe, not even a continent. Contrary to floods, which can be measured and quantified, drought very often seems to retain qualitative connotations. In this respect - maybe even more here - it is essential to define precisely what one wants to learn about a drought and to which particular points and characteristics one must pay most attention. We therefore regard drought not as a definable entity in itself but as a "prime mover" which has attributes or consequences. It is these consequences, in particular the hydrological consequences, that we focus upon and define in this report. No flood, even the most catastrophic and memorable, has been responsible for as many victims as exceptional droughts because their destruction is confined to the valley bottoms while droughts characteristically strike at immense areas at the same time. One can study drought on its own, or drought and its consequences on agriculture, economy and human society: this can lead to several different characterisations as already given in the previous subsection. In any case it is evident that the notion of drought is relative, but its chief characteristic is a decrease of water availability in a particular period and over a particular area rather than a general decrease in water availability. It is thus this abnormal distribution, which must be considered typical of droughts. Water which falls in the form of rainfall of course reappears in due time in the form of river flow, of groundwater, or else is held. for subsequent release after melting as snow or ice. Because of the different inbuilt delays in the parts of the land phase of the hydrological cycle the different manifestations of the drought (i.e. lack or absence of rainfall) are not simultaneously felt. Thus it is conceivable that a certain month can be considered dry by a climatologist and an agriculturist who are primarily interested in rainfall, but normal, or even above average, by the hydrologist who is more concerned with flows. That is why it is necessary to closely define, not only the factor considered (rain or flow), but also the duration and period studied. The definition of a base period is essential when drought is to be described in terms of 'pluviosity' or 'hydraulicity'. These two terms which are in standard French usage (pluviosité and hydraulicité) have been retained as a convenient shorthand for rainfall and runoff totals in some period expressed as a ratio or a percentage of the long term average value over the same period. A normal annual pluviosity (close to unity) can hide the fact that the year could include an abnormally dry and an abnormally wet period. By analogy with the treatment of floods - also an abnormal distribution of flow - one tends to regard the minimum instantaneous or daily flow reached during the year as a characteristic or measure of the drought severity of that year. In fact the two phenomena are fundamentally different. The flood, in particular its peak value, is a transitory phenomenon due to multiple and random causes such as the intensity and the duration of the rainfall, the permeability of the ground etc. The low flow is a phenomenon which evolves much more slowly and which is very intimately tied to structures of great inertia, such as the total volume of stored sources and the summer evapotranspiration on the catchment. On physical grounds it is difficult to claim that the minimum flow in a year is totally independent from those of previous years even if the river at the point of measurement had run dry. However, in practice one finds that the daily flow sequence presents a picture which does give a minimum apparently independent of its neighbours. This is normally due to the superposition of man-induced effects, e.g. operation of dams, flood gates, pumped diversions, on the relatively smooth natural hydrograph. Any flow, but particularly the minimum flows, can thus display notable artificial variations; an extreme lack of water can occur completely separately from a drought in the commonly understood sense of the word. Besides this requirement for adjusting the flows for
2 artificial variation one must not forget another important difficulty concerning recorded flows: the measurement of low flow is often delicate, which explains frequent gaps and jumps in the published data, especially for the past decades. In many cases, the accuracy of the measurement is often mediocre, especially over a small time scale. What are the main numerical characteristics that attach to low flows? Three numbers are needed to define them: a. the minimum flow value averaged over n consecutive days (n = 5, 10, 20, 30 or more days); b. the dates of their occurrence; c. the frequency attributable to the phenomenon. This last mentioned number will be assigned either by using the long term data of the site or by analogy with a comparable record. A number of ways of describing frequency are in common use :
a. a simple ranking, i.e. the ith lowest in N years of record; b. an empirical probability of non exceedence based upon sample values, i.e. i/N; c. a more thorough approach based upon a statistical analysis of all the annual minima considering a correct choice of statistical distribution and plotting position;
d. alternative methods of describing frequency express the probability in percentage terms of else as return period being the average recurrence interval between non exceedances of the selected low flow. An ideal representation of low flow severity occurring in a drought would consist of a set of low flow maps each one being for a given value of n (the duration over which flows are averaged) and a given frequency (for example the median condition - not exceeded by one half of low flows, the quinquennial and the decennial low flows - not exceeded by one fifth and one tenth of n-day annual minima). It must be appreciated though that the production of such a set of maps requires a dense gauging station network over the basins, and observation series extending bàck over several decades. Unfortunately this ideal is seldom realised. Start and end of droughts
Droughts differ from other meteorological phenomena in their temporal aspect. It is difficult to tell at what date a drought started, what date it ended, and thus how long it lasted; what is certain is that this duration can be relatively long by comparison with other meteorological events. The start of a drought, which can only be determined to within a bracket of 1 to 2 months, depends very much on ones own point of view which is not necessarily the same for the agriculturist as for the hydroelectrician. A drought, in the most general sense, does not start immediately at the termination of the last useful rainfall. For hydrological drought, the commencement of the phenomenon may be much delayed because of the damping effect of underground reserves which continue to support the flows, at least for a while after the cessation of rainfall. The end of drought is more visible and thus easier to determine, particularly when abundant rainfall saturates the ground, raises the flow in rivers, and rebuilds underground reserves. Numerical criteria deduced from the empirical study of a certain number of cases have been proposed to define the end of a prolonged period of drought. To summarise, it is not drought itself which is strictly defined but important attributes or characteristics; examples are given above but many more characteristics and methods of quantifying them are given throughout this report. Particular broad classes of drought are given in the next subsection. 1.2.3 Various aspects of hydrological droughts Six types of drought may be distinguished based upon variations in the duration, season of year, or severity:
1. A three-week to three-month runoff deficit during the period of germination and plant growth. This could be catastrophic for farming that is dependent upon irrigation drawn directly from the river without the support of reservoirs.
2. A minimum discharge significantly lower or more prolonged than the normal
3 minimum but not necessarily advanced much in its position relative to the growing season. Because the germination period is not affected this type of drought is of less consequence to agriculture.
3. A significant deficit in the total annual runoff. This affects hydropower production and irrigation from large reservoirs.
4. A below normal annual high water level of the river. This may introduce the need for pumping for irrigation. This type of drought is related to Type 3 - deficit in annual runoff. 5. Drought extending over several consecutive years as with the "Secas" of Northeast Brazil. Discharge remains below a low threshold or the rivers dry up entirely and remain dry for a very long time.
6. A significant natural depletion of aquifers. This is difficult to quantify because observation of the true level of the aquifer is disturbed by the over-utilization of groundwater during the drought. The second, third, fourth and fifth categories of drought are concentrated on in this report. The first category was considered, to some extent, in the WMO report "Drought and Agriculture", while the sixth category, difficult to assess, is closely tied to the others. It should also be noted in this context that if there is a substantial lag between natural aquifer recharge and the level in the wells, then short duration droughts may be damped out and reduced in importance. An effective study of drought type 5 in tropical countries requires the largest and smallest as well as the average yearly flows to be considered. The return period assessment should consider the deficits in individual years and overall.
4 2. Characteristics of hydrological drought
2.1 GENERAL
Six different types of hydrological drought were described in Section 1.2.3. Each one was expressed in terms of a different variable, e.g. total volume of runoff, minimum discharge at an instant and over a period, maximum river level and minimum aquifer level, but nevertheless each presents common features which will be discussed in this chapter. In particular there are processes within the hydrological cycle acting over all time scales which tend to maintain a drought once started - this 'persistence' phenomenon is discussed in Section 2.2. Within all serious droughts there are localities which are particularly hard-hit and others which are relatively spared. Also one finds differences in severity in different portions of the drought. Examples of space and time heterogeneity are described in Section 2.3. Longer term phenomena giving rise to increases in the likelihood of drought have been pGStUhted in recent years and these are discussed in Section 2.4.
2.2 THE PERSISTENCE PHENOMENON
2.2.1 Introduction and summary
Having reserved the term 'drought' for events which affect man's activities, it is therefore a truism to state that droughts persist - ifthey did not their impact on our activities would be minimal. However many have claimed that there is a tendency for drought conditions to persist over longer periods than can be explained by chance alone. This section describes sources of interannual persistence (2.2.2) and weighs the evidence for and against the non-randomness of drought recurrence (2.2.3). Particular reference is made to large scale studies of regional, especially Sahel, data to try to isolate the particular variables which display persistence most markedly (2.2.4). Within-the-year persistence is less contentious-although not all mechanisms are well understood (2.2.5) . A considerable body of evidence points to annual rainfall and runoff being independent random quantities. Against this there exists the universally recognised phenomenon of long runs of below (and to a lesser extent, above) average years such as during the recent and continuing Sahel drought. Following sub-sections present the evidence for and against interannual persistence and attempt to reconcile some of the opposing evidence. It seems likely that hydrological memory is most marked in below average periods i.e. the probability of 'dry' following 'dry' is proportionately greater than other contingencies. It also appears that the phenomenon is more visible in large data assemblages than in individual station records. 2.2.2 Sources of persistence Hydrological drought considered as an aspect of the total hydrological cycle is bound to exhibit some degree of persistence because of the large inertia of some processes within the cycle. The prime example is the release from aquifer storage which contributes to, and in the dry season may totally account for, river discharge. The long residence and response times of such sources build a smoothness into hydrological response. Hydrologists will of course need no reminding that local effects so far disturb this underlying smoothness as sometimes to lose it altogether in random fluctuations. Although aquifer storage is the major source of inertia in the land phase of the hydrological cycle, it is necessary to realise that analogous forces operate in the atmospheric phase. There is a considerable meteorological literature describing those climate anomalies (departures from normal), often associated with drought, the classics among which are the southerly shift of the intertropical convergence zone which is associated with intertropical
5 drought, and the 'blocking' tendency associated with temperate zone drought. These climate anomalies are themselves sustained by medium and long term departures, some of which are well established such as uncharacteristically low or high sea surface temperatures. The sea-surface temperature often reflects the heat content of the water in the upper levels of the ocean. The temperature anomaly may at times penetrate to depths of a few hundred metres to provide long- lasting "reservoirs" of heat - or cold - analogous to a water supply reservoir. This heat and its gradients, in turn, can be exchanged with the atmosphere to help govern the positions of troughs and ridges in the atmospheric flow patterns (Namias, 1975). Another well established causative factor concerns the polar or equatorward shift in ice packs. There are others, slightly more speculative, concerning feedback mechanisms (see Section 2.2.5.2) and variations in incoming solar energy. 2.2.3 The evidence for and against interannual persistence Attempts to quantify and test for persistence fall into two main groups. The first group uses sequences of annual rainfall or river flow data to compute the first order serial correlation coefficient, P1. This is tested against the null hypothesis that the sample derives from a population whose true pl value is zero, i.e. stochastically independent. Table 2.1 lists some results from the main exponents of this approach.
Conclusions_about Author Data average Pl
Yevjevich (1964) 140 runoff records worldwide O. 18
II 446 runoff records in western USA o. 20 II 1141 rainfall records in western USA Not different to p=o
Hardison (1966) 180 runoff records in USA 0.17 Brunet-Moret (1975) 179 rainfall records in tropical N. Africa Between O and O. 17
Sonuga (1977) 14 rainfall records in N. Nigeria Not different to p=o
Table 2.1 Experience with first order serial correlation coefficients from annual rainfall and runoff data
In every case the conclusion is broadly that persistence as measured by this index is a relatively minor feature. Although much used the estimation of Pl, either by the standard formula of WMO (1966a) p60 or the robust method of Brunet-Moret (1975), is not without problems of computation and interpretation. Trend due to natural causes or data inhomogeneityA such as might result from a change in site or observational practice will tend to inflate the Pl value artificially (Potter, 1979). The presence of serial correlation can introduce a downward bias in the estimate from small samples. For Yevjevich's samples the bias amounts to only 0.03; and was empirically recognised. But, if the population fscc is 0.3, the estimated fscc obtained from samples of size 25 would group around an average of 0.21 (Wallis and O'Connell, 1972), a considerable bias. Brunet-Moret's (1975) study was unusual, firstly in adopting a different method of estimating Pl as the coefficient, A, of the first order Markov or autoregressive equation: xi+l = AX^ + ~i+l where xi, xi+l are the rainfall totals in years i and i+l zi+l is a random term; and secondly in investigating the causes of variation in A. A trend line was drawn which shows that persistence is least for wetter sites, being 0.10 to 0.17 for values below 500 mm (the annual average rainfall for which persistence was most marked) dropping to near zero for 1500 mm rainfall stations. The second school of persistence analysts uses the statistics of runs, i.e. the length of successive years of below (or above) average conditions. Tests are described in Brooks and Carruthers (1953, p310 et seq), WMO (196633) and Clarke (1973). Such tests are particularly attractive as they conform directly with the subjective impression about droughts. The tests above use random sequences to base the null hypothesis but Saldarriaga et a1.(1970), Millan (1972a, 1972b), Sen (1976), and Gottschalk (1977) tabulate properties of runs from Markov
6 processes with various levels of serial correlation. Table 2.2 summarises the results of some run analysis, some of which allow for serial correlation.
Author Data Conclusion
Millan (197233) Colorado river runoff (USA) Long run found Jenkinson (1973) Sahel rainfall between 12' and 14 ON Appeared random Bunting et al (1976) West African Sahel rainfall Appeared random Sonuga (1977) 14 N Nigerian rainfall stations Appeared random Walker et al (1977) Sahel rainfall in 16' to 1835'N band Long runs found Gottschalk (1977) Swedish runoff Long runs found Kraus (1977) Similar region to Bunting, and India Long runs found Chervin et al (1981) Sahel and Soudano-Sahel Very long runs of 15 years found - present drought still continuing to 1980
Table 2.2 Run analysis results
Sonuga (1977) also employed 'Hurst h' or 'rescaled range' analysis which has been prominent in recent hydrological literature and whose relevance to drought hydrology lies in its close association with reservoir design calculations. Wallis et al (1973) provides tables for testing the 'Hurst h' statistic against an independent random Normal process. Clarke (1973) presents an algorithm for computing Hurst h from a data sequence. However, the values found fell within the bounds that are expected for an independent series, see also Potter (1979) for a similar finding from 49 United States river flow stations. 2.2.4 Can the conflicting evidence be reconciled? There is more evidence of persistence in runs analysis than in serial correlation analysis. A detailed scrutiny of the various studies into Sahel rainfall gives the necessary clues to resolving this paradox. Jenkinson (1973) analysed Sahel rainfall records between 12' and 14'N and found little difference from a random sequence. Walker et al.(1977) extended the analysis to more stations but confined their study to a drier northern group in the 16' to 1835'N band. Also instead of treating each raingauge separately they formed an index of the average of the percentage of each to its 1945-1974 normal (average pluviosity). Two runs of extraordinary length appeared, one wet, from 1950 to 1959, and the other dry, from 1965 to 1974. The authors make the important point that the test is not conclusive because the analysis would not have been carried out if the drought had not occurred, in other words a measure of pre-selection of the data means that they were not viewing a randomly selected segment. Nevertheless, the appearance of the data contrasts greatly with the earlier findings of little or no persistence. The questions must be asked whether this is a chance occurrence (2.2.4.1, 2.2.4.2); whether it is truly the result of confining the search to the more northerly data set (2.2.4.31, or whether it is du? to the use of an index based upon widely scattered raingauges and so is less subject to local and temporary relieving storms. These points are addressed in the following subsections. 2.2.4.1 Can processes with low P1 display long runs? An initial question to be answered is whether the extraordinary long runs which admittedly could not reasonably arise from a random and independent process could after all be expected from a process with a moderate value of Pl. Gottschalk (1977) has studied run lengths below particular thresholds for Markov processes. Graphical and tabular material is presented which suggests that simple persistence models would give very low probabilities indeed (<< 0.01) for runs of more than five years below the median with serial correlations considerably in excess of those that are observed in practice. The other authors listed above Table 2.2 present similar conclusions.
7 However, all those authors' results are based on the same assumption that the data derive from a Normal process i.e. that the distribution of flow in year i+l,Qi+l depends on that in year i,Qi, only in respect of its location but not its variability, this being independent of Qi.In equivalent terms the probability of Qi+l departing from Qiby an amount A i.e. Prob (Qi - A < Qi+l < Qi+ A) is a constant irrespectiveAof whether Qiis high, average or low. This same assumption is implicit in the use of tests of Pl, the estimated first order serial correlation coefficient. There is overriding evidence that this pattern of inter-year dependence is far from the truth. Gottschalk himself concluded that there was in the annual runoff of Swedish rivers 'a tendency ..... that the 'memory' in the dry stage is larger than that in other cases' or in statistical terms:
2.2.4.2 Evidence for persistence being more evident after extreme years It is here postulated that the presence of long runs in the Walker et al. (1977) data is due to persistence which is manifest mainly in extreme years (thus accounting for indifferent Pl values). Two methods of analysis are presented which focus on this problem specifically. Katz (1978) attempted to reconcile Kraus' (1977) findings of significant persistence with those of Bunting et al. (1976). Katz subjected Kraus' data, which consisted of an areally averaged annual rainfall index for subtropical Africa, to Tukey's (1977) exploratory data analysis technique the 'boxplot', to display the distribution of rainfall conditional on whether the previous year was wet or dry. Figure 2.1 is derived from Katz (1978) and shows that the range of values of areal rainfall in years following very dry years is considerably larger than that of rainfalls following very wet years, there being little overlap between the two interquartile ranges. The author uses this graph to conclude that persistence is of less note than intrinsic variability in developing an action plan for combating drought. The lesson to be learned here though is the difference of .67 standard deviations (from -0.37 to +0.30) between the medians of the two distributions representing conditions following dry and following wet years, and the reduced range following wet years by comparison with that following average years.
Conditional Diatiibutiona
Fig. 2.1 - Conditional discharge of subsequent year's rainfall given three bands of current rainfall: 1. All years. 2. Dry years. 3. Wet years. (Derived from Katz, 1978).
A second procedure that likewise focuses on particular parts of the record was used first in climatology by Craddock et al. (1962) to study the structure of temperature records. The scheme involves the division of the data values into quintile classes and then counting occasions when in subsequent time intervals the data passes from quintile class i into quintile class j. The results appear as a 5 by 5 transition matrix containing the number of transitions, the expected values for random data being 0.04 times the total number of transitions. Tests can be performed on these data much as in contingency tables or transition matrices. However, the major benefit is the ease with which one may observe tendencies in the extreme. Two subsequent uses of the procedure concerned monthly quantities; Murray (1967) demonstrated that such persistence as appeared in his data was confined to the extremes and this was supported by Gordon et al. (1976) whose prime interest was in the possibilities the technique affords for forecasting (Section 3.3.1.5). Chapter 4 of Davy et al. (1976) advocated a variant on the contingency table to forecast annual and seasonal river flows in the Sahel region. River Senegal annual runoffs are used to exemplify the procedure and the transition matrix clearly displays more persistence following extreme years than following average years. Table 2.3 shows updates of Kraus' data analysed by quintiles. The sample size is somewhat low and tercile or quartile analysis may have been more appropriate, but a small increase in
8 persistence in the extremes relative to elsewhere within the distribution is apparent.
Quintile class in following year
I II III IV V
I 4 5 O 2 3 Quintile class in initial II 5 2 4 1 1 year III 3 1 2 4 3
IV 1 4 6 2 1
V 1 1 1 5 5 Table 2.3 Quintile analysis of Sahel rainfall showing frequency of transition from one year to the following year. Class Iis dry and Class V is wet. Expected occupancyof each is about 2.7. Persistence of extremes is indicated by tendency for higher numbers to occupy top left and bottom right at the expense of opposite corners
2.2.4.3 Do drier and more northerly Sahel Stations display more persistence? Walker's hypothesis (op cit) is rooted in the belief that a soil moisture anomaly will tend to persist through feedback mechanisms (Section 2.2.5.2). As stated in Table 2.2 they indeed did find evidence of greater persistence in a more northerly group. Likewise, it was mentioned in Section 2.2.3 that Brunet-Moret found the coefficient of persistence, A, to be highest in the 500 mm annual average rainfall zone, and this dry zone is of course to the north. However, it is not possible to generalize the result that persistence is related to dryness. The fact that the climate zones in West Africa do not run parallel to lines of latitude (see Section 2.3.2.3) is an added complication. Against this Yevjevich (1964) stratified the discharge and rainfall stations in his western United States data sets according to average annual runoff and rainfall and observed the opposite effect; that persistence was marginally greater in the wetter area. Of course the results may not be comparable as the regions are not comparable. The physical feedback mechanisms mentioned above and described more fully in Section 2.2.5.2 may be expected to make their presence felt more on a within-seasonal than an interannual scale so it is on this former level that rainfall persistence should be tested. Seasonal persistence is treated in Section 2.2.5.
2.2.4.4 Do station groups display more persistence than individuals?
An intuitive case can be made for the supposition that the average of station groups is more likely to display the persistence phenomenon on the grounds that the area represented is much larger so that local and short lived disturbances will be suppressed and hence both serial correlation and run lengths will be increased. A question that must be asked though is whether this increase is not merely a statistical fiction resulting from spatial averages, like time averages, having larger correlations than the fundamental data from which they are constructed. To elucidate this point, consider annual rainfall totals which have P1 of 0.1. The serial correlation between successive two year totals is a little under 0.2. Hardison's (1966) formu la
R = r(i-rN)'/{N(1-r)' + 2r(N-1) - 2r2 (N-rN-')} where r is the correlation between adjacent units R is the correlation between adjacent values consisting of N units accumulated will convert a p1 for single year value of 0.3 to a two year value of close to 0.5. The theory of spatial processes as applied to hydrological and climatic data has not reached the condition where it has answered such difficulties. It is common practice for climatologists to pool stations to create a regional rainfall index and KTaus' (1977) work is of particular interest because of its focus on recent subtropical drought. A quotation is highly relevant to the subsection title: "Rainfall records are more "noisy" than those of other geophysical variables. This applies particularly to regions which receive their water through convective storms. A few isolated perturbations can, and often do, produce transient excess precipitation locally without necessarily breaking a continuing drought. In general, the overall duration of a subtropical drought period can therefore not be documented from the analysis of single station rainfall records. Çtreamflows are more suitable.
9 Alternatively, the records from several stations have to be combined. Such a combination cannot be carried out by simple addition or arithmetic averaging of local rainfall values. The number of stations which are operative in a given region may vary from year to year. Some may have useful records which are
shorter, however, than the overall period which is the subject of analysis .....'I Two long term regional rainfall series from 1911 to 1974 are created from northern Africa and northwestern India data. The richness of these, like the Chapter 5 series, in long runs - 9 years of deficit from 1966 to 1974 and two full decades of surpluses - contrasts with Bunting's finding, based on individual station analysis, that there was no evidence of departure from a random independent process. Another author who has adopted a similar approach is Hastenrath (1976) who has prepared composite long term precipitation records for (a) the United States Central Great Plains, (b) Central American-Caribbean region, (c) the Brazilian 'Nordeste', (d) Coastal and (e) Interior Ecuador and (f) the African Sahel. The objects of this work were to find teleconnections and also to recognise patterns in the global circulation that foreshadow drought. However, the point is made that the series are not random and extreme weather conditions affect extended regions even though departures vary irregularly over the area. 2.2.5 Within year persistence The definitions of drought severity which have been adopted in Chapter 1 have for the most part concerned annual totals. However, for the problem of forecasting events within a drought, some knowledge of seasonal persistence is important. The forecasting problem is dealt with in Section 3.3 and here we simply review the relevance of the techniques to within year problems and briefly record some examples from the literature. First, though, we review the physical inertia and feedback mechanisms that are responsible for persistence at this time scale. 2.2.5.1 Hydrological subsystem The major inertial influence in the hydrological system is that due to the release of flow from stored sources such as aquifers. Under a simple theoretical river flow model (Weiss, 1973; Bernier, 1963) the correlation between values s time steps apart is tied very closely to a river's recession constant. Indeed under the simplest formulation the correlation at lag s is equal to the ratio of the discharges s time units apart along the recession. Thus to fix ideas one may assume as a first approximation that the correlation coefficient drops to 0.5 at the 2 'half life' of the recession and to 0.25 (i.e. .5 ) at twice this period. This refers strictly in the theory to instantaneous discharges and, although results are given by Weiss for averages over periods such as one month, the linkage as quoted here suffice to emphasize the importance of catchment size, presence of lakes and swamps, and porosity, storativity and transmissivity of the rock strata and soil material, these being the factors that control the recession characteristics. Feedback mechanisms within the hydrological subsystem are seldom discussed, at least in the same terms as in atmospheric models. However, the laws which govern the ratio of actual to potential evaporation are of this form. The more prolonged the dry period the more reduced this ratio becomes and the more effective is any rainfall increment in restoring internal storages within the soil profile. This is a negative feedback which will introduce 'antipersistence'. Examples of countering positive feedbacks are not SO forthcoming although it might be speculated that certain paths once established might increasingly favour a particular response but these are not allowed for in current conceptual hydrological models where for the most part model parameters depend only on the current value of state variables and not on their history. 2.2.5.2 Atmosphere and ground/air interface systems Hare (1977) presents a useful and succinct account of the climatic causes of drought and the material below borrows heavily from portions of that reference which relate to within year persistence. The climate system has to be viewed as a global system involving not only motions of air masses but also large water and ice bodies, and the land surface including its vegetative cover. The system thus formed is so complicated in its interactions that simple intuitive guesses as to how responses to external agencies or how shifts in one aspect may perpetuate themselves or react on others are not possible. Some mechanisms within the atmosphere sea system are known to have lifetimes measured in months and also can be expected to exert a strong influence on the supply and location of humid air, and are thus prime candidates for explaining seasonal persistence. Sea surface temperature anomalies are of this type and numerical modelling with General Circulation Models (GCMs) support this supposition as does a wealth of observational evidence (Barnett, 1978), (Namias, 1975). Experiments performed by Gilchrist (1975) and Shukla (1975) with GCMs have linked Atlantic Ocean and Arabian Sea temperature reductions with reduced
10 rainfall over North Africa and India. Anomalies of the global circulation including the position of the jetstream are also of this type and the literature on the topic is particularly rich; Chapter 3 of this report provides entrees to the large literature. Sea and atmospheric anomalies are probably not separate entities although the linkages, long speculated upon, are still open to controversy (Barnett, 1978; Chiu, 1979). Feedback mechanisms that are able to perpetuate drought conditions also are increasingly studied (see pp 50-59 of Hare, 1977, for an introduction). Gilchrist (1978) emphasizes the danger of simplistic assumptions about feedback mechanisms. Ellsaesser et al. (1976) offers a useful summary of hypotheses that have been advanced on physical and energy balance grounds but in most cases are not supported also by GCM tests that invoke other processes than those immediately involved in the feedback loop. These and others are summarised below, but the speculative nature of many of them must be borne in mind. All assume randomly occurring dry starting conditions. a. Rainfall is said to be reduced by the overseeding of clouds by dust particles from the surface; b. Dust reflects heat away from the earth so increasing subsidence and in turn reducing rainfall;
C. Albedo increase lowers surface temperature and decreases lifting, so further reducing rainfall; d. Albedo increase resulting in heat loss locally and a consequential temperature gradient which induces a circulation such as to restore equilibrium with warmer surroundings and inducing subsidence which depresses rainfall (Charney' s (1975) hypothesis) ; e. Decreased availability of biogenic nuclei for raindrop formation due to reduced plant cover; f. Increased albedo due to reduced soil moisture leading to an alteration in the proportion of net radiation needed as latent heat (Walker et al.,1977; Ratcliffe, 1977);
9- In many parts of the world an important source of moisture to supply rainfall is the evaporation of soil moisture from neighbouring areas (Schickendanz, 1976). Since the soil moisture itself derives from previous rainfall, we might expect wet periods to persist longer than chance alone would indicate and dry periods to persist even longer, especially in areas where soil moisture is almost the only moisture routinely available. There has been much recent interest in the role of and sensitivity to soil moisture in GCMs (Rowntree et al.,1981; Mintz, 1981; Walker and Rowntree, 1977). 2.2.6 Conclusions about persistence Serious drought is perceived as a run of deficit years but the results of run analysis contrast with searches for persistence based upon serial correlation. However, it appears from more detailed scrutiny of year to year transitions that persistence is concentrated mostly in the extremes and this highly non-Normal behaviour invalidates conventional significance testing procedures. The evidence for greater persistence in particular regions is not conclusive but it does appear highly plausible that the persistence phenomenon is seen more clearly when regional rather than individual station data are employed. The causes of persistence appear to be still a speculative matter although many outwardly plausible explanations have been proposed to explain both within and between year memory.
2.3 SPACE AND TIME HETEROGENEITY
2.3.1 Introduction Space and time variations are bound to be present within any drought. For example, river discharge, soil moisture and aquifer level deficiencies are not the same from point to point within the afflicted area, nor do the values of such variables remain at a uniform level during the drought. From an economic viewpoint this spatial inhomogeneity is beneficial and allows some agriculture to survive. In 1972 during the Sahel drought there was sufficient grass cover in some areas but often no water supplies from small ponds and wells. But the few areas with both grass and water permitted part of the cattle stock to be saved. The recognition and understanding of sources and extent of heterogeneity are important prerequisites of drought management and forward planning.
11 On the other hand, this spatial heterogeneity renders any overall quantification of the drought very difficult. In this section methods are described for displaying and quantifying the variable behaviour in space (2.3.2) and in time (2.3.3) of the primary hydrological variables. 2.3.2 Spatial heterogeneity 2.3.2.1 The cause of spatial variability Because we regard drought as a 'prime mover' responsible for water supply shortages of many possible types, any given drought pattern defined in climatic terms will convert to a variety of water shortages dependent upon small and medium scale weather patterns, topography, conditions prior to the drought, soil variability and properties, and man's environmental effects and requirements. The following subsections consider the degree of variability and methods of describing and quantifying it - maps of course being the most important tool. 2.3.2.2 Spatial extent of drought Less severe droughts, including those of the first type listed in Section 1.2.3 and particularly those where snow melt is not an important source of supply, generally cover relatively small areas, say between 10,000 and 100,000 km2. However, it quite frequently happens that other areas at the same latitude suffer simultaneously. When the spring runoff is the result of snowmelt then the drought may be regarded as belonging to the third type and the drought area tends to be more extensive and its consequences are more severe. This third type also includes those tropical regime cases where there is normally only one or two well defined rainy seasons for which vast areas around the globe may be simultaneously affected. Such was the case in the 1958 drought, and the current (1980) drought which began in 1965, which in Africa alone has affected 12 million km2 both north and south of the equator. Yevjevich (1967) has found that for other parts of the world the affected region is often elliptical. Particular droughts of this third type will be discussed in more detail in Chapter 5. But even for these extensive droughts a close scrutiny clearly indicates that the relative water deficit is far from uniform over the entire area. Relatively small areas may be spared and others may suffer larger deficits. The disturbing influence of mountain ranges may be responsible for local and ephemeral effects on the rainfall regime. Isolated rainstorms may alleviate the drought briefly as happened in the Western European drought of 1976. An intense but very local storm near Paris with a return period in excess of 10 years mitigated, in that area, the effect of the drought. Similarly in south west England isolated thunderstorms permitted seed germination and plant growth locally. The same phenomenon was observed during the Sahel drought in 1972 and 1973, and larger areas may be spared with no evident explanation. 2.3.2.3 Mapping drought intensity by means of rainfall data Local events, which are typical aspects of spatial heterogeneity usually have their main effect on small stream runoff. Because few of such streams are gauged it is usually more reliable to map rainfall data to analyse spatial heterogeneity despite the likely deficiencies in the raingauge network. These maps may portray rainfall depth or deficit in depth or proportional form, or alternatively may express the deficit in terms of probability. Two examples of such precipitation maps are presented here; others appear in subsequent chapters of this report. Figure 2.2 shows part of the African tropical zone affected by the drought commencing in 1968 and shows the rainfall deficit for 1972 (Sircoulon, 1976). Figure 2.3 (CBM, 1978) shows rainfall deficit probability for an area in southern Australia for 1977 illustrating the application of a slightly different technique with a greater network density and in a different climate zone. Rainfall deficiency analysis requires a sufficient number of good rainfall records and where the density of stations is low the contour map must be qualitatively completed by aerial observations made of vegetal cover. The African map shows contours of equal relative deficit from the average annual precipitation for the desert, the Sahel and the adjacent tropical dry hydrological zone in West Africa, the total length being 4500 km. Of course the drought extended far south and east of this area. An apparent decrease in the relative deficit from north to south is not very significant. The decrease is matched by a corresponding decrease in the year to year variability of annual precipitation, the coefficient of variation also decreases from north to south. If, like the Australian map, contours of equal deficit probability had been shown instead of equal deficit, then a large part of this apparent trend would be eliminated (see Section 2.3.2.4). However, what is relevant here from the African map, is the conclusion that the western and eastern
12 e, m id LI 5 id
13 WA
WESTERN AUSTRALIA 10 months Apiil 2 January 4 Serious Deficiency
ALL OTHER STATES 7 months July - January Severe Defncieno O- WO Mo JO0 400 500Km
Fig. 2.3 - Regions of rainfall deficit in Southern Australia during 1977. (From Drought Review Australia, Bureau of Meteorology, Number 102).
parts of the mapped region were more severely drought-stricken than the central area. Some important areas were relatively untouched by the drought; central Upper Volta and the area east of Niamey for instance. One small region in south Mali and south-east Chad experienced only a slight deficit. Soil variability was cited at the outset of this section as a source of spatial hetero- geneity but even on soils of the same permeability in different regions, equal rainfall deficits may not necessarily produce the same soil moisture deficits. For example, near the 10' latitude in Chad, a rainfall deficit of 20 per cent is required to give the same soil moisture deficit that would follow a 40 per cent deficit near the 14' latitude. This is because the precipitation in the north is much more concentrated in time than in the south and because the vegetal cover is not SO dense. It is thus necessary to study heterogeneity along bands which have constant climate and which slope slightly relative to lines of latitude, for instance Dakar at 15' was similar to N'Djamena at 12'. This was the situation in 1972 and for other years the contour pattern was no more complex although regions that were less and more severely affected did vary in position. On the Australian map, Figure 2.3, rainfall deficiency is slightly differently described using the following criteria. A ten month period is used; April 1977 to January 1978 (July to January for the West Coast) and rainfall percentiles are mapped. A 'severe drought' obtains when the rainfall is among the lowest 5 per cent of recorded rainfalls over the period. A 'serious drought' is defined as the situation when the rainfall is above the 5 per cent but less than the 10 per cent conditions. The map shows that the south west and south east coastal areas were more drought stricken in this period than the centre. Victoria, southern New South Wales and southern Queensland present marked spatial heterogeneity apparently not directly connected with the influence of mountains. In droughts in Western Europe the spatial heterogeneity is more marked than in Australia because of the more complex regime of air masses. An example of this spatial heterogeneity is shown on Figure 2.4 for the 1976 drouqht in Enqland and Wales (Hamiin and Wright, 1978).
14 O
Fig. 2.4 - Return perbd of occurrence of 5 month rainfall in England and Wales, April to August 1976, based on a random starting date analysis. (Map presented in Hamlin and Wright, 1978, prepared from information supplied by the Meteorological Office, U.K.).
2.3.2.4 Problems in mapping hydrological variables
A contour map of runoff deficiency is more difficult to draw because, as has already been mentioned, the spatial heterogeneity which is so characteristic of important droughts is apparent mainly on small streams of a type that are not commonly gauged. The pattern of contours would differ from that of rainfall for two main reasons: a. Soil permeability influences drought severity because impervious soils produce runoff from quite small rainfalls; b. The time distribution of rainfall influences runoff; the more concentrated the rainfall the higher the proportion that runs off. An instance of the latter occurred north of Ouagadougou in Upper Volta when the 1961 rainy season presented a significant global deficit but because two big storms accounted for the main part of the seasonal rainfall the annual runoff on small basins was significantly above average. As a consequence,, on small streams the pattern of runoff deficiency contours does not in general follow those for rainfall deficiency except where the pervious and impervious soils are randomly distributed and the pattern of rainfall deficiencies is uniform. In the runoff from large catchments, 100,000 km2 and above, this source of heterogeneity is largely absent. In 1972 the Senegal river draining the western tropical belt, and the Chari, draining the east showed more severe deficiencies (0.01 and 0.02 annual non exceedance probabilities) than the Niger draining the centre (0.05 probability). This accords well with the pattern observable on the rainfall map but, of course, reflects none of the finer detail of the contour pattern. Endoreic lake level behaviour is more attenuated still. Minimum levels of lakes such as Lake Chad on the Chad, Niger, Nigeria border, Lake Eyre in South Australia and Lake Faguibine in Mali are related to the sequence of low yearly rainfalls and would bear little resemblance to the rainfall deficit experienced in a single season. This hysteresis phenomenon is further described in Chapter 4 where it is shown that historic droughts are often well embodied in lake level records, especially the lakes of endoreic basins. For instance, by using information On Lake Fertg a 1,500 year drought record has been reconstructed (Bendefy, 1973). Palaeo- ciimatoiogists also find lake levels a rich source of evidence for past climatic Variations (Street-Perrott, 1981).
15 2.3.2.5 Statistical treatment of spatial variability In order to quantify spatial heterogeneity of drought the spatial correlation coefficient of monthly-or annual rainfall is used, in particular the rate of decay of the correlation coefficient between the rainfall of satellite stations and a central station as the distance between the two increases. Some examples for various parts of the world follow. Europe: Generally the monthly correlation decays to 0.5 at about 500 km distance, a faster rate of decay in summer and slower in winter, the controlling factors being the influence of , convective processes and the direction of movement of prevailing weather systems. The role of mountains is limited Figure 2.5 illustrates the type of decay that is met (Tabony, 1977).
Fig. 2.5 - Correlation coefficient of monthly rainfall in England and Wales with that of Kew for two seasons. (Prepared from data presented in Tabony, 1977, and reproduced with the permission of the Meteorological Office, U.K.).
India: Mountains have an important effect on the rate of correlation decay during the monsoon period. In the Himalayas the correlation decays rapidly to near zero but in central India significant correlation coefficients are met at distances of the order of 500-800 km (Flohn, 1968). West Africa: The influence of mountains is usually not so pronounced on total rainy season rainfall correlation decay rates. In the plains significant correlations may be found for distances of more than 3,000 km along lines of latitude (Rodier, 1960). Generally, though, the correlations between stations in the Sahel, tropical dry belt and equatorial zone are not significant. However, between the north and south tropical dry belts significant correlations are found. It is also observed that the spatial coherence is greater during dry years.
United States: A recent study in the Great Plains area showed a correlation decay rate such that over a distance of 100 km the correlation decayed to 0.8 and over 500 km to 0.27 (Tase et al., 1978). The correlations in this case were computed from monthly residuals from a monthly mean and standard deviation. 2.3.3 Temporal heterogeneity 2.3.3.1 General features of temporal heterogeneity Two time scales must be considered when treating time heterogeneity: the 'drought' time scale from several weeks to several months; and the 'dry period' time scale spreading over several years. In the first case when dealing with a shorter time scale one is concerned with how the different parts of the drought-affected area may experience rainfalls of different depths and at different times, and also with the storms that may to some extent interrupt the drought. In the second case one is concerned with how the isohyetal pattern varies from one year to the next. Two examples are given in Section 2.3.3.2; one concerning the United States west coast drought over the 1976 and 1977 winters. The second one concerns the 1960-1965 dry period in Australia, more specifically the two years 1961-1962 within it (Hounam et al.,1975). In both examples the severity is represented by contours of rainfall deficit. We consider in section 2.3.3.3 the temporal variability of runoff sequences.
16 2.3.3.2 Case studies of temporal variability
At the outset of the west coast USA drought, in the 1975-1976 winter, conditions were most severe in the northern half of California and northern Nevada while central Oregon and Washington were less affected. In the following winter, 1976/77, the drought spread to cover the whole of California, Oregon, Washington, Idaho, Nevada and Utah, and parts of British Columbia (Canada) and Arizona. Rainfalls in 1977/78 were above average everywhere in the region. In the Australian example, the entire central part of the continent was badly drought- stricken in 1961 while eastern and western areas received above average precipitation. In 1962 a very different pattern was observed; only a part of Western Australia was affected, rainfall depths were average in the central area and wet or very wet in parts of the east. As the drought progressed through 1963, 1964 and 1965 each year presented a different pattern of deficiencies. Other examples of time variations of the pattern of deficiencies are given in Chapters 5 and 6. 2.3.3.3 Runoff variability
Time heterogeneity of runoff deficiency is broadly similar to that of rainfall deficiency with the provisos listed in Section 2.3. Consequently it is easier to analyse it using the discharges of large rivers wherever possible. Some examples are given below of series of deficiencies and excesses of annual runoff quantities during long and severe droughts. The first example concerns two Sahel rivers during the drought commencing 1968: (a) a small mostly impervious catchment, the 'Kori' of Badeguicheri (Niger) and (b) a larger and mostly permeable catchment, the 'Ba Tha' (Chad). Their average annual rainfalls are 470 mm and 500 mm respectively. Table 2.4 shows the rainfall deficits and excesses.
Observation Average Deficit or excess in % Area Station period discharge km years m3/s 1968 1969 1970 1971 1972 1973 1974
Kori of Badeguicheri 825 35(l) 1.34 -87 -57 +15 -77 -66 -47 +380
Ba Tha at Ati 45,290 20 19.1 -74 -34 +87 -72 -75 -27 +24
Note: (1) Part estimated using rainfall runoff correlation
Table 2.4 Runoff variation at two stations
Note how, in 1974, the rainfall excess had a much more marked effect on the 'Kori' of Badeguicheri. All vegetation had disappeared a long time before the onset of the rainy season. The wet year, 1970, in the middle of a run of dry years is also to be noted. Other tropical zone examples will be quoted in Chapter 5. The second example concerns the same drought, but this time in India, represented in Table 2.5 by the Godavari at Polavaram (299,320 km2) . Within the run of dry years shown in. the table, 1970 (as in the African Sahel), 1973 and 1967 were slightly wetter years, and 1969 was an average year.
Observation Average Deficit or excess in % Area Station period discharge km2 years m3/s 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974
Godavari at Polavaram 299,320 74 3,041 -46 -17 +16 -30 -1 +6 -41 -50 +12 -5
Table 2.5 Runoff deficits and excesses for the Godavari (India)
2.3.4 Conclusion
Like space heterogeneity, time heterogeneity can also provide a welcome breathing space for the affected population, but also makes forecasting and planning more complicated.
17 Paris (49' N; 2'30' E)
Marseille (49'30' N; So E)
Budapest (47' N; 19' E)
300 I
Duluth (47' N;92' W)
Moderate --- 800 1- I Heavy oOo Very severe v . Zinder (I4O N; 9' E) (Lines show rainfall above mean.)
Bornbxy (19" N:73O E)
1300 r I
Perth 13?'S: 116" E)
Fig. 2.6 - Annual rainfall variations for selected stations (x = mean rainfall; 0 = Standard deviation calculated for the period 1931-1960).
18 It is well known that the big droughts affect large areas of the Earth simultaneously, especially those caused by perturbation of large air masses as in the tropics, but this is not necessarily reflected in the simple correlation between series such as given in Table 2.4 and 2.5. The seeming random interruption of the dry sequences at different times at different sites reduces the level of correlation whose testing is in any case very complicated (see Section 2.2.4). The year 1970 seems interesting by virtue of its generality but many more examples can be quoted of the opposite, exemplified by the year 1974 which was wet in the Sahel but dry in parts of India. This aspect of drought analysis, just as the indexing of spatial and temporal variability in a region, remains an area of applied hydrological research.
2.4 DROUGHTS AND CLIMATIC CHANGE
2.4.1 General Are droughts nowadays more or less frequent than in the past? Is significant trend discernible in the evolution of this phenomenon? Much has been written in recent years on the possibility of climatic change (e.g. Bryson, 1974). Any change clearly has important consequences to mid- latitude and subtropical drought. In mid-latitudes drought is most severe when the shortage occurs during the growing season although water supplies for domestic and industrial use are also affected by winter drought. In tropical areas where relatively well defined dry and wet seasons alternate, droughts threaten not only agriculture but also human life, subsistence agriculture being the rule in those latitude s . The following subsections review the various forms o'f natural climatic change that have been postulated, change due to anthropogenic (man-induced) causes is treated in Section 3.2. In particular the ramifications of these changes to drought incidence are discussed. 2.4.2 Drought occurrence since 1850 Drought incidence in the recent epoch, since instrumental records began, can be quantified using a variety of schemes relating annual climatic values to long term normals. Chapter 4 describes the various methods that have been used. In this section droyght -intensity is measured in terms of numbers of standard deviations below the mean, e.g. x - O, x - 20 etc. (see Figure 2.6). While such a measure is reasonably successful at quantifying drought intensity in enabling comparisons to be made between different locations and between different years, it does not describe that other important feature of drought, its spatial variability. No drought affects the entire world or entire hemisphere, although in 1893 and 1921 a band spanning much of the mid-latitude of the northern hemisphere (Eurasia and United States of America except the south) was drought afflicted. Similarly around 1913 and 1970 drought extended over much of North Africa between the Mediterranean and Sudanian zone (Dorize, 1976). The spatial variations as exemplified in Figure 2.7 cannot be simply indexed SO the subsequent paragraphs discuss periodic and other climatic trends in terms of point or fixed regional values...... 1961 ...... 1964
&$A 1963 V&L 1965
Fig. 2.7 - Moving droughts:an Australian example. (Reproduced from Gibbs and Maher, 1967).
19 2.4.3 Are droughts periodic? The possibility that droughts (and other natural events) may occur as regularly periodic occurrences has exercised a very strong hold over man's imagination and the search for periodicity has provided a considerable source of research endeavour. Well over 1000 scientific papers have appeared linking climatic events with the 11 year single cycle of sunspot numbers alone. This basic cycle has spawned several others related to it; the 22 year double or Hale cycle, linked by Flohn (1968) to blocking frequency, and a 44 year pseudo cycle linked by Sanson (1954) to rainfall variations in France exclusive of the Mediterranean area. Others at about 80 and 160 years are claimed (Chapter 8 of Gribbin, 1978) which are supported to some extent by isotope studies of ice cores (Duplessy, 1978, in Gribbin 1978). Despite the absence of a well established physical link between sunspot numbers and climate some quite compelling evidence has been presented for a tendency for droughts to occur at some particular phase of the sunspot cycle. Perhaps the best known example is the cycle of droughts in the Great Plains area of the United States which suffered droughts in the early 1850s, 1874, 1890, 1913, 1930s (the 'dust bowl' era) and mid 1950s. Palmer (1965) was led to speculate about a cycle of drought recurrence which could produce another serious event in the early to mid 1970s. In the event those years and subsequent ones have seen nothing on the promised scale. Another well known example is the level of Lake Victoria which, between the closing years of the 19th century up until 1925, seemed to follow the sunspot numbers trend. The subsequent behaviour up to 1961 was of much increased noise (interpreted by some as a 5Js year half sunspot cycle), then a rapid rise followed by a slow but broken decline. Despite the enormous visual departure from cyclic behaviour, it is still common to see these data quoted in support of a solar weather link. Similar analyses (Giraud et al.,1976; Boudet, 1972; and Landsberg, 1975) revealed some cyclicities in tropical west African rainfall but Bunting et al. (1976) failed to find any in 11 stations also in West Africa although confirming an 11 year cycle for rainfall at Addis Ababa. Figure 2.8 shows a tendency for severe droughts at Paris to occur at the time of sunspot minima although the two most notable events of recent decades, 1959 and 1976, were not of this character.
Wolfs numbers 200
O0 O 00 O O O moo O0 O0 O O O00 O000 ,900
Fig. 2.8 - Solar activity and severe droughts in Paris since 1800. The curve indicates the variations in Wolf's relative sunspot number and the points indicate severe rainfall droughts in Paris.
Overall the literature presents a very mixed picture showing great contrasts between negative and positive conclusions both seemingly objective in their methods. Section 3.3.2.3 highlights such cases with riverflow sequences and other similar disparities are noted in Section 3.1.2.7 and in a recent review by Pittock (1978) who draws particular attention to statistical inadequacies in many investigations. Section 3.1.2.7 highlights the problem from the point of view of sun-weather linkages while attention is also paid to cycles in Section 3.3 with respect to climate forecasting (3.3.1.8) and runoff forecasting (3.3.2.3). Other periodic components have been cited, in particular the Milankovich cycles. These are of much longer duration, measured in tens of thousands of years and act through variations in the earth's orbit and attitude to alter the effective solar constant. Mason (1976) presents past evidence and long term future forecasts based upon the Milankovich cycles which are held to be partially responsible for glaciations and warmings. The quasi biennial oscillation, like the sunspot cycle, lacks a generally accepted linking mechanism but has been used informally in climate forecasts (Parker, 1976; Dyer, 1979;
20 Thapaliyal, 1979). Many other cycles are reported in the literature. Most arise empirically as statistically significant spikes in spectral analyses of climatic and hydrological time series. Before accepting them as genuine, great caution must be exercised considering the following factors: a. Persistence in the data (Section 2.2) affects the base line from which statistical significance is measured, and much reduces the significance. b. Non Normality of the data renders the significance tests doubtful.
c. The non replication of the cycle in subsets of the data or in other local time series should sound a warning. d. Inadequate specification of the null hypothesis on which the significance is founded. The significance test as normally stated refers to the probability of a chance occurrence of a periodicity in the sample even if it is not present in the generating population; but this spike must be stated a priori. Thus if the spectrum consisted of independent spectral estimates at 100 frequencies 5 of them would exceed the 95 per cent significance level entirely by chance. This is an important and much neglected consideration.
e. Moving averages and filters applied to the data affect the significance levels. f. Preselection of the data in terms of choice of records which optimise the effect, and reduction of the length of the record in order to show a particular effect will lower the significance of that effect. Examples of these factors in climatic literature are given by Pittock (1978) while practical advice on allowing for the effect of many of them may be found in WMO (1966a). 2.4.4 Droughts and climate in motion
Up until the middle of the present century the earth's climate has probably not been affected by man so the secular trend of a global warming during the 1890 to 1950 period was probably a natural event. This trend has not continued to the present day as, in the northern hemisphere at least, there has been a cooling trend over the last two decades (Kukla, 1977). However, some have criticised the estimates of temperature on the grounds of sparsity and continental bias in the network (Parker, 1980 and 1981). Droughts and air temperature are related to each other and as indicated by Figure 2.9 follow the same variations. The same phenomenon is well marked in subtropical areas: when monsoon rainfall is deficient the air temperature is higher, evapotranspiration increases, soils dry and the entire warming process may increase through the feedback processes mentioned in Section 2.2 to accelerate the drought process.
Temperature (annual mean)
Number of A .- droughts
1841-50 189 1-1900 1941-50
Fig. 2.9 - Mean annual temperature at Leningrad compared with total rimer of droughts in each decade in European part of USSR.
Clearly such trends cannot continue unabated indefinitely and reversals must occur eventually even if these do not take place with strict periodicities. Explanations for these irregular vacillations concern the effect on atmospheric transparency of volcanic dust
21 particles, chemical and physical changes in the ocean and atmosphere and so called 'intransitivity' in the atmospheric circulation mechanism. This latter theory suggests that the global circulation does not have a single rest state but has several equilibrium conditions. This can perhaps also be perceived elsewhere in the hydrosphere where temperature changes once initiated by random events persist through the high thermic inertia of water and so adjust boundary conditions controlling the subsequent climate. The effect of such phenomena on drought occurrence is a matter of speculation only and can best be approached by appealing to the evidence of the past (Section 2.4.6). 2.4.5 Anthropogenic climate change Man induced effects on the climate are referred to with other 'effects of man' in Section 3.2 so will only briefly be mentioned here. Industrial pollution, dust and smoke particles, heat and most importantly carbon dioxide, may affect the climate through chemical and physical action in the troposphere and stratosphere. Agricultural particles may create dust, alter albedo, impoverish local moisture supply and carbon dioxide sinks all of which are potentially capable of altering the climate. Agricultural settlement of arid regions, for example Israel and Kazakhstan, can introduce, but also use, moisture (Rosenan, 1963). Consequences of such changes have different signs and act differently over the globe. It is believed that the overall trend will be towards a global warming (Kellogg, 1977; 1978) although this is difficult to reconcile with the recent northern hemispheric cooling (Kukla, 1977). It is possible that the momentum of anthropogenic warming has been overestimated but the expectation must remain that in the decades to come man induced trends will overtake natural fluctuations. 2.4.6 Climate history Probably the most reliable method of predicting the future pattern of events is by reviewing the range of occurrences of the past. Techniques for climate reconstruction vary very much in their abilities to discriminate in time and space (see Kutzbach, 1975, for a general review). For hydrological drought studies it is preferable to have a time discrimination of a single year, because longer averaging periods illuminate only average climatic conditions. This limits the choice to historical documents (Section 4.21, ice layers, sediment cores and tree rings. Among the elements studied from these sources are layer and ring thickness, pollen constituency and isotopic composition. Figure 2.10 shows an early example of the
!II 1 Wider rings
Narrower rings I I I i
Solar activity
\%9 \9."
Fig. 2.10 - Variations of the solar activity and tree ring width (Acacia giraffae) from Namibia. (After Walter, 1936).
22 association of ring width with drought incidence and solar activity (Walter, 1936). More recent examples have applied ring width to extending river flow records in the Colorado basin (Stockton, 19771, and temperature and precipitation.records in North and South America (La (La Marche , 1978) . Such studies indicate that the climate of the past 10,000 years has been relatively warm and in common with other such interglacial periods from the distant past will give way to a colder climate. Fluctuations on the shorter scale of about 100 years have given rise to more transient coolings and it could be that the cooling trend since the 1940s marks the initial stages of a repetition of the so called Little Ice Age when between the 16th and mid 19th centuries mid latitude temperatures were some 0.5OC lower than the maximum achieved in the 1940s. The temperature has since dropped back about half of the rise to 1940 with some very recent indications of a halt to this cooling trend. 2.4.7 Climate change and drought incidence The climate change literature focuses on the problem of climate normals; drought even under a different climate regime is expressed as a departure from those normals. So while climatologists may speculate on the consequences of global warming (see Section 3.2.1.2, and also Figure 2.9 for support for average rainfall reduction) in terms of decade averages, hydrological consequences depend on short term factors including, in mid latitudes especially, the within year pattern of rainfall occurrence. Scrutiny of historic drought assemblages do not reveal a clear cut association with periods of greater or lesser warmth. Perhaps, as suggested by Bryson and Lamb in many of their publications, the strength of the circulation is the more important climatic indicator for drought studies. The associated high variability accounts for the observation that periods of excess drought occurrence have also been periods in which many other adverse climate effects were common.
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WMO. 1966a. Climatic change. Tech. note no. 79, h'M0 no. 195. Geneva, (TPlOO), 79 p.
WMO. 1966b. Some methods of climatological analysis. Tech. note no. 81, WMO no. 199. Geneva, 53 p. (TP103). Yevjevich, V. M. 1964. Fluctuations of wet and dry years. Part II - Analysis by serial correlation. Fort Collins, U.S.A., Colorado State University, (Hydrology paper no. 4), 50 p.
Yevjevich, V. M. 1967. An objective approach to definitions and investigations of continental hydrological droughts. Fort Collins, U.S.A., Colorado State University, (Hydrology paper no. 231, 17 p.
26 3. Factors responsible for the droughts
3.1 PARTICULAR ASPECTS OF THE DYNAMICS OF THE AIR MASSES
3.1.1 Introduction In Chapter 1 we have endeavoured to define the term 'drought' and emphasise that, despite being contingent upon its impact on society and on the economy, a primary characteristic of hydrological drought is an extended period of below normal precipitation. With this representation of drought it is obvious that drought can occur almost anywhere in the world because natural variability of precipitation is a universal, statistical characteristic of climate. This part of the report will be concerned largely with drought in temperate latitudes, although some reference will be made to tropical latitudes. In addition, the following material will be concerned largely with the statistical, synoptic and physical aspects of drought on time scales of a month to several years. At the start it should be made clear that there are many unsolved 'mysteries' of drought. While some physical understanding has been achieved for droughts that last from a month to a season, spells of years characterised by drought are poorly understood, and thus remain on the agenda of research climatologists. At the start, we must stress that drought is the statistical chance combination of persistent and persistently recurrent meteorological events. Thus, if a high-pressure area and associated dryness enters a given region for a few days, a similar weather situation is apt to recur several days later, and recur again and again. The net effect is to produce, in wind and weather patterns, deviations from the mean that are highly anomalous. The central question, therefore, is "Why do these drought producing patterns have an affinity for certain areas at certain times"? We shall see that the drought problem is not local in character but, rather, is almost global because of interconnections between parts of the general circulation of the atmosphere. From time to time we shall give references to certain droughts and to certain related phenomena. The literature on drought is voluminous. The Russian literature includes a number of studies of droughts and sukhoveis (foehn winds) in their European territory (Dzerdzeevskii, 1957) with emphasis on gaining an understanding of the mechanics of such phenomena and the effect upon agriculture. Another source, a drought bibliography by Palmer and Denny (1971) contains hundreds of references to articles on drought. Regional studies have appeared by the hundreds and are exemplified by the work of Friedman (1957) and the California Department of Water Resources Report on the 1976-1977 California Drought (1978). During the 1970s, the subject of drought has been catapulted into the news and into the scientific commuhity because of severe events like the Russian drought of 1972 (Kats, 19731, the two year back-to-back West Coast drought of 1976 and 1977 (Namias, 1978a, 1978b1, and the European d5ought of 1976 (Ratcliffe, 1977, Miles, 1977) all dealt with in Chapter 6. Of course, the Sahel drought, treated in Chapter 5 of this report, has been documented by many individuals including Rodier (19751, Landsberg (1975) and others. 3.1.2 Some physical factors associated with mid-latitude drought 3.1.2.1 Subsidence and its causes Perhaps the most characteristic signature of regional drought over most areas of temperate latitude is the presence of warm, dry air in the middle troposphere. This warmth aloft can be associated with slow sinking motions (subsidence) of the order of several hundred metres per dray. Effectively, the sinking motions and associated adiabatic heating and low relative humidity inhibit precipitation, because it is ascending air motion with attendant adiabatic
27 cooling and condensation that is responsible for most precipitation. Besides, during warm seasons dry and warm air aloft discourages the growth of cumulus clouds, both because of the static stability of the air and because of the entrainment of moist cloudy air with the dry ambient air. The phenomenon of subsidence was first described and quantified by the great meteorologist Margules (1906). Margules showed that sinking motions would not only lead to warming of the air adiabatically and to stablised lapse rates ('lapse rate' refers to the decrease in temperature with height in the atmosphere. 'Stable lapse rate' means that the air is relatively cool and dense at low levels but relatively warm and light aloft so that any tendency for upward motion or mixing is resisted), but he also showed that during subsidence there are frequently horizontally dlverging air masses that lead to even more stable lapse rates. Examples of the manifestations of subsidence on the vertical temperature distribution are shown in Figures 3.1, 3.2 and 3.3. Figure 3.1 shows conditions in the core of the severe drought in California during the winter of 1976. Figure 3.2 shows similar plots for the Great Britain drought during the summer of 1976 and Figure 3.3 for the Moscow summer drought of 1972. Note the relative warmth aloft in all three cases. This warmth can be accounted for by subsidence of roughly several hundred metres per day. Associated depths of rainfall are given in tables in the lower left of these charts. The question arises as to what features of the general atmospheric circulation are responsible for the subsidence and why is there persistent recurrence of these drought-producing features (Tschirhart, 1969). In the first place, it may be shown that, particularly during warm seasons, subsidence is associated with high pressures mainly in mid-troposphere and, at times, at the surface. It is well known that because of surface friction, the air flowing round these high pressure areas is forced to leak out of the bottom layers, - in the so-called boundary layer. To satisfy the principle of continuity, this outflowing air must be replaced, and the replacement usually comes about through sinking of the air masses aloft. But this frictional divergence is not the only cause of subsidence. The dynamics of long waves in the westerlies (Rossby waves) demands areas of convergence and divergence of air aloft (Bjerknes, 1937). The converging air results in increased pressure aloft that is frequently compensated by divergence and lowering pressure below. The piling up of air aloft, often associated with a high tropopause, is frequently found in the deep warm anticyclones associated with drought. Not infrequently, particularly during cold seasons of the year, sinking motions are induced in the
OAKLAND, CALIFORNIA U.S.A.
'O/
9-
JANUARY 8-
7-
6- -E 1 -c 3 5- wx
4-
3-
2-
Rainfall l- 1976 9 rnm Namal 111 rnm
I I I
-50 -40 -30 -20 -10 O 10 ~
TEMPERATURE (OC)
Fig. 3.1 - Upper-air temperatures in the core (Oakland) of the California drought for January 1976 and the normal temperatures. Numbers. beside temperature plots give e relative humidities. Rainfall amounts are given in the lower left.
28 CRAWLEY, ENGLAND
I0I
JUNE
TEMPERATURE (OC 1
Fig. 3.2 - Upper-air temperatures in the core (Crawley) of the British drought in June 1976 relative to the more normal June 1974 temperatures. Numbers beside temperature plots are relative humidities. Rainfall amounts at nearby Kew are given in the l.ower left.
MOSCOW, USSR IQ
9
7
4
3
2
RAIN LCL I 6Omn '71 840m 24mmn'72l2oom ii8m 'T3 KXY)m , , S5,79.5%' \ O 3 TEMPERATURE (*Cl
Fig. 3.3 - Upper-air temperatures in the core (Moscow) of the 1972 Russian drought compared with those of July 1973 and 1971. Numbers beside the temperature plots give, from left to right, relative humidities for 1973, 1971 and 1972. In the lower left, LCL refers to the lifting condensation level.
29 mid-tropospheric northwest currents of air behind long-wave troughts. Thus, in these regions, if the flux is sustained or recurrent, substantial deficiencies in precipitation, and hence drought, can occur. In addition to the dynamics and thermodynamics associated with subsidence, one must consider the source and trajectory of moisture. Obviously tongues of moist air emanating, let us say from the Gulf of Mexico, given proper systems involving ascending motion, can produce heavy rains, while descending dry tongues, flung southward from continental sources, can lead to deficient precipitation. An an example of what we have said, we refer the reader to Figure 3.4 which shows the pressure distribution and its standardised anomaly associated with the California West Coast drought of winter 1975-1976 and the beginning of the European drought. Notice that anticyclonic conditions and positive anomalies of pressure dominate the drought areas. In Figure 3.5, one of the months of the especially severe Dust Bowl drought of 1936, we see the upper-level anticyclone and the associated extreme warmth over the Central Plains of the United States. The warmth was in part associated with the lack of cloud and precipitation during this period, and the consequent high insolation received. In Figure 3.6 is shown the moist (MI tongues and
Fig. 3.4 - The 700 mb heights (metres) and standardized departures from the long-term mean for winter 1975-1976. (SLP: sea level pressure; DM: departure from the mean - in millibars).
30 AUGUST 1936
700 mb
FigL. 3.5 - (Upper) Average contours of the 700 mb surface for August 1936, a drought month. (Lower) Average temperature departures from normal (OF) for August 1936. (DN: departures from normal).
31 Fig. 3.6 - Mean isentropic chart for the potential temperature surface 31SaK for August 1936. Height contours (broken lines) are labelled in metres above sea level. The average moisture content of the air at this surface (in g/kg) is shown by solid lines. Moist and dry tongues are labelled M and D. Inset shows the departure from normal of precipitations in inches.
Fig. 3.7 - Observed 790 nb height pattern (contours labelled in tens of feet) during July 1972 when the great Russian drought occurred.
32 (D) dry tongues associated with the 1936 case. It is important to note that over the Central and Southern Plains of the United States, dry air originally from Canada spirals into the drought affected area, sinking as it progresses. In the upper right of this figure is shown the deviations from normal of precipitation. Some of these features are described in an article by Wexler and Namias (1938). Likewise, the Russian drought of the summer of 1972 (see Figure 3.3) was associated with a strong upper-level anticyclone as shown in Figure 3.7. 3.1.2.2 The role of high pressure cells Before we leave this portion of the report, it should be noticed that there are oceanic high-pressure cells companion to those over the drought-affected areas. For example, in the 1936 drought (Figure 3.51, it will be seen that strong high-pressure areas exist over the Atlantic and Pacific as well as the Continental United States. Other cases of this kind will be shown later. It will be pointed out that each drought-producing cell appears to require for its sustenance companion cells remote by thousands of kilometres. These cells are in fact the great centres of action first described by Teisserenc de Bort (1881). Their explanation as reflections of the mid-tropospheric prevailing wind patterns, was first given by Rossby and collaborators (1939). Essentially, these high-pressure areas (upper-level ridges) together with the low-pressure troughs comprise the upper-level westerlies. Positions of these long waves determine to a large extent climate and variations in climate. Figures 3.4, 3.5 and 3.7 each illustrate that drought is associated with a ridge or a cell of high pressure at the 700 mb level in the atmosphere. The figures are monthly or seasonal averages; this does not imply that the indicated patterns persist throughout the month or season, but, rather, that such patterns frequently recur owing to the influence of near-stationary waves. 3.1.2.3 Teleconnections between pressure anomalies Because the high-pressure centres-of-action over the hemisphere are interrelated or teleconnected, there may be three to five of these anomalous high-pressure cells present in any one season. This is especially true during the warm season when the prevailing westerlies are farthest north. Thus, if one averages the zonal wind speed for various latitudinal belts over much of the hemisphere, one will find deviations from normal in the position and strength of the westerlies during droughts. An example is shown in Figures 3.8 and 3.9 for the summer of 1955, where the westerlies from O' westward to 180' were displaced north of their normal position and were stronger in high latitudes than normal. In Figure 3.9 note that the height departures are generally negative in the low latitudes, positive in the mid-latitudes and negative again in the polar regions. This indicates a general poleward shifting of the pressure zones - normally high at about 30' latitude and low at about 60'. During this summer, the Central Plains Region of the United States was frequently affected by hot, dry conditions as was northern Europe from Scandinavia through Great Britain.
Fig. 3.8 - Main 700 mb zonal wind speed profile in the Western Hemisphere (0' westward to 180') for August 1955. An abnormally contracted circumpolar vortex was associated with above normal westerlies north of 45'N and subnormal westerlies to the south. Note the strong easterlies in the subtropics.
33 Fig. 3.9 - Mean 700 mb contours and height departure from normal (both in tens of feet) for August 1955. Note continuous zonal band of positive anomalies in the middle latitude associated with contracted circumpolar vortex.
34 Another example of the synergistic effect of companion anticyclones occurred during the summers of 1952-1954, when the Southern Plains region of the United States was exceptionally dry and warm as shown in Figure 3.10 (Namias, 1955). This drought was associated with three anomalously strong upper-air anticyclones (Figure 3.11); one over the central north Pacific, one over the central north Atlantic and one over the south central United States. Under each of these anticyclones dry conditions prevailed although for obvious reasons only the continental drought received much attention. We can illustrate the teleconnection process with the help of cross-correlation charts as shown in Figure 3.12. Here we correlate the 700 mb height in the Atlantic High or the Pacific High with all other points 5's of latitude and 10's of longitude apart. Figure 3.12 shows that when the Atlantic and Pacific Highs are strong during summer, the central United States anticyclone is also likely to be strong. 3.1.2.4 Self generating mechanisms
While the oceanic cells make it more likely for the continental high-pressure cell to emerge and persist, the latter may have its own self-generating properties. While these are not completely understood at present, there appear to be a few theories backed up by data. One of these is that the land, rendered hot and dry during drought, heats the air above it and further enhances the upper-level isobaric surfaces. A second possibility is that under dry conditions there is likely to be an increase of fine dust particles in the air, and these would lead to high cloud droplet concentrations whenever cumulus clouds are formed. As Twomey and Squires (1959) have pointed out, this mechanism could make it more difficult for precipitation to form. A third theory that has recently received prominence has been suggested by Charney (1975). Charney suggests from numerical modelling studies that high albedo in dry areas (particularly deserts) 'contributes a net radiative heat loss relative to its surroundings and that the resultant horizontal temperature gradients induce a frictionally controlled circulation that imports heat aloft and maintains thermal equilibrium through sinking motion andadiabaticcompression'. Seasonal persistence is further discussed in Section 2.2.5. Whatever the mechanisms involved, there is some statistical evidence suggesting that dry, warm springs over the plains of the United States have a tendency to be followed by hot, dry summers (Namias, 1960). Further, there is some evidence presented in the same report that hot, dry summers in the plains have a tendency to persist from one year to the next. 3.1.2.5 The role of sea surface temperature We still have not fully answered the question of why these high-pressure cells persist over long intervals. Within the past few years, there have been some studies suggesting that the coupled atmosphere-ocean system may account for some of this persistence. As an example, we show in Figures 3.13a, b and c sea-surface-temperature (SST) anomalies associated with the 1952-1954 summer droughts described previously and the 1975/1976 drought in Europe. It will be noted that anomalously warm water is associated with anticyclones and consequently the gradients of SST to the north of these warm pools was enhanced. This gradient in SST, transferred to the overlying atmosphere, could account for the strong high-latitude westerlies in these areas. In turn, the high-latitude strong westerlies, if periodically super gradient, can frequently transfer air southward and thereby converge to maintain the high-pressure areas to their south in the manner suggested by Rossby (1937). Similar comments may apply to the Russian 1972 drought as implied by Kats (1973), and to the 1976 drought described by Brochet (19761, Ratcliffe (1977) and Namias (1978b). Thus, the clue to predicting mid-continent U.S.A. and European drought may lie partly in the coupled air-sea system. 3.1.2.6 Long waves in the westerlies Another mechanism for the initiation and maintenance of drought involves the positioning of the long waves in the westerlies. As pointed out by Rossby (1937) and later demonstrated theoretically b9 Smagorinsky (19531, there are favorable sites for the generation of forced perturbations in the westerlies. Thus, from a climatological standpoint, the east coast of Asia during winter is almost always characterised by a strong trough produced in part by diabatic heating of cold Asiatic air masses moving over the warm Japanese current and by mountain effects of the Tibetan Plateau. Similarly, the North American Rockies force a trough to their east through complex dynamic effects on the westerlies. The east coast of North America provides another source of troughs in a mechanism similar to that of the Asiatic trough. Once established these troughs tend to set up high-pressure ridges downstream at a wave length determined by the strength of the zonal westerlies (Rossby, 1937). The upper wind pattern over the Atlantic and Europe is conditioned in large part by the North American trough as described by Bolin and Charney (1951). While these considerations apply to the long-term climatological state, this state is never observed during any one month or season. Since it is the anomalies that are responsible for
35 Fig. 3.10 - (Upper) Average temperature departures from normal (OF) for the three drought summers (June, July, August) of 1952-54. (Lower) Percentage of normal precipi- tation for the summers 1952-54.
36 Fig. 3.11 - Mean contours of 700 mb height (solid) and lines of equal height departure from normal (broken) for the three summers 1952-54. (In tens of metres).
37 700mb SUMMER MONTHS TELECONNECTIONS (CROSS - CORRELATIONS)
Fig. 3.12 - (Upper) Teleconnections between 700 mb heights over the field as correlated with a point in the North Pacific (labeled 1.00). (Lower) Same with a point in the North Atlantic (labeled 1.00). Note in each case the positive correlation with 700 mb heights over the central United States.
38 TEMPERATURE ANOMALIES SUMMER 1952
Fig. 3.13a - Sea surface temperature (SST) and temperature anomalies (OF) for the summer of 1952.
39 TEMPERATURE ANOMALIES SUMMER 1953
Fig. 3.13b - Sea surface temperature (SST) and temperature anomalies (OF) for the summer of 1953.
40 TEMPERATURE ANOMAL1ES SUMMER 1954
Fig. 3.13~- Sea surface temperature (ÇST) and temperature anomalies (OF) for the Summer of 1954.
41 Fig. 3.14 - Winter (December, January and February) 1976-1977, 700 mb contours (solid lines) and isopleths of departure from nomal (broken lines), both labeled in tens of feet.
42 Fig. 3.15 - Sea-surface temperature anomalies and isopleths of 700 mb height anomalies (intervals of 50 feet) in winter 1977. Stippling indicates 1°F or more above normal. Slant shading indicates l0F or more below normal.
drought, one must adequately explain the interannual variations and the positions and amplitudes of the long waves. Obviously, if one could forecast a strong trough - let us say in the central Pacific during winter - he would be in a good position to forecast dry conditions along much of the west coast of the United States because of the probability of a strong high-pressure ridge in that area. Precisely this condition was observed during the strong drought in the winter of 1976-1977 as is shown in Figure 3.14. It is possible that the mechanism which maintained the strong trough and responsive west coast ridge was due to the establishment of a strong anomalous gradient of sea-surface temperature (SST) between the warm eastern Pacific and the very cold central and western Pacific as indicated in Figure 3.15. It was hypothesized by Namias (1978d1, that this gradient in SST would not only sharpen atmospheric fronts and troughs, but also produce more southerly wind directions in the zone of temperature contrast. These upper level winds in turn steered cyclones northward toward Alaska rather than eastward toward the west coast. In this manner persistent recurrence of these abnormal storm tracks resulted in drought along the west coast. Other contributing factors are described in the same article. Somewhat similar conditions appear to have been responsible for some of the droughts that occurred in Northern Europe during the period 1958-1960 (Namias, 1964) with forcing of the long waves being associated with similar sea-surface-temperature variations in the North Atlantic. Another type of drought which occurred over the northeastern United States during the springs and summers of 1962-1966 appears to have been maintained in part by a sea-surface- temperature gradient just off the east coast where cold-water masses along the continental shelf were adjacent to anomalously warm waters to the east (Namias, 1966). This condition appears to have led to prevailing cyclone tracks displaced off the coast, and hence to northerly subsiding winds over New England and the middle Atlantic States, resulting in a most unusual drought situation (Spar, 1968). 3.1.2.7 External factors responsible for drought Runs of successive years with drought are extremely difficult to explain. The section concerning persistence (2.2) explains that observed runs far exceed a length which can be readily expected given the calculated serial correlation coefficients. It is unlikely that the atmosphere itself has repetitive mechanisms o.ver such long time scales and it is more likely that persistent anomalous surface conditions or other external forces operate.
43 The most immediately popular of these external forces is the receipt of solar energy both in terms of its quantity and its quality. Proponents of solar-weather relationships believe that the sun-spot cycle and alleged accompanying solar variations are responsible for spells of drought. Hypotheses of this kind have appeared in the meteorological and hydrological literature for many decades. The latest efforts in this direction are associated with the 22 year double sun-spot (or Hale) cycle (Mitchell et al., 1978). However, some meteorologists have been very sceptical of solar-weather relationships pointing to the absence of both clear consistent statistical and physically causative linkages. A recent scholarly article by Pittock (1978) puts this contrary position from the meteorological standpoint, and Rodriguez-Iturbe et al. (1968) show it from a hydrological viewpoint (see Section 3.3.2.3). The difficulty that faces the outsider who wishes to follow this aspect of the literature is nowhere petter exemplified than in that concerning cycles in long historic United Kingdom climatic sequences. Shapiro (1975) noted in the variance spectrum Qf a temperature record dating back to the mid-17th century 'absence of an enhancement of the spectrum at periods near 11 years', or indeed 22 years. Dyer (1976) analysing the same basic data set but sub-dividing it by months found indications of 'oscillations in the ranges 10-12 and 22-25 years'. Mason (1976) analysed the same data but employed a different estimator and found a peak at 23 years but at 11 years the peak was barely discernible. Annual rainfails over a similar period showed no cycles of these lengths. Gray (1976) also searched for cycles in a long term rainfall record and found a weak peak at 11.5 years in one but not in another, nor in parallel climatic records used to check the reality of the cycles. Miles et al. (1978) employed a temperature record which was strongly dependent upon the record discussed above, with some smoothing. They used other long time series including sun spot activity (Wolf number), CO2 and a dust veil index. The results provided 'somewhat contradictory indications about the relations between Wolf number and teniperature', different subdivisions of the data yielding inconsistent results both in terms of sign and value. The other variates used by Miles et al. (1978) also affect the solar input and might account for temporary but consistent departures from the normal. These are discussed in sections concerned with climate change (2.4) and man's influence (3.2). These and other external factors complicate the physical understanding of the drought problem because one must model not only the atmospheric, oceanic and cryospheric behaviour but also must know something of the effects on these systems of external forcing, a research problem of major proportions. 3.1.3 Tropical droughts 3.1.3.1 West Africa Droughts in the tropics are more serious in areas that have a long dry season, e.g., the Sahel. If rainfall has been deficient during the prior wet season, the dry season will be warmer and the dryness reinforced. Rainfall in the tropics is often associated with a monsoon mechanism and an impairment of this mechanism brings drought (Garnier, 1976). The causes may be as follows : a. Meteorological factors: The wet trade winds that originate in the southern hemisphere cannot penetrate to the Sahel because the intertropical convergence zone (ITCZ) is south of its normal position; see Figure 3.16. It is the dessicating northern trade winds (harmattans) that dominate the Sahel. b. Dynamic causes: There are two complementary mechanisms that cause the southern trade winds to penetrate the North African continent: one is a northward "push" controlled by the St Helena anticyclonic cell. This cell moves northward in May or June - the southern hemisphere winter. The southerly winds on its east side then tend to cross the equator and thrust northeastward under the equatorial air mass; the other mechanism is the target of these winds, a sort of "suction" area - the Sahara thermal trough (thermal equator) between 16' and 24'N in western Africa. Each of these two atmospheric centres, the St Helena anticyclone and the Saharan trough, provides an influence proportional to its depth. The St Helena cell tends to govern the southern trade winds to a height of 6000 to 7000 metres while the Sahara thermal trough extends upward normally only to 1500 to 2000 metres tending to induce air from the friction layer when the air mass flow aloft permits. Thus the Saharan trough can be effective in drawing the low-layer moist-air mass northward at the time when the St Helena cell is strong and displaced northward.
44 While an anticyclone normally dominates mid-levels above the Sahara surface trough, it sometimes weakens when a long-wave trough in the northern hemisphere westerlies penetrates North Africa. This can happen during a 'low index' situation (the word 'index' here refers to the strength of the westerlies in mid-latitudes. During periods of low index, the flow may be weak at many longitudes and large-amplitude wave or even cellular patterns may predominate). Satellite photographs reveal the circulation structure of Figure 3.17 where southwesterly flow is shown along the eastern side of a trough. The monsoon air mass seems temporarily to become thicker just where the upper-level trough axis crosses the ITCZ. Finally, easterly waves can develop in the monsoon air mass and cross Africa from east to west accompanied by 'shear lines' of ascending motion with heavy rain showers and gusts - see Figure 3.16. The conjunction of these various factors seems to be necessary if rain is to fa' in western tropical Africa. If any one of the mechanisms fails, the chance for rainfa,- in the Sahel is reduced. Thus Sahelian droughts are caused by: a. The Saharan trough. This varies only slightly from year to year but during the latest severe drought it had shifted southward with lowest pressure near 15'N - see Figure 3.18 taken from Lamb (1975). This displaced trough was characterised by a weak pressure gradient and a weak ITCZ.
b. Easterly waves. These are imperfectly understood but they do not seem to occur during periods of tropical drought. c. Increased albedo. Charney (1975) shows that progressive degradation of vegetation increases the surface albedo and perturbs the radiation balance to provide a positive feedback mechanism for drought. Other positive feedback mechanisms which have been postulated are listed in Section 2.2. These regional or local factors are probably less important to Sahelian drought than d. Extra-tropical factors. These are involved in the general atmospheric circulation. The westerlies of both hemispheres play prominent parts in the initiation of west African droughts. In the southern hemisphere these are: a weakening and southward displacement of the St Helena anticyclone. This allows the sea-level pressure gradients in the Gulf of Guinea to become weaker than normal. The weakening and southward displacement often seem to be connected and they may be related to a weakening of the Antarctic polar front. During the recent drought in the Sahel moving troughs decreased in number and strength and took more southerly tracks. The Argentine daily
6000mr I I I I SAHARAN ANTICYCLONE 3000 2000 1
'Ooo t
I UV31 iOFI !%HARAN; I TROUGH (Jul.) I
Fig. 3.16 - Schematic diagram of the structure of air masses over western Africa and the rainfall process in the Sahel region.
45 'isohypses of the '\d' pianetary wave "trougt
progreççofmonçoon air-mass (in mid-trop- f pheric level 1. "Cirrus'' clouds particularly
thick monsoon air - mass
Fig. 3.17 - Conjunction of the surface low and a trough in the upper-level wave over western Africa.
weather maps (Carta del Tiempo) showed an important weakening of the cyclonic activity over the Weddell Sea during this period (Dorize, 1974). For three consecutive years, 1970-1972, a strong positive anomaly of sea-level pressure over Antarctica is indicated in Figure 3.18 taken from Lamb (1975). This represents a weakening of the westerlies in the south Atlantic. Unfortunately, the meteorological data in the high latitudes of the southern hemisphere are scattered and heterogeneous but we may assume that anomalies of the pressure field are related to anomalies of the surface temperature (in this case a decrease in temperature gradient between the Antarctic continent and ocean). Indeed, during wet periods in the Sahel the reverse of the above condition appears; there is strong cyclonic activity along the Antarctic margin, vigorous activity which seems to recharge the St Helena cell and shift it equator-ward; southern hemisphere trade winds are then deflected northward into the African continent; cold fronts sometimes cross the equator and cross the Gulf of Guinea into Africa. In short, a wet Sahel is related to a high circulation index of the south Atlantic westerlies (Dorize, 1974, p. 409). e. In the northern hemisphere during a dry Sahel, the mid-tropospheric anticyclone over the Sahara expands southward; this shortens the span of the Hadley cell, moves the ITCZ southward and strengthens it. Rainfall increases in low latitudes but the Sahara dryness spreads into the Sahel. These circumstances probably are associated with a high zonal index of the northern hemisphere westerlies.
Winstanley (1973a and 1973b) indicates the connection between heavy monsoon rainfall in tropical Africa and a high frequency of westerly winds over the British Isles. Namias (1974) has shown that the upper-level pressure over Great Britain is a good indicator of precipitation in the Sahel. The northern hemisphere circulation may be of secondary importance to the monsoon circulation. Southern hemisphere circulation, on the contrary, is of crucial importance.
46 30
O
30
60
Fig. 3.18 - Anomaly (mb) of sea-level pressure during 1970-72 as compared with normal 1901-50. (After Lamb 1975, Fig. 6).
HIGHLANDS AREAS (HIMALAYAS, TIBET) DEPRESSION IN LOW AREAS DYNAMIC DEPRESSION BUILT BY CYCLONIC CURVE OF JET STREAM
Fig. 3.19 - The hydrodynamic effect of the Himalayas on the summer jet stream. (After Yin 1947).
47 Flohn (1964) and Koteswaran (1958), however, suggest that the 'easterly jet stream' at about 150 mb contributes to extended dryness in the Sahel. This easterly jet arises in tropical Asia by reason of the strong thermal contrast between the warm air mass over Tibet and the cooler air mass to the south. The jet axis is found at about 18'N over India and at about 14'N over West Africa. The easterly jet seems to induce an atmospheric sinking motion on its right side. This makes the monsoon air mass in low layers more stable leading to drought. On its left side the easterly jet induces an ascending motion that increases the activity of the ITCZ between 7' and 14'N. Thus, if the easterly jet is accelerated by a stronger thermal contrast between Tibet and India, the pluviometric contrast between the two sides of the 14'N parallel would increase in Africa (Pedelaborde, 1976). 3.1.3.2 Drought in India Monsoon rainfall in India and Pakistan is irregular in space and time and this can cause important economic disturbances. Droughts are a particular threat in the northwest (Rajasthan, Gujarat, Punjab ... ) where mean annual rainfall is lower than 700 mm. The geographical setting for the Indian monsoon is unique. The peninsular region of India is flat and offers little resistance to the penetration of air masses from the south. Northern India has high orographic barriers on three sides; west, north and east. This isolates India from the rest of the Asian continent. Some of the principal features of the summer monsoon are as follows:
a. The air mass: equatorial moist air spreads over all India and Pakistan northward to the Himalayas. Polar fronts cannot pass the Himalayas to disturb this air mass. b. The pressure field: characterised by two cells at about 65OE. In the southern hemisphere the Mascarene anticyclone at 25'-3OoS diverts the southeast trades toward the equator; and in the northern hemisphere a surface heat low over Pakistan corresponds to a trough aloft resulting from a southward deflection of the westeklies by the Himalayas. The cyclonic curvature around the orographic barrier is quite apparent in the 500 mb charts (Figure 3.19). c. The atmospheric mechanisms: During the first half of June the summer monsoon 'bursts' into the Indian peninsula spreading to the 23rd parallel. This sudden invasion is associated with the formation of the aforementioned trough in the upper levels. Thus, the strong trough of Figure 3.19 sets in throughout the troposphere encouraging the northward flow of the moist monsoon air mass thousands of metres thick (Yin, 1949). This action probably is associated with a slackening in the strength of the westerlies, the trough over Pakistan deepens and in higher levels - 500-200 mb - an anticyclone forms over Tibet sometimes with a second cell to the southeast of Tibet (Pedelaborde, 1970). This high-level anticyclone signifies relatively warm air over Tibet to correspond with the cooler air of the monsoon air mass to the south. This sets up the easterly jet mentioned earlier. Here, where the jet is formed, however, the climatic consequences are the reverse of those for the end of the jet over Africa. Here, on the right side, the jet builds a divergence area at high levels SO that upward motion and rainfall is induced in the lower layers. On the left side the opposite effect occurs.
The ITCZ, which has weakened as it has been carried north from the equator, is terminated by the descending air on the left of the jet but the ascending air on the right compensates for this. In actuality synoptic charts often show several easterly jets over India. There seems to be an incessant oscillating movement of the ITCZ. When it reaches its northern most extremity it is damped out but a new ITCZ forms in the south. The movement of the ITCZ is produced in part, perhaps, by a strengthening of the Mascarene high (although this would be difficult to prove owing to the sparsity of data in this area). The movement might also be due to an inertial mechanism as described by Pisharoty (1963). The latent heat from condensation of the ITCZ moisture at mid-levels establishes small anticyclonic belts that tend to stabilize the monsoon air mass. Thus there is negative feedback that tends to damp out the monsoon each time it builds up to strength and the summer rains become very irregular in both space and time. Drought in India occurs when the monsoon fails. This happens more frequently in August and September as a result of weakening of the Mascarene high in the southern hemisphere SO that the ITCZ is weakened or fails to penetrate the Indian peninsula. When the ITCZ is strong, a
48 I010 nn E Y O w œ 2 1009 KI œ a
1008 O
1007
RAINFALL (% NORMAL)
Fig. 3.20 - Correlation between summer (June through September) rainfall in Rajasthan and sea- level pressures at Amini-Divi, Laccadive Islands, period 1916-55.
- DROUGHT WETNESS 1004 -i I+ 1918 I I olYltrO I I I 1002 I -\l936i I E O! I U I ---.a \, 19QO I w 1000 I 19.48 192' E 1933 998 & 19; v> - 1 W I 1954 I! 0 a 996 - I a I - I I 994 - - 1 40 50 60 70 I 80 90 100 110 1201 130 140 150 160 %' 75 125 RAINFALL (% NORMAL)
Fig. 3.21 - Correlation between summer (June through September) rainfall over Gujarat State, India, and sea-level pressure at South Georgia Island, period 1917-55.
49 trough is well marked between i0°-200N as indicated in Figure 3.20. Indian droughts are associated with insufficient lowering of pressure in this area. The cool moist southern hemisphere trades have difficulty crossing the equator when the Mascarene cell is weak. This cell is fed, in part, by air discharged by storms along the Antarctic polar front and a weakening of the westerlies in the south Indian Ocean threatens a failure of the Indian monsoon. Here again the sparsity of data makes this situation difficult to evaluate. But droughts in the Indian peninsula do seem to be associated with a low index in the southern westerlies. The pressure at South Georgia Island, 54OS, 36OW, see Figure 3.21, correlates well. This relationship showed at the time of the severe drought in 1972 - when the Sahel was also affected - the pressure weakened just to the south of Madagascar and strengthened along the Antarctic margin. These features lasted throughout the period 1970-1972 (Lamb, 1975). Some of the changes in the monsoon over western India have been associated with influences in the northern hemisphere. The mid-tropospheric easterlies in summer over the Hindustan plain sometimes turn to a northwesterly flow. This blocks the monsoon air mass and it occurs when there is a low-index situation in the northern hemisphere westerlies that allows a long wave- length ridge to build north of India. Thus, the ridge over the Urals in 1972 encouraged northwesterly winds on its eastern side over Pakistan and this was associated with deficient rainfall throughout the area (Figure 3.22). The opposite anomaly was observed in 1939 and 1976 when a trough appeared over the Urals and a ridge over western Europe. At these times monsoon rains were heavy in the Indian peninsula. Thus, the low index situation of the northern hemisphere might diminish or enhance the monsoon depending on the position of the blocking high.
Fig. 3.22 - 500 mb chart-mean (tens of metres) for August, 1972. (Taken from a Soviet weather chart).
50 Regional and local factors also may enhance a drought once it has become established. The albedo change discussed under the Sahelian drought is one example. Another is the local reduction of the thermal gradient that sustains the easterly jets. Some limited ‘zonal’ droughts in the Indian peninsula are probably due to this. 3.1.3.3 Drought in South America The Nordeste region of Brazil is the classic drought prone region of South America. Unlike the Peruvian coast there are no cold currents in the ocean, nor upwelling effects. However, an atmospheric mechanism for drought in this area has been put forward by Namias by establishing a connection between it and a blocking anticyclone in the north-east of the USA (see also Hastenrath, 1976). Another atmospheric mechanism that can be hypothesised is when the Santa Helena and Azores anticyclone cells close up (following, for instance, a more intense westerly atmospheric circulation in both hemispheres). In such circumstances a sinking motion in the low layers of the atmosphere will occur causing drought in northeastern Brazil (Figure 3.23). The shape of the Brazilian coastline intensifies this effect (atmospheric divergence due to a hydrodynamic effect). Coastal regions are seldom submitted to drought because the equatorial current in the ocean induces instability in the air mass.
...... Subsidence area
H Dynamic ;inricyclone
---==-AAtniorplieric tlow
Fig. 3.23 - Drought in Northeastern Brazil suggested by divergence effect due to subtropical anticyclonic cells (dynamic anticyclones).
51 3.1.3.4 Australian drought
Severe and extensive drought in Australia seems to occur when there is a low atmospheric circulation index in southern hemisphere westerlies. Thus the 'Walker circulation' is weak and the following connections are generally established: (a) warming of the central equatorial Pacific; (b) weakening of equatorial westerlies; (c) appearances of the El Nino phenomenon bringing rainfall to the Peruvian coast, and (d) onset of drought in Australia, especially Queensland and New South Wales (Linacre and Hobbs, 1977). Anthropogenic factors probably amplify this situation, particularly the destruction of grass by overgrazing. 3.1.3.5 Drought in South Africa Drought frequently afflicts South Africa and seems to be connected with two typical atmospheric circumstances: 1. When an anticyclone is situated over Transvaal or south Mozambique as shown in Figure 3.24. In these regions the phenomenon of subsiding air involves drought and in other regions, the anticyclonic margin such as the Cape, the atmospheric circulation brings north winds with a dry continental air mass and which also brings drought. 2. When an anticyclone is situated in the Atlantic as shown in Figure 3.25 (low pressure between South Africa and Madagascar is often associated with this situation). A pressure-field of this sort conducts an atmospheric flow from north to south. During the southern winter, ie May to September, this oceanic air mass is cold and consequently lacks humidity. When these conditions persist drought can occur inland due to the mountains' foehn effect.
3.2 INFLUENCE OF MAN
3.2.1 Introduction
Man's influence on drought incidence is felt in two ways; through anthropogenic effects on the climate (Section 3.2.21, and by virtue of the way that his operations and land use practises alter the hydrological regime, for example, by affecting infiltration, modifying stream flows and altering the quality. The avoiding action that can be taken through intelligent land use practise is referred to in Chapter 7. 3.2.2 Anthrowoaenic effects on climate This subsection briefly describes the influence that various of his activities, industrial and agricultural, have on climate outlining the mechanism and where possible referring to the direction of the change insofar as it may influence arought occurrence. The topics which follow are all alleged to induce 'climatic change' (see Section 2.4 for treatment of natural climatic change and drought) and like natural climatic change the results at the basin level and the impact of departures from mean behaviour of climate have proved very difficult to predict. A further general point about many of the following topics is their speculative nature, ifnot of the immediate reality of the particular activity, then of the ultimate effect on the climate. 3.2.2.1 Individual mechanisms Table 3.1 summarises five major causes of anthropogenic climate change and their magnitudes according to recent opinion. The complexity of the topics is such that only the briefest indication of the effect can be given and there are many caveats and exceptions. Primary references for these details may be reached via the following reviews: Budyko et al. (1975), Kellogg (1977, 1978a and b) and NAS (1975). The emission of carbon dioxide is commonly regarded as the most important of these effects and much of the modelling of consequences that has been carried out focuses on that aspect. Schneider (1975) reviewed efforts to that time to predict the effect of doubling the CO2 concentration noting the differences that arise due to different accounting or neglect of processes. Values between 1.5OK and 3'K rises are found in the favoured models for such a doubling. Mason (1979) reports more recent modelling at the UK Meteorological Office which uses more detail (11 levels) and a more realistic treatment of cloud and revises the earlier figures down to a 0.4OC increase above the present average. It is pertinent to ask whether the surface warming implied by the Table 3.1 factors are already reflected in a global temperature increase. Surveys have been carried out of both stratospheric and tropospheric temperature at various levels. As far 'as the temperature near s2 ...... Land above 1,500 metres m.s.1.
H High pressure area (schematic position)
L Low pressure area
.----AAtmospheric flow
I 100 20° E 30' 000
Fig. 3.24 - Drought in South Africa on account of an anticyclonic weather and north atmospheric circulation.
Fig. 3.25 - Drought in South Africa on account of an oceanic cold and dry air-mass.(South atmospheric circulation). For key see Fig. 3.24.
53 mu) ma, id0 m.c a,c, 4 CI alid Q O O3 cici
c ci .4 F rn 3i- .a, m a-ci O c4m id O Pc w.4 -0-l a5 OCiC4F- a, vm 7> cidg -m4 O d: L! c 2 m h- id 4 'GA ma;m o O a, u) mo40-lci4 m urlmm moo L! Lncirl5U a, 4 cc d.4 L! a.4 U id2 m
c a
54 the ground is concerned the general consensus is that a cooling trend since the middle of the present century has continued into the present decade. A short reversal in the early 1970s has given way to a further cooling (Kukla et al., 1977). Against this background it is necessary either to invoke a more enhanced natural cooling or else abandon the magnitude of the anthropogenic effects in favour 'of smaller values. A different approach to the test of the reality of the link is to be found in Miles et al. (1977) who uses multiple regression to try to isolate the effect of individual factors. Over the long term CO2 does have a significant positive correlation implying a near 2OC warming due to a CO2 doubling; however, the increase is very much against the apparent temperature trend since 1940. Recently Parker (1981) has questioned the reality of this trend by showing that it may be an artifact due to the continental bias in temperature recording stations. 3.2.2.2 Drought effect
With the reigning doubts surrounding the importance and scale of the individual factors it will be no surprise to discover that the situation regarding their consequences to temperature and precipitation at particular locations is even more speculative. The primary methods that have been employed are to use general circulation and energy budget models into which the revised global conditions are entered as boundary conditions, or else to try to recognise past epochs in which the conditions were similar, or extrapolate from them. Kellogg (1978a) attempts to answer the question 'what would a warmer earth be like?' and seeks methods for simulating and reconstructing the climate of a warmer earth. The overall impression by comparison with past warmer climates is of greater wetness and hence fewer droughts. The central plains area of the United States is an exception to this rule. Manabe's (1975) atmospheric model of the consequences of doubling CO2 gave a much enhanced hydrological cycle with increased precipitation and evaporation at almost all latitudes. Drozdov (1974) also statistically modelled the precipitation and temperature field following a polar warming basing his conclusions on an extrapolation from recent similar events. The precipitation anomalies are separated between summer and winter rainfall. The Sahara for example is expected to receive a reduction in winter rainfall following a 2.5OC polar warming but an increase in summer rain in the Sahara and Sahel following the expected l0C summer warming. The effects in North America and in peninsular India appears small in both seasons. As in the other studies, the size of fluctuations about the revised mean is not investiqated. 3.2.2.3 Local effects on the climate Both in Section 2.1.8.2 and 3.1 we have introduced the role of man's activities in possibly enhancing or prolonging drought conditions once they have become established. These are the inhibition of rainfall by overseeding of clouds by anthropogenic dust; the rejection of heat by dust; the albedo increase following man induced desertification processes; and the reduction of biogenic nuclei following overgrazing. All are at present research suggestions and the component of the effects due to man's influence have not been quantified.
3.3 POSSIBILITIES OF FORECASTING DROUGHT
3.3.1 Meteorological methods of forecasting From the above description of some of the physical aspects of drought, it must be clear that the forecasting of these catastrophic events is fraught with difficulty. However, on the time scale of months and seasons, it has at times been possible to give some indication of the inception of drought and also its termination. This is really part of the long-range- forecasting problem into which we cannot go in detail. The reader is referred to a monograph by J. Namias and to his reviews of the subject (Namias, 1953, 1968, 1978~). The procedure involves a combination of statistical-physical-synoptic methods. The following subsections summarise the procedures. 3.3.1.1 Analogue methods
Analogue methods, where a similar situation to the present month or season is found in past data. In this method it is assumed that the weather of the future will undergo the same behaviour as that of the known past. Needless to say, it is difficult to find 'good' analogues because the length of the historical record does not comprise many decades. There is help, however, from new techniques such as the use of eigenvectors which are now being used to select analogues.
55 3.3.1.2 Linear regression methods
A linear regression equation of the form
may be used for drought forecasting where Y is the pressure or temperature at a point or over a region at some point in time or averaged over some duration such as a month or a season of interest. The variable Y may take the form of an index summarising the total weather situation derived using the principal components method. The forecasting X variables must be observable at the time the forecast is to be made and like the Y variable can include prior values of pressure and temperature, summarising indices, sea surface temperature, wind, ice extent, and even sunspot number or some other cyclic variable. The estimation of the coefficients bo, blr etc is by least squares and makes use of a run of back data, typically 30 years for seasonal or annual forecasts. Standard texts should be referred to for details of the estimation technique concerning the use of transforms, dummy variables, selection of variables and 'stopping rules' for the addition of new X variables. There are many pitfalls in the development of regression equations, many of them stemming from the use of serially correlated data which complicates the testing procedures for statistical significance. Another problem has been highlighted by Torranin (1972) who used an optimal subset selection routine to develop regression equations for forecasting west coast precipitation in the United States from Pacific Ocean surface temperatures. The fact that the predictor temperatures are not selected randomly but are searched for in a way which maximises the correlation coefficient upsets the significance test. Torranin found that close to three times the expected degrees of freedom were used up in this type of search routine which lowered the significance of the forecast relationship to the point where the ostensibly useful relationship could not be distinguished from a chance occurrence. Clearly a physical basis is implied by the method but the form of the relationship itself is essentially non-physical. Bunting et al. (1975) show a forecast of monthly and seasonal rainfall south of Sahel based upon the previous Niamey 700 mb geopotential anomaly. Dyer (1979) uses the phase of the southern oscillation and the latitude of the subtropical high pressure belt in a regression equation to predict rainfall along the east coast of southern Africa. Arzhelas et al. (1977) use a variant on the regression method by classifying monthly precipitation over the southern part of European USSR into dry, normal and wet classes. A dry month, for example, occurs when less than 80% of normal rain falls over 70% or more of the region. A discriminant function is developed as a linear combination of several variables including a zonality index, snow cover, Barents Sea ice and other sea and air temperature variables. The sign of the value of this discriminant function indicates in which class the subsequent month should fall. The dry summer of 1975 was correctly forecast by the procedure. 3.3.1.3 Teleconnections Specific teleconnection charts, selected by season and grid point, are used to specify conditions remote from key areas where forecasting seems more straightforward. These charts show, for the 700 mb level, the correlation coefficients at all grid points with the given grid point (examples are shown in Figure 3.12). Other, more general, examples of teleconnections are : a. links between sea surface temperature and inland weather (Section 3.1.2.3; Namias, 1963; Shukla et al.,1977);
b. wind in East Africa and monsoon in India (Findlater, 1977; Raghavan et al., 1975 and 1978); c. general circulation and sea surface 'signature' antecedent to deficit seasons in Brazil, central America and Africa.(Hastenrath, 1976 and 1978);
d. location of ITCZ and jet stream (Section 3.2; Winstanley, 1973). These teleconnections do not in general provide firm forecasts which are specific to points but nevertheless provide valuable general information on the likely progress of events within a drought. 3.3.1.4 Statistical and kinematic methods
Statistical and kinematic techniques are used to predict long-period trends in wind systems making it possible to track and extrapolate the long waves in the upper level westerlies for periods of months. These methods are too extensive to describe at length here, but details may be found in Namias (1953, 1968 and 1978~). Once the pressure and wind patterns are
56 predicted, using these and other methods, equations have been developed which estimate the temperature and precipitation associated with these patterns (Klein, 1965). This procedure may be applied to monthly as well as to seasonal prognostic charts (Klein, 1962). 3.3.1.5 Contingency tables The use of tables which give the probability of an event in a given class interval at tiqe t + 1, contingent on the situation at time t has already been mentioned in Section 2.2 where it was suggested that they provided a more flexible method of expressing persistence than the correlation coefficient. That section showed examples and references, in particular Gordon et al. (1976) where it is claimed that the method produces a monthly temperature forecast which is as accurate as the official long range forecast. The method may be applied equally to temperature or precipitation, the two being strongly related at a given location and season. For example, over the plains of the United States in spring and summer, anomalously warm periods are dry, and cool periods wet. This is indicated in Figure 3.26 which shows the correlation between temperature and precipitation from a few decades of summer records. 3.3.1.6 Use of air-sea interactions Some new methods using large-scale air-sea interactions as primary guides are encouraging because the sea has a long time-constant relative to the atmosphere. The macroscale thermal character of the sea surface thus appears to be easier predicted than the atmosphere over time scales of a month to a season. In fact, such predictions are routinely made. The second step is to compute or 'specify' the atmospheric circulations compatible with the predicted anomalous sea-surface temperatures. These correlations are specified over the oceans, but teleconnections then make it possible to infer the circulation in continental areas. These can then be translated into anomalous temperature and precipitation. Detailed accounts of this procedure appear in many papers by J. Namias (e.g. 1976).
Fig. 3.26 - Correlations in summer months between temperature and precipitation derived from 200 stations over the United States using the period 1934-1973. Isopleths of correlation are drawn for each 0.2 with the highest negative values most heavily shaded.
57 3.3.1.7 Statistical time series forecasts Some authors have been tempted to use entirely statistical techniques that assume the series of annual precipitation values follow a particular process such as the Box Jenkins ARMA (Autoregressive Moving Average) model. Little success can be expected from such procedures because of the known high level of randomness in climate series. A recent example (Rodda et al. 1978) fitted the model to England and Wales winter rainfall, calibrating the model on 51 years of data. The confidence interval was wide and the forecast series is muted. The exceptionally dry winter (1975/1976) within the recent European drought was overestimated by 43 per cent. In a similar investigation (Gray, 19761, a series of decade mean rainfalls for south east England was analysed; periodic cycles having first been removed, an ARMA model was applied to the residuals. This accounted for only eight per cent of the original variance. Dyer (1977) applied an autoregressive model Y(t+l) = aoY(t) + qY(t-1) + .. . . . apY (t-p) and exponential smoothing Y(t+l) = aY(t) + a(l-a)Y(t-l) + ..... a(i-a)PY(t-p) to provide one-step-ahead forecasts of rainfall in South Africa. Neither technique explained sufficient of the variance to provide useful forecasts. At its very simplest this technique reduces to a two variable correlation between a current and a past value of a variable of interest. This has been applied with some success to seasonal forecasting, for example Winstanley (1974) and Bunting et al. (1975) both of whom forecast Sahel area rainfall during the latter part of the wet season from that observed during June. Care must be taken to avoid spurious correlations, for example, by including the predictor month within the more extended forecast period. One should be aware, also, that statistical significance has not been demonstrated for the above methods and the results might be simply fortuitous. 3.3.1.8 Extrapolation in time using cyclicities As stated in Sections 2.3 and 3.1.2.7, the existence of many cycles in climatic records remain unproven. However, workers who have identified significant cycles subsequently use them to extrapolate forwards in time. Examples that have already been quoted are Gray (1976) and Dyer (1977) for the United Kingdom and South African rainfall. In both cases, the cycles were suggested by the data. The former study suggested low decade mean rainfall during the 1980s and 199Os, the latter forecast a possible period of drought in the mid-1980s. A more rudimentary form of forecast concerns the use of recurrent events. For example, Palmer (1965) speculated on a 19 year interval between droughts on the central plains of the United States based upon events centred on the late teens, the mid thirties and fifties. He was led to foretell drought for the period 1972 to 1975, a period which in the event continued the high yield era. Drought did in fact occur at various locations within the area of concern between 1976 and 1978 but not on the space or intensity scale of the earlier events (see Section 6.5). 3.3.2 Hydrological methods of forecasting drought The onset of a drought is quite clearly a function of previous weather conditions so cannot be forecast from consideration of river flow or aquifer conditions alone. However, these can provide valuable aid in forecasting the future progress of the drought once it has begun. The available techniques divide between those based upon the recession or natural depletion rate of water bodies, and those based upon statistical methods such as contingency tables and linear regression. A brief survey of these procedures follow in Sections 3.3.2.1 and 3.3.2.2. The WMO Guide to Hydrological Practices gives a fuller exposition of the forecasting techniques. Annual runoff series can be analysed in the same ways as pressure and temperature anomalies to try to discover trends, cyclicities and recurrent features (Section 3.3.2.3). 3.3.2.1 Recession based methods In river basins with a well established wet and dry season the forward extrapolation of the recession curve from the flow condition at the outset of the dry season provides a normally safe method of fixing the future course of the flow (Canceill et al., 1977). The onset of the rainy season at the close of the low flow period is clearly defined retrospectively but difficult to forecast. Observations of the displacement of the ITCZ may help with this forecast but clearly recession extrapolation provides at best a limited method of forecasting.
58 An individual practical application will have to recognise the particular circumstances that may cause the recession to differ from the average, for example, man-made influences such as pumping for irrigation, local differences in catchment wetness leading to base flow support arising from particular subcatchments, seasonal variation induced by phreatophytes and swamp vegetation, and the behaviour of effluent streams. It nay be possible, if the hydrological response of the river is sufficiently well understood, to allow for the effect of observed or forecast rainfall following the issue of the forecast (Roche, 1963).
3.3.2.2 Regression methods These follow a very similar pattern to those described in Section 3.3.1.2 but in the case of river flow they often use soil moisture and climatic factors such as rainfall and temperature as explanatory variables. In many cases the previous rainfall history is divided into separate seasonal variables and the relative weight of the terms helps to identify the major causative time lags within the rainfall runoff system. In strongly groundwater fed rivers dip well measurements provide a useful explanatory variable representing the total storage below ground. Low flow in rivers whose flows are derived from the melt of the previous season's snow are forecast using snow variables such as water equivalent, or where a stable relationship is found, the areal extent of snow cover. It is common for long term flow forecasts to be expressed in statistical terms. Thus the forecast has the following appearance: given a current discharge of 100 cumec the discharge two weeks hence will be: less than 80 cumecs with probability 0.1, between 80 and 100 with probability 0.3, between 100 and 120 with probability 0.2 etc. Methods for this type of forecasting include contingency tables (transition matrices) and conceptual or regression models applied to hypothetical rainfall sequences of known probability. 3.3.2.3 Cycles in annual streamflow Just as with climatic records, the presence of cycles or recurrent features within the records would allow future droughts to be anticipated. There is a very considerable literature on the topic and the evidence is mixed. Rodriguez-Iturbe et al. (1968) failed to find significant correlation with sunspot numbers using 16 river flow records from various parts of the world. On the other hand, Smirnov (1974) found significant spectral peaks and sunspot correlations ' using 50 filtered gauging station records from 46 rivers in the USSR. Maps are shown of the phase variation across the country. It is interesting to note that the River Neman was common to both studies with apparently opposite conclusions (although Smirnov states the effect is weakest in the European USSR). Rodriguez-Iturbe et al. (1968) and WMO (1966) refer to the caution required in interpreting spectra from filtered or smoothed time series. A similar study was carried out by Agarkov et al. (1971) who divides the eastern USSR territory into regions according to cyclical structure. Cochrane (1960) gives an informal demonstration of a supposed linkage between the rate of change of sunspot activity and three-year-running average of river flow in a widely separated set of basins. Accurate forecasts are obtained for two out of three years of Lake Nyasa levels, the third year, 1959, was very dry and the extremity of the drought was underestimated. Restricted use only has been made of the extrapolation procedures referred to in Section 3.3.1.8. Rozhdestvenskiy (1966) has applied a dynamic averaging technique on which to base the forecast one year in advance. The author makes the point that the forecast is of the mean point about which the random fluctuations are centred. A similar technique has been used to define possible future trends in Poland based upon the so-called 'trend factor' being the proportional derivation of a given annual runoff from the long term average up to that year (Prus-Chacinski, 1976). 3.3.3 Review of accuracy
The verificatiori of a long-range forecast in general leaves much to be desired. If chance skill is set at 50%, long-range forecasts of temperature for the United States and perhaps for other countries over the world is no mo.re than 65%. Precipitation skills are somewhat lower - on the order of 60%. No verifications have been worked out for the special case of drought, although these would probably be of the same order.
59 3.4 REFERENCES TO CHAPTER 3
Agarkov, S. G.; Druzhininin, I.P.; Konovolenko, Z. P. 1971. Cyclical structure of the hydrological series and the nature of individual components. IAHS Publ. no. 100. Warsaw Symp. on Mathematical Models in Hydrology, p. 85-90. Angell, J. K.; Korshover, J. 1978a. Global ozone variations: an update into 1976. e. Wea. Rev., vol. 106, no. 5, p. 725-737.
Angell, J. K.; Korshover, J. 1978b. Comparison of stratospheric trends in temperature, ozone and water vapour in north temperate latitudes. Journ. Appl. Met., vol. 17, p. 1397-1401.
Arzhelas, G. F.; Sverev, N. I. 1977. Method of forecasting the precipitation deficit and excess in the southern part of the European USSR. Trans. Hydromet. Sci. Cent. of USSR, no. 172, 1977, p. 3-7 (in Russian). Soviet Hydrology, vol. 16, no. 2, p. 120-122.
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Budyko, M. I.;Karol, I.L. 1975. Man's impact on the global climate. Proc. WMO/IAMAe Symp. on long-term climatic fluctuations. Norwich 18-23 August 1975. WMO no. 421, Geneva, Switzerland, p. 465-471.
Bunting, A. H.; Dennett, M. D.; Elston, J.; Milford, J. R. 1975. Seasonal rainfall forecasting in West Africa. Nature, vol. 253, 20 February 1975, p. 622-623.
California Department of Water Resources. 1978. The 1976-1977 California Drought, A Review. Sacramento, USA, 95802, 228 p.
Canceill, N.; Forkasiewicz, J.; Margat, J.: Thiery, D. 1977. La prévision du régime naturel des nappes d'eau souterraine appliquée à leur gestion. Colloque National 'Les eaux souterraines'. B.R.G.M. Nice, 27-28 October 1977.
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64 4. Methodology for the study of drought and exceptional low river flows
It is first essential to recognise the type of drought under study and then to perform various analyses in order: to summarise and to quantify its characteristics (severity, duration, inclusion of any short normal or wet contained periods); to classify the drought relative to past events (which requires the collection of as much back data as possible), and lastly to estimate the return period of characteristics of this.and other droughts on a given river. Techniques for solving these important problems are considered in this chapter. The first section reviews simple indices which may be used as a broad definition of the severity of the drought and subsequent sections suggest procedures for more detailed analysis of aspects of the drought. The second section is concerned with means of obtaining information on historic droughts from documentary folk memory, and geomorphological sources. Sections 4.3 and 4.4 enumerate procedures for the frequency analysis of precipitation and river discharge for the purposes of drought study. Section 4.5 discusses the use of statistical distributions which applies to drought data of all kinds. Section 4.6 presents some practical procedures for prediction (not real time forecasting) of low runoff characteristics of a basin from the controlling features of the catchment, primarily the geological and pedological characteristics.
4.1 CHOICE OF INDICES FOR DEPTH OF DISCHARGE AND PRECIPITATION
In order to summarise the degree of severity of a given drought period it is not sufficient to give a simple qualitative appraisal. A numerical index is necessary to characterise the intensity of the event. Many indices are used to describe features of drought suited to different water uses and hydrological regimes and it seems neither possible nor desirable to attempt to standardise. The simplest index and the one most used is to compare the depth of precipitation (or of runoff) for a given duration: week, month or year, with the long term mean or standard period normal value of this same duration. So a given year will be designated dry or wet if the ratio of the precipitation for this year to the mean yearly depth of precipitation is less or greater than unity. This ratio to the normal value determines the pluviosity (pluviosité) when discussing rainfall, and 'hydraulicity,' (hydraulicité) of the period when discussing river flow. This simple dichotomy between 'wet' and 'dry' needs further refinement for all but the most straightforward applications in order to grade levels of dryness. The actual numerical value of the hydraulicity or pluviosity will serve this purpose but it is generally better to classify on the basis of probability. One scheme which is commonly seen in the European drought literature (e.g. Serra, 1960 and 1964) is to divide the ranked pluviosities, hydraulicities or depths into the following five classes.
a. very wet: exceedance frequency between: O and 15% b. wet II II 11 15 and 35% c. normal : II II Il 35 and 65% d. dry I, I II 65 and 85% e. very dry: I, II II 85 and 100% Variations on this basic theme employ standard deviation or interquartile range classes from a central value. Chapters 5 and 6 use a simple definition of very dry year as one between 80 per cent and 100 per cent exceedance frequency in order to form a regional drought summary based upon a number of flow records. The rather similar concept of quintiles has already been introduced in Section 2.2. More complete descriptions of drought require the introduction of the duration aspect.
65 This may be the duration of consecutive rain free days or duration when discharge remains continuously below some threshold value. Intensity and duration aspects can be compressed into single indices using run lengths, run sums, deficit volume, residual mass curves, and storage yield requirements, all of which extract both volumetric and durational aspects from the hydrograph. These more detailed topics are covered in sections 4.3, 4.4 and 4.5.
4.2 USE OF HISTORICAL AND GEOMORPHOLOGICAL INFORMATION
Historical information derived both from early documents, from the memories and traditions of inhabitants, and from geomorphological traces such as mud varves and other palaeoenvironmental indicators can provide invaluable extra data augmenting that contained in the conventional record. However, the search process is more difficult for droughts than for floods because the latter, with their high velocities and damaging inundation, leave traces both physical and in local memory which remain a long time after the flood water has receded. The only analogous drought related event is the drying up of a river or lake. The following section reviews sources that have been used to provide drought information. 4.2.1 Historical documents and folk memorv Documentary information is found most commonly in countries with a long established tradition of public administration. Local memory and traditions have to be tapped in non-documented regions. Both types of drought information tend to be generalised consisting frequently of remarks on crop failures but occasionally also descriptive statements on the absence of rainfall. Examples of collected material for the United Kingdom are Symons (1888) and Brooks et al.(1928) who have drawn on sources from the earliest time. Potter (1978) provides a structured procedure for searching British historical documents for hydrological insights and lists years in which drought has been remarked upon in Britain since 1066. Historical climatology in Britain and worldwide has benefited immensely from the work of Lamb (in particular Lamb, 1977, but see Anon, 1979, for a complete list of his works). Ingram et al.(1978) refer to European sources and critically review and compare reconstructions of climate for the study of extreme events including droughts. Le Roy's (1971) book is another useful general source of background information on droughts of the past. Reports of travellers and early explorers and colonising settlers often can augment folk memory in more recently developed countries. A prime example of what can be achieved is shown by Nicholson (1978) in a synthesis of data from very many sources to build up a generalised picture of Sahel climate over the past 500 years. Several droughts occurred in the Sahel between 1680 and the present; those of 1680 to 1692, 1730's to 1750's (killing half the population of Tombouctou), in the 1790's and 1828 to 1839. The overall character of the area is said to have been more humid than nowadays, at least up until the last century, with a tendency to aridity following. Other references to historical African drought data are Tilho (1947), Tixeront (1963) , Plote (1974) and Servant (1974). Bryson et al. (1967) and Seth (1963) have made use of similar sources plus the results of archaeological researches to describe Indian climate, and to trace the increase in dessication linked to man's activities. Foley (1957) has studied the occurrence of drought in Australia from the earliest years of settlement using rainfall statistics, wheat yield, and sheep and cattle losses. His list has been brought up to date by Coughlan et al.(1976). Lamb (1967) has used historical data to develop a rainfall index for southern South America from 1500. Landsberg et al. (1968) have produced a composite record of annual temperature and precipitation for eastern United States from 1738 although, while not common, instrumental records of such lengths are to be found for parts of Europe. One problem that invariably arises when using historic data is how it ranks in quantitative terms with conventional data from the instrumentally recorded period. Usually it is not possible to find transfer functions between, say, agricultural or social damaqe, and annual rainfall or runoff mainly because a deficit during a critical period may result in disproportionate crop reductions. Nevertheless, it is usually possible to detect how the worst recent events rank in relation to the worst historically. Examples of how available qeneralised information may fit in with precise hydrological requirements are:
Historical information Current requirement Period of low or zero flow in well-known Deficit period for irrigation scheme drought. Behaviour of long-established (need to take account of modifications irrigation works to river) Stoppage of mill operations Insufficiency of water for hydro-electric plant
66 Drying of river bed, lake or pond in Similar current need connection with river system
Ability to ford a river or emergence of Minimum stage for navigation (precise rock or rocky island point must be ascertained)
' 4.2.2 Geomorphological and other palaeoenvironmental indicators As stated above the drying of a lake is a good drought indicator which may allow current conditions to be set in an historic context. The first stage of lake drying permits the growth of vegetation; submerged tree roots thus delineate the ancient shore line and indicate the severity of previous drier periods. In the second stage of drying sand dunes begin to appear, SO submerged sand dunes likewise offer proof of more severe droughts than those of the current period. Such submerged sand dunes are known in Lake Chad but presumably this technique could be extended to many lakes or depressions. Chouret (1974) has mapped the outline of Lake Chad during several recent droughts. The maximum level during a dry year is easily ascertained because, as the river remains within banks, a distinctive mark is left on the more or less argillaceous steep river banks. This was particularly noticeable during the recent drought along the main Sahel rivers. There are many other indicators of past conditions, many proving the existence of drier conditions than at present. However, for the most part these are related to conditions in the remote past, perhaps 30,000 to 1,000,000 years, and do not permit adequate time resolution to make useful comparisons with the present for water resource purposes. A valuable summary of 17 palaeoclimate data sources is given in KÜtzbach (1975) against each of which is listed the period open to study and the minimum sampling interval. Apart from the written records only tree rings and, locally, layered lake sediments offer a sufficient time resolution to be useful for hydrological drought studies. . A valuable recent review of the application of tree ring study to climate reconstruction in general and drought chronologies in particular is given by La Marche (1978) who, in turn, gives many other relevant references. Construction of rainfall and river flow sequences for areas of South America is demonstrated based upon correlations of 0.65 and 0.73 respectively in the calibration period with tree ring data. This should be sufficient to indicate runs of deficit years although too low to provide a quantitative annual ranking with recent conventionally recorded data. 4.2.3 Droughts of the immediate past from indirect evidence In the most arid of areas (below 20 mm annual rainfall) where there is no local population, vegetation occupying the beds of ephemeral streams can indicate whether flow has occurred in the preceding one or two years; the better its condition the more recent the flow event. Of course, in less arid areas too the condition of tree growth in the basin indicates the climate of the immediate past but this does not necessarily extend to river flow. Sand dunes can assist in making similar judgements depending upon the extent or otherwise of graminiferous and short-lived vegetation. Such indicators as these are sensitive to man's influence either direct or through stock foraging. A distinction must be made between the results of drought and the results of desertification due to anthropogenic causes as occurs in the vicinity of wells or settlements.
4.3 ANALYSIS OF PRECIPITATION DEPTH AND OTHER CLIMATIC VARIABLES
4.3.1 General
Six types of hydrological drought were described in Chapter 1, mostly in terms of runoff volume and the levels of surface and subsurface water bodies. While drought study should preferably focus directly on such variables it is often not possible or practical to do so and we are obliged to substitute precipitation variables, not least because raingauge networks are denser and considerably longer than most river flow or aquifer level series. Annual rainfall has already been used for illustrating many points in this report, e.g. persistence, and temporal and spatial variability. Important points to bear in mind when considering the methods reviewed in the following subsections are:
a. The complexity of the transfer function between rainfall and drought impact; and
b. The ability of the land phase of the hydrological system to smooth out the spatial and temporal variability of the rainfall process.
67 The first problem is less severe in subtropical droughts where, for practical purposes, the annual rainfall is compressed into a single season. In the temperate zone the details of the incidence of the rainfall within and prior to the drought period can have an important impact on the state of aquifers and on agricultural output. Thus southern Britain's most severe drought of the past 300 years occurred in a calendar year, 1976, of average total rainfall. Discharge measurements play an important part in operational decision making and water resource management while the drought persists, as well as providing the necessary data for retrospective analyses such as return period assessment. Rainfall data, while a useful indicator of drought severity to the lay public is of less immediate operational value in drought management because of the uncertainties introduced when converting it to more relevant hydrological variables. Therefore, the applications described in subsequent paragraphs relate mainly to the planning role of retrospective assessment of drought severity. 4.3.2 Annual rainfall
To index drought severity it is necessary to quantify the shortfall of rainfall. This may be done using the concept of pluviosity, by considering percentiles, or by distribution fitting. Other indices are based on runs and run sums of annual rainfalls. The following subsections summarise these methods. 4.3.2.1 Pluviosity The concept of pluviosity has been discussed in Sections 1.2.2 and 4.1. Its application is not limited to annual periods but can be applied to any duration. A closely related measure is the fractional deviation from the average (R - R)/R which differs by one from the pluviosity. Tabony (1977) used this measure which he termed 'meteorological' drought to rank events in a long climate record. 4.3.2.2 Percentiles
A drawback of the pluviosity concept is its inability to describe adequately the relative severity of deficits between widely scattered locations. The problem arises because the variability of rainfall varies spatially and so, for instance, a 30 per cent shortfall (70 per cent pluviosity) will be experienced much more frequently in a region of great variability than it will in a region of lower interannual variability. For this reason it is more meaningful to compare drought intensity at different locations by using measures of shortfall which allow for the different variability at the two locations. This may be achieved in several ways. One was demonstrated in Section 2.3 where rainfall was standardised using the standard deviation (s): N C (Ri - E)' 1 1' where: N is the number of years of record Ri is the rainfall in the ith year R is the average rainfall over the N years.
An individual annual rainfall is then expressed in terms of the number of standard deviations, above or below the average.
zi = (Ri - E)/S The size of the standard deviation is found to be quite well related to the mean value. Landsberg (1975) shows a selection of rainfall frequency curves from around the world on which the slope of the curve is proportional to s and he notes that records from monsoonal zones have the steepest slope, corresponding to greatest variability. The ratio s/R, termed the coefficient of variation, may thus be the more stable quantity. Another way of allowing for different variability involves the use of percentiles. The individual values, Ri, are ranked and divided into equally sized groups. If ten groups are used then the division between the groups mark the 'decile' points. Quintile division at the 20 per cent, 40 per cent, 60 per cent and 80 per cent points result from dividing the ranked data into five groups: this was employed in Section 2.2 concerning persistence. Australian experience with deciles has suggested that the first (lowest) decile group encompasses most drought occurrences (Gibbs, 1975). The Australian Commonwealth Bureau of Meteorology have developed a 'drought watch system' based upon monthly percentile values (see subsection 4.3.3). The Bureau has also prepared world maps of deciles for individual years between 1950 and 1970 (Maher et al, 1974): Figure 4.1 has been prepared from the first two deciles for 1970. Serra (1960) used a rather similar criterion for his analysis of long term rainfall in France, selecting the 15th percentile as the level below which 'very dry'
68 Fig. 4.1 - Distribution of decile ranges of annual rainfall, 1970. (Prepared by Commonwealth Bureau of Meteorology, Australia). conditions pertained. Dhar et al (1979) used the same threshold for analysis of drought in India. 4.3.2.3 Distribution fitting In principle fitting a statistical distribution to the rainfall data allows a more precise interpolation of decile or other percentile values; also, the distribution's parameters may themselves be mapped or employed in drought severity indices. In regions of moderate and high rainfall the distribution of annual rainfall values appears quite symmetrical and the Normal distribution is commonly employed despite having an unbounded lower tail. Thom (1966) states that locations with rainfall totals as low as 250 mm are well described by this distribution which has many advantages in application. Under the Normal assumption the first decile or 10 percentile point is 1.28 standard deviations below the mean and the 15th percentile is 1.04 standard deviations below the mean. Reports are also found (e.g. Brunet-Moret, 1975) of the use of the log Normal distribution (equivalent to a Normal distribution fit to the logarithms of the annual rainfall values). The Gamma distribution has also been employed in cases where the data appear positively skewed. The same author has produced for ORSTOM a series of reports dealing with aspects of distribution fitting (Brunet-Moret, 1974 and 1978; Brunet-Moret and Roche, 1975). Another approach is to try various transformations to return the data to normality. A very powerful technique that can be recommended for this purpose is the Box and Cox (1964) transform:
XT = (XT - 1)/T T # O XT = log, x T=O
where: XT is the transformed value T is the transforming index
This encompasses all power transforms: e.g. T = 0.5 is proportional to square root and T = 2 to the square transform; harmonic transforms where T is negative, for example T = -1 is the reciprocal transform; and the logarithmic transform when T = O. Thus, skewness values for trial T values can be evaluated and a T chosen which zeros the skewness. Alternatively the transformed data can be plotted on Normal probability paper (Section 4.5.2) and the T values which best linearises the data is chosen. Skees et al. (1974) reviews the problem and provides several references. The presence of persistence disturbs distribution fitting procedures which almost invariably assume independence among the data. In Section 4.5.2 the effect of persistence is explained and an example of its effect on analysis is given. Beran (1979) gives guidelines as to the maximum amount of correlation that can be tolerated before special adjustments become necessary . Whilst statistics such as the mean and standard deviation hold important positions in summarising the climate regime the importance of a graphical plot of the histogram must also be emphasised. In arid areas where the histogram will be positively skewed the mean value will probably exceed both the median (fifth decile) and the modal (most likely) value so it may not
69 provide an adequate baseline for drought definition.
4.3.2.4 Run length and run sum
A run of years below average, or below some threshold such as a decile, is clearly a more serious state of affairs than an isolated occurrence. The problem of describing the probabilitv of runs in the light of the known persistence has been introduced in Section 2.2 which highlights the unsolved problem raised by the particular form of persistence which mostly affects extreme values. The run sum is an extension of the run length idea in which the accumulated deficits of rainfall below the threshold become the variable of concern. Several of the references of Section 2.2 deal also with run sums particularly Yevjevich (1967) and Miiian (1972): see also Murota et al. (1972) for theory applicable to non Normal rainfall variables. On a shorter time scale the length of runs of rainless days is often analysed. For example, Sneyers (1960) and Szigyarto (1960) have fitted distributions to such lengths in Belgium and Hungary and more recent investigators have constructed very complex probability models to the wet day:dry day process. 4.3.3 Analysis of rainfall in other durations The difficulty of using annual rainfalls as a drought indicator has already been referred to and much extra information is retained if the monthly pattern is incorporated into the index. An obvious approach is to isolate the particular period of the year which is critical for the aspect of drought under investigation. Examples are rainfall depth during the germination and growing season for the study of agricultural drought, and winter period rainfall for water supply drought in the temperate zone. A graphical presentation which enables such information to be readily extracted is shown in Chapter III of Davy (1976) reproduced here as Figure 4.2. This shows the variation around the year of the second decile value of monthly rainfall (i.e. exceeded by 80 per cent of months).
Tombouctou 250. - Sikaçso
200. ___ Linguere
Fig. 4.2 - Monthly rainfall exceeded by 80% of months for three Sahel zone rain gauges. (Reproduced from Davy et al, 1976).
Month
The 'Australian drought watch system (op cit) operates on monthly rainfall percentiles; a severe deficiency exists if the total rainfall for the ten last consecutive months does not exceed the five percentile value for that same period and a serious deficiency is referred to the ten percentile (first decile) value. Daily reporting raingauges from up to 800 sites nationwide are employed in preparing a monthly drought review which shows for each of 107 districts the depth of precipitation in the current month required to bring the rainfall over a ten month period up to the first decile level, as well as the probability of this being achieved on the basis of past records. This first screening is used solely to identify regions at drought risk. This having been established, raingauges from the denser 800 Station network are brought into play. A drought situation report and map is produced early in the month giving the rainfall deficiency status, severe or serious, based on the previous ten months rainfall total. An example is shown in Chapter 2, Figure 2.3. Retrospective analysis of a particular drought is likely to focus on a specific period, for instance in the case of the recent United Kingdom drought from May 1975 until August 1976. The question arises whether the sample of data used to assess its return period consists of other low rainfall 16 month periods irrespective of their starting date, or whether it is more correct to limit the sample to precipitation depths within those particular months, May of one
70 year until August of the following year. The return period by the latter method will be greater. The solution to the problem lies in the different meanings that would be ascribed to the event. The population of all 16 month periods is the more natural one on which to base the return period assessment if the drought consequences are not markedly dependent on the start and end dates. When considering such long periods this would usually be the case. Nevertheless, much of the analysis that has been carried out has compared the 1975/76 drought with others starting and ending in the same months (CWPU, 1976). An exception is the map shown as Figure 2.4 for which random start months were employed. 4.3.4 Spatial description of drought rainfall As stated in Section 2.3, spatial heterogeneity is an invariable feature of droughts. Unfortunately, spatial variability is very difficult to quantify. A spatial statistic analogous to the temporal autocorrelation is used expressing the decay of cross correlation between two points as the distance between the points increases (Figure 2.4 of Chapter 2). Another method which is more immediately interpretable is the average of annual arithmetic differences between pairs of raingauges, given in Appendix A of Davy et al. (1976). Startlingly large differences between short period rainfalls at adjacent sites within the Sahelian zone are also shown by means of scatter diagrams. The summarising statistic, the average of annual arithmetic differences between two sites, is used to support the view that rainfall when averaged over longer terms and over large agrometeorological regions is meaningful because interannual variability is typically much greater than station differences. The shape of the area covered by drought is a little explored aspect. Tase et al (1978) present an interesting approach using simple shapes built up from squares of side 100 miles (163 km) in the Great Plains area of the USA. A random sampling technique was used to estimate the annual probabilities of rainfall below a given truncation level affecting areas of different sizes and shape. The correlation between pairs of raingauges within a region can be used to identify characteristic isohyetal patterns using principal components (termed empirical orthogonal functions in meteorological literature). Linear combinations of the rainfall totals are found which yield a single number per time period encapsulating some quality of the rainfall such as, for example, the strength of a precipitation gradient away from the coast (see Dyer, 1975). 4.3.5 More complex climate based indicators The limitations of rainfall based indicators in indexing drought severity have led to the construction .of indices which incorporate other climatic and environmental factors. Landsberg (1975) gives an historical survey of such indices, the earliest and simplest being the Lang rain factor index which is the ratio of rainfall to temperature in some period. Many were intended for mapping aridity using normal values for the variables but are nevertheless capable of acting as drought severity measures by substituting annual values. Hounam et al. (1978) also gives an exhaustive treatment of drought indices. One of the best known is Palmer's (1965) drought severity index. This is calculated from monthly rainfall and evaporation data and although based conceptually on a soil accounting procedure it includes empirical adjustment or weighting factors to summarise the previous history. Its magnitude correlates very closely with drought severity as perceived by farmers in the mid-West (Saarinen, 1966) and has been applied widely within the USA where it is incorporated in official water supply outlooks issued by the US Department of Agriculture. An example of its extension outside the USA is given in Balme et al. (1979). A feature of Palmer's system is that it includes a definition of drought commencement and ending and allows drought terminating rainfall and its probability to be estimated. Annual drought indices based upon annual rainfall and evaporation have been analysed by Yevjevich (1964) and Tabony (1977). The former study regarded the resulting net rainfall as entirely analogous to river discharge which was t.hen used to search for serial correlation and run properties. The latter study distinguished between: 'Meteorological drought' being the fractional deviation of rainfall less potential evapotranspiration from its average (Section 4.3.2.1); 'hydrological drought' based upon an estimate of effective rainfall Re which is produced by the UK Meteorological Office's operational soil moisture deficit mapping service; and 'grassland drought' which is the difference between actual evaporation for short rooted vegetation and the potential evapotranspiration. Long records at Kew near London enabled Tabony to compare historic drouqhts under the . three definitions. The classic 1921 drought was the most severe one year meteorological drought but was exceeded in severity by 1959 as a 'grassland' drought. Data for the 1976 drought were not available at the time the full calculations were made but it was thought to occupy first rank over a range of durations.
71 4.4 ANALYSIS OF DISCHARGE
It is of prime importance to use river data of as high an accuracy as is possible. Relevant types of data include river level and discharge, rating curves, duration of periods of no flow, and indirect information such as is described in Section 4.2. As far as possible, it is very desirable to visit the site during the drought in order to make special purpose discharge measurements and observations of the catchment and river condition. 4.4.1 Special purpose measurements during low flow periods
By definition, river flows during droughts are lower than those in the commonly observed range and in such instances one might be tempted to extrapolate the hydrograph downwards to low flows either along the rating curve or along the recession curve. Downward extrapolation of the rating curve can introduce sizeable discrepancies if the water level is far below the range that is well established by current meter gaugings even when the defined rating at moderate flows is precise and the river bed is stable. Errors of 100 per cent are possible even in these favourable circumstances. It is very unusual to find a riverbed control which remains stable at flows as low as 20 or 50 l/sec. Stations without artificial control structures are frequently insensitive and with the type of observational practices that apply to general network stations, it is difficult to record the water level to better than * 1 cm. These factors increase the error still further. Extrapolation of the recession curve may be less dangerous but during drought periods the recession constant may change suddenly to a greater or smaller value. Every opportunity should be taken to obtain special purpose discharge measurements during the period of low water. Measurements can be made at intervals of a week, two weeks or a month depending on the normal duration of the low flow period, and between these discharges the recession curve is used for interpolation. Staff gauge observations and autoqraphic water level recorders are used only to check that there were no small increases of water level between two gaugings. This is the best way to circumvent the lack of sensitivity of stations. When discharges become very low, it may be necessary to find a more suitable current meter site than the usual cross section. For instance, during the recent Sahel drought, the minimum discharge in the Niger immediately upstream of Niamey dropped to 0.6 m3/s compared with a median value of 28.4 m3/s and it became absolutely essential to look for a new site for gauging these extreme low flows because the water velocity was too low for current metering at the usual station. If possible, provide a stable section, and, if necessary, confine flows to a part of the cross section. All observations must be used to evaluate mean and minimum discharges during low flow periods which is often delicate. It is also necessary to try to evaluate the amount of water abstracted from the river which, during drought, generally become more important than under average or wet conditions; this is even more delicate. In the case of a drought lasting several years in rivers with one well defined rainy season, an accurate assessment of the maximum annual value is important but this would seldom involve any extrapolation of the rating curve; it is often sufficient to know the maximum water level. Because the order of magnitude of discharge during the rainy season is so much larger than that of the dry season a rough evaluation of low flows is sufficient for an accurate estimate of annual runoff. There are only two special difficulties: the approximate evaluation of the very low discharges and a precise determination of the no-flow period. As for the determination of the duration of the no-flow period, station observers should be trained to note the day when the river flow stops (NB not the day when the bed is dry because some ponds remain in the river bed, especially in the vicinity of a well chosen staff or recorder site). Just as for large floods, good information is obtained when the staff responsible for making observations is familiar with what he is expected to do at the time of the drought. 4.4.2 Analysis of discharges of rivers with one defined rainy and one defined dry season Very often the annual hydrograph, including that part related to low flow, is controlled by the rainy season runoff volume or by the volume of snow accumulated during the cold season. These volumes will be quite well correlated with the annual maximum discharge. But in tropical wet countries and in many equatorial countries, there may be small floods during the dry season and in temperate cold countries summer thunderstorms may occur which may affect the low water hydrograph. The simplest case occurs when the low water period is not interrupted by such floods. Depending on the application the analysis should focus on annual runoff, annual maximum discharge or low discharge period. The last named includes the analysis of the length of spells when the discharge remains below a given threshold, and also the study of the deficit volume of runoff below this threshold, as well as the minimum instantaneous or average discharge over a
7 2 specified duration. In the case of a river used for irrigation or water supply, but without any reservoir of significant capacity, the analysis of minimum discharge is the most important one. When the low water period is interrupted by floods, the methods used are very similar to those described in Section 4.4.3 concerning temperate areas. When there is a reservoir capable of storing close to the annual runoff volume, the analysis of average annual discharge is more appropriate. Before the series of characteristic values such as maximum yearly or average yearly discharge is analysed, it is important to check the quality of the data and to use all available information. Restricting analysis to only complete years can lead to a very short record from which it will be difficult to determine drought frequencies. When analysing a mean annual discharge series containing some incomplete years, if,in an incomplete year the observed runoff total is close to the usual annual total, it is advisable to infill missing months with the long term average after checking against rainfall data or the data of neighbouring gauging stations. Significant correlations with nearby discharge or rainfall-stations can be used to improve estimates of missing values and so obtain the maximum length of series of data to be analysed. When there is evidence that the missing data occurs in a year that is not too far from average, or is a wet year, the gap should always be filled. If not great caution is needed in applying regression techniques for data extension purposes as, in this instance, it is possible to 'dilute' the information contained in the data by the addition of synthesised years. The basic problem is that the variance of the extended data is always less than that of the recorded data. This does not matter much for estimating mean values, but can be critical when inferring return periods of rare droughts. The effect is to exaggerate the return period of a recorded low flow when estimated from the frequency curve of the synthetically extended sequences. Simple guidelines for deciding on the level of correlation which will improve estimation are given by Fiering (1963) and for a more complex situation where random additions are used to restore the variance in the extended series in Gilroy (1970). A review of the problem as well .as graphical solutions are given in NERC (1975). The most basic formula states that regression should be employed to extend a short record of length n using a longer one when the correlation coefficient r exceeds (n-2)-3. However, this formula relates to estimates of the mean. For frequency estimates for practical cases the correlation coefficient should be at least 0.7. 4.4.3 Analysis of discharge for rivers without well defined rainy or dry seasons For rivers of the temperate and mediterranean zones Low flows tend to occur in the summer months. Drought may occur in any season although most generally in summer when domestic water supply and pasture may be affected. Hydrological droughts during spring affect agriculture more significantly, especially where the agriculture uses irrigation schemes. It may be that a given year will include both a severe drought and months of very abundant discharge. The correlation between minimum monthly discharge or minimum daily discharge and average runoff during the hydrological year is generally very low ox indeed not significant. M. J. Hamlin and C. E. Wright (1978) describe three methods of analysing the 1975-76 drought in Britain: 1. The first and simplest one is to plot the continuous monthly flow hydrograph showing on the same graph the long-term average and the minimum value of the mean monthly discharges. This facilitates evaluation of each monthly deficit relative to the historic average arid minimum and so gives a visual impression of the severity of drought. On such a graph it is easy to discern drought during the critical time of year and drought occurring at a time when the damage is not so important.
2. A second and similarly simple method is to establish tables or graphs showing the cumulative monthly flows in m/month or the cumulative monthly mean values in m3/s for the period of drought together with the corresponding long term average values. 3. The third method, which is more complete, is to consider the lowest flows of different durations prom 10 days, 1 month, 3 months, 6 months, 9 months up to 1 year and to derive, for each duration, the return period of the corresponding runoff quantity. The details of return period assessment are given in Section 4.5.2 and consist of finding the minimum of each duration in each year of record and using this sample of data to define low flow frequency curves for each duration. Depending upon the water resource application the annual minimum values may be derived from consecutive or non-consecutive periods. Another variation lies in the use of minimum 30 consecutive days or minimum calendar monthly data. The latter, of course, will not be as
73 small as the former but a simple relationship between the two has recently been found (Institute of Hydrology, 1979). The rule is that the T year return period one calendar month low flow is equal to the T year return period 45 day low flow. In general the equivalence is obtained by adding 15 days to the number of days in the calendar month period, so 77 days is equivalent to two calendar months etc. A third variant which is discussed by Hamlin and Wright (1978) lies in the distinction between fixed and variable starts to the drought period entering the analysis. For operational decisions within a drought which commenced in, say, April it is relevant to compare it with the discharges which occurred in other years also using April as the commencement of the period of interest. On this method of accounting a drought of current interest will appear to be rarer than on the a posteriori procedure which uses the minimum discharge period of the requisite duration irrespective of the start month. The fixed period accounting procedure makes it easier to present for each month a report on the severity of the drought for different parts of the country. This is currently done for rainfall in Australia (Drought review Australia, Bureau of Meteorology), and for river discharge in other countries. The 'hydraulicity' of each month: i.e. the ratio of the mean discharge of the month under consideration to the average is broadcast by Electricit6 de France. Another class of discharge variables that may be used to assess drought severity are based upon periods when the hydrograph dips below some threshold of interest, for example the 80 percentile flow (Figure 4.3). The durations when the hydrograph is below the threshold and the volumes of runoff may both be subjected to statistical analysis to assign a return period to that particular aspect of a given drought. The deficit volumes are closely related to reservoir storage needs for regulating the flow in the river at the threshold value. Direct supply reservoirs require a more complex hydrograph treatment which allows for the possibility of carryover from one period to the next if the reservoir does not refill (McMahon et al.,1978).
,I I I II I II I I II I I I II I I_ I l I II I IA I II I
Fig. 4.3 -. Definition of low flow speii DsDuratlon in daya O< deilcit perlod below the threshold dlacherge. I durations and deficiency V.Voiurne in cumec-day8 below the threshold in deiiclt period. volumes. (Reproduced from Time - Days Institute of Hydrology, 1979).
4.5 FITTING STATISTICAL DISTRIBUTIONS TO DROUGHT DATA
This section is concerned with the choice of suitable distribution for fitting to drought data such as runoff and rainfall totals over various durations. Methods of fitting are also reviewed and a preference for graphical procedures is expressed. Plotting formulae and interpretation problems are also discussed. 4.5.1 Choice of distributions It is important to realise that there is no single universally applicable statistical distribution suitable for all rivers of the world or for all of the various indices used to describe and quantify drought. The closeness of fit may be excellent for average or dry years but poor for the wetter years. In fact sample lengths normally available to hydrologists are far too short to make realistic comparisons so one usually selects the distribution on grounds of expediency and empiricism. A very fundamental point is that the basic geometry of the distribution pust accord with that of the data which is being fitted. For example, if the data have a zero lower bound (or perhaps some higher lower bound for strongly aquifer fed streams) then this property should be reproducable by the distribution chosen, such as two or three parameter log-Normal
74 distributions. If the mode (most frequent value) of the data is not at the left hand side of the frequency histogram then there is little point in using the exponential distribution. One may compromise to the extent that the distribution is fitted to a portion of the data only but it is important to realise that this is the case. Annual runoff values often have low skewnesses, lower than the distributions mentioned above; so the Normal distribution is a popular choice despite not being bounded below. The use of the Normal distribution and transformations of it for fitting to rainfall data has already been described in Section 4.3.2.3. Just as the Gumbel and the Frechet distributions are a natural choice for flood data because they are tailored to the properties of sample maxima, the Weibull distribution is an attractive choice for data which can be interpreted as minima of samples. This distribution has been used in a number of drought studies including Matalas (1963), Joseph (1970) and Institute of Hydrology (1979). It was advocated by Gumbel (1958) and termed by him the third asymptotic distribution of smallest values. In its two parameter form, it is conventionally written as p(x) = XY-' exp(-x Y we) /e) o 8002 Spey at Klnrara. Dur8tlon Idmysl. 180 O 901 70tOOO0 eo* . O '*:I. xx O . I e t A + e A e '0t 0.5 1.0 1.5 2.0 2.5 31) Fig. 4.4 - LOW frequency curves for different Plotting Poaition w. durations. (Reproduced from Institute 2 2.5 5 10 25 50 100 of Hydrology, 1979). Return Perlod Yeen. 75 A third advantage of the graphical procedure over analytical ones is that the commitment to a particular distribution is reduced. Thus while it may be convenient to employ a particular form of probability paper to linearise the plot and to enable small exceedance and non-exceedance probabilities to be read off-conveniently, there is no absolute necessity to fit a straight line if the assemblage of data suggests a curve is more appropriate. This is particularly important with low flow data in temperate regions where it is often difficult to fit the minimum flows from the dry and the wet years simultaneously. For interpolation or moderate extrapolation (say T < 2N where T is return period and N is record length) the choice of distribution is in any case less critical than the use of the correct plotting positions for the extreme low points (for drought runoff). Plotting formulae which imply that the return period of the extreme value is approximately equal to NI the record length, are almost certain to lead to a line which is too steep and hence underestimate return period of a given drought, or alternatively overestimate drought runoff of a given return period. The general formulae for a plotting position are where: i is the ranking from smallest to largest N is the sample size F is an estimate of the non-exceedance probability of the ith smallest value y is the plotting position P-l is the inverse of the cumulative distribution function, i e. the scaling which linearises the probability paper The correct choice of value of a comes from a consideration of order statistics of the parent population (Cunnane, 1978). For the Normal distribution and its transformations the best value of a is 3/8, so F = (i- 3/8)/,(N + 34) ; t = O-' (F) where: t is the Normal plotting position expressed as units of standard deviations from the mean O-' is the inverse of the standard Normal integral (this inversion is unnecessary if Normal probability paper is available) For the Gumbel and extreme value distributions the recommended formula uses a = 0.44, so F = (i-0.44)/(N + 0.12) y = -loge (-loge F) for the Gumbel distribution W = y (1-e-YlY) for the Weibull distribution An important feature of these formulae is that the return period they imply for the largest and smallest ranking values of a sample of N is of the order of 1.6 NI a value entirely in accord with probabilistic argument. The Hazen formula (i- 0.5)/N does not accord with the basic pattern of such formulae but is much better than the other commonly used one, the Weibull formula (NB this has nothing at all to do with the Weibull distribution) where a = O F = i/(N+i) which should not be used with conventional probability paper because it causes best fit lines which are much too steeply sloping. Another reason for preferring graphical methods of fitting distributions is the relative ease with which historical information can be incorporated. Thus, if it is know that the most severe drought in a short record, say 25 years, is also the most severe, for example, in d much longer span of 70 years then the plotting position is revised accordingly. Thus the background information provided by an analysis of the type shown in Section 4.2, can be formally introduced into the return period assessment. Some particular points about the preparation and interpretation of probability plots follow. The points, once plotted on probability paper, commonly appear as a series of steps each consisting of three or more points describing a gently sloping line followed by a step down to the next set of gently sloping points. This step pattern is due to the correlation between points and the gentler slope has no hydrological significance so must not be used to extrapolate the best fit to high return period line. Ideally, the entire set of points ought to be used to determine the best fit line but some curvature is often found in practice. In this event it is better to fit a line to the points in the portion of interest, e.g. from the median to the high return period end. 76 On many rivers covered by Section 4.4.2, particularly tropical or snow fed temperate zone rivers, droughts can extend over several years. It is wise to consider the whole period of drought so incorporating those periods when the drought is broken by some average or even wet years. Data points consist of runoff totals over one or more years and so are bound to be correlated one with another. This correlation may be the result of storage within the hydrological system or simply the manner in which the sample is collected, overlapping periods being the prime example. The effect of correlation is felt mostly in the estimates of high return period events. The tendency will be for data sets which include correlation to be less variable than those which are entirely randomly drawn from the parent and SO return periods will be overestimated. The effect is a function of the type of correlation structure, the amount of correlation and the distribution of the data. A typical adjustment for a simple structure type with inter year correlation of 0.3 will reduce the apparent return period from 100 years to 80 years (see Beran, 1979). 4.6 POSSIBILITIES OF DROUGHT PREDICTION BY CORRELATION WITH GEOLOGICAL AND OTHER CHARaCTERISTICS OF BASINS 4.6.1 General The previous sections have presupposed the existence of adequate data at the locations of interest. This will be the case when the problem concerns drought over a very wide area; normally the hydrometric network can provide at least a few stations on which analysis can be based. But many drought related problems are site specific, in other words one wishes to know the low flow frequency properties at a site at which, for example, abstraction for irrigation or domestic water supply is proposed. To overcome this difficulty at least two approaches are possible for transferring the data from gauged basins. The first is informal and consists of the choice of an analog catchment which, as far as can be judged, shares a common hydrological regime with the catchment of interest. The second is more formal and entails the development of relationships, often using multiple regression, between drought flow characteristics and important characteristics of the basin such as its extent, climate, morphology, soil and rock materials. The outcome of such an exercise will be a set of regression and dimensionless relationships which enables the statistics of salient low flow characteristics to be predicted even on ungauged basins. Sections 4.6.2 and 4.6.3 describe the technique originally developed for United Kingdom rivers since extended to mediterranean and African countries. Section 4.6.4 describes a procedure developed for Sahel rivers which is based upon the average behaviour of rivers within particular landscape units as determined from soil and geological properties. 4.6.2 The UK scheme for estimating low flow characteristics of ungauged catchments The UK Low Flow Studies Report (Institute of Hydrology, 1979) deals with many of the low flow properties described in previous sections. For clarity the remainder of this section will concentrate on annual minima over various durations and demonstrate how a frequency diagram of the type shown in Figure 4.4 may be synthesized at an ungauged site. A useful device for interpreting and visualising period minima, especially periods of several years, is to regard them as instantaneous minima from a derived process obtained by passing a moving average through the data. The length of the moving average is the required period of accumulation. This device enables such concepts as the five-year-return-period three-year runoff to be readily interpreted. It does not, of course, circumvent the problem of analysing dependent data series (Sections 2.2 and 4.5.2). 4.6.3 Relationship with catchment characteristics It is not practical to predict an entire diagram such as Figure 4.4 from catchment characteristics; a single representative valuewas therefore selected, the mean annual minimum ten day low flow, MAM (10). Regression relationships were developed between this value and catchment characteristics based upon the data of about 500 gauging stations. 'Internal' relationships were also established linking together the different frequency curves corresponding to various durations and also linking together different return periods (Section 4.6.4). As shown on Figure 4.4 the flow axis has been standardised by dividing by the long term average discharge. This standardisation had the effect of eliminating catchment size and climate variables from possible regression relationships. However, very marked differences are apparent between the statistical behaviour of streams; on some MAM(10) is 80 per cent of the average flow while on others it is much less than 10 per cent. On closer inspection it was found that catchment geology was the determining factor, and success in predicting low flows 77 Fig. 4.5 - Base flow index and catchment geology for Central Northern England. 78 depended totally on an ability to quantify geology and to estimate this numerical geology index at an ungauged site. This was achieved in the United Kingdom by taking advantage of the relatively dense river gauge network. At each of 1100 sites the hydrograph record was used to calculate a 'Base Flow Index' as the ratio of base flow runoff to the total runoff. In impermeable catchments this ratio is low, 0.1 to 0.2, while on strongly aquifer fed rivers the index approaches unity. This variable was very successful in prediction equations and has been mapped and correlated with superficial and solid geology features so a value can be obtained anywhere in the country. Figure 4.5 shows the variation of base flow index in a region of diverse geology. 4.6.4 Internal duration and frequency linkages Having calculated MAM(10) from the base flow index it is next necessary to estimate MAM(D), the mean annual minimum of other durations. The spacing between the curves of Figure 4.4 was found to depend on MAM(10) itself. High values of MAM(10) arose from permeable catchments so the curves were close together. The converse was found with impermeable catchments. Catchment climate has an effect too. An expression was found which enables users to estimate MAM(D) for any required value of D. The final stage is to calculate the drought flow of any required return period; the mean corresponds to the two year return period in the Weibull distribution, this being the one selected for fitting to the data. A number of approaches were tried and the one explained below was found to give the most satisfactory results. When the discharge values were standardised by dividing by the mean annual minimum of their particular duration, all frequency curves reduced to a fairly restricted pattern of behaviour as indicated on Figure 4.6. Despite the relative narrowness of the band these curves still span a range which has important practical consequences. For example, a particular river and duration may follow a Type 1 curve for which the 50 year return period flow is about one-third of the mean annual minimum, while another may follow a type 10 curve for which the 50 year return period low flow is two-thirds the mean annual minimum. It is thus important to recognise which type curve applies to a particular circumstance. 1.O -n Y x 4 x 0.8 L P E 0.6 c>O u icm cn 0.4 E .I- h 7n '$ 02 F Fig. 4.6 - Multiplying factors to scale the mean z annual minimum flow to low flow of O other return periods. For example, if O 0.51 1.0 1.5 I 2.0 2.5 I 3.0 Table 1 gives curve number 5, the 100 i Plottlnp Porltlon W ; ! ! year return period flow is 0.4 x mean .. 2.0 2s 5 io 25 so 100 low flow. (Reproduced from Institute Rotuin Porlod - Yom of Hydrology , 1979) . Table 4.1 shows how this was achieved. At short durations, say D less than three months, the value of MAM(D) itself determines which type curve is appropriate, the flatter type curve corresponding to permeable catchments where MAM(10) is a high percentage of the average flow (ADF). However, as the duration increases this progressively loses importance and a single curve is approached.. This is wholely reasonable because, as the duration increases the year to year variation of flows becomes more and more a function of the year to year'variation in climate, in particular total annual rainfall (the coefficient of variation is relatively uniform across the United Kingdom), and less affected by detailed catchment effects such as geology. 4.6.5 Low flow estimation for ungauged catchments in the Sahel zone In arid and semi-arid tropical African countries the two to five month runoff volume is a commonly used drought index relating to reservoir filling probability. ORSTOM hydrologists were able to capitalise on the Sahel drought data from numerous representative and hydrometric 79 Duration D in days 1 10 30 60 90 180 MAM(10) as 5 1 1 1 percentage of 10 2 2 2 average daily flow 15 4 4 3 20 5 5 3 25 6 6 4 30 8 8 6 40 10 9 8 50 10 10 9 60 10 10 10 Table 4.1 Curve number for required duration and 10 day mean annual minima network basins and to develop, with the help of the CIEH, a method for estimating the 10 or 100 year return period dry runoff (Rodier, 1975). From an exhaustive study of daily precipitation and the use of simplified models (Girard, 1975) combined with the data of representative basins (some of them collected during the 1965-1978 drought) it was possible to define the runoff frequency distributions for six 2 surface area classes (less than 2 km , 2 to 10 km2, 10 to 40 km2, 40 to 500 km2, 1000 to 10 O00 km2, more than 10 O00 km2) for basin types defined essentially by their geological substratum, their soils, and by the catchment slope if significant. For instance, basins with crystalline subsoil are represented by three basin types: a. The 'ABOUGOULEM' type with 80 per cent of relatively permeable soils and 20 per cent permeable or extremely permeable soils. b. The 'BARLO' type with 35 per cent permeable or extremely permeable soils and at least 25 per cent impermeable soils. c. The 'CAGARA' type where the substratum is overlain by impermeable vertisols or impermeable clay sandy soils for 90 per cent of the area. A further subdivision was made based upon annual precipitation; 300 mm up to 750 mm in 2 steps of 50 mm. Thus the following results would be found for annual runoff from a 5 km catchment in a 500 mm annual rainfall area: ABOUGOULEM (slight slope) median runoff 22 mm, 10 years dry runoff = 7 mm 11 II II ABOUGOULEM " 35 mm, lo " = 12 mm II II II BARLO " 80 mm, 10 " = 35 mm II II CAGARA West " 100 mm, 10 " = 45 mm In total eight basin geological types were dëfined with substratum of sand, crystalline rock, sandstone and marls, sandstone, schist, sands and marls. For practical application generalised graphs have been established such as Figure 4.7. The interpolation between the various annual precipitation depths and curves for various basin types largely use the concept of annual runoff coefficient: i.e. ratio between the annual precipitation and the annual runoff for a given frequency. The most difficult operation is the classification of the basin for which hydrological characteristics are unknown into one of the categories or between two categories. The same study was carried out for subdesertic and desertic basins and for tropical dry basins (Rodier, 1976). Graphs such as Figure 4.7 perhaps could not be applied to all tropical dry countries but the same underlying methodology of regionalising the available data should be universally applicable. 4.6.6 Concluding remarks The successful conclusion of both studies described in this section was due to the ability to quantify geology. The procedure can be fully recommended and its practical and scientific value to drought studies is very evident, the facility to make estimates of drought severity at ungauged sites being a very common requirement. Before these techniques can be applied it is necessary to have a good hydrometric network and the availability of methods of transferring and regionalising low flow information cannot be used to justify the sad decline in hydrometry that has been noted in several countries. 80 Q N Lo 81 4.7 REFERENCES TO CHAPTER 4 Anon, 1979. Publications authored or Co-authored by Professor H. H. Lamb. Climate Monitor, March 1979. Norwich, England, Climatic Research Unit. Balme, H. N.; Mooley, D. A. 1979. On the performance of modified Palmer Index. Proc. Symp. hydrological aspects of drought, p. 373-383. Delhi, December 1979. Beran, M. A. 1979. The effect of dependence on the assessment of hydrological drought severity. Proc. Symp. hydrological aspects of drought, p. 103-111. Delhi, December, 1979. BOX, G. E. P.; Cox, D. R. 1964. An analysis of transformations. Journ. Royal Stat. Soc., Series B,vol. 26, p. 211-252. Brookes, C. E. P.; Glasspoole, J. 1928. British floods and droughts. 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J.; Hounam, C. E.; Maher, J. V. 1976. Drought: a natural hazard. Symp. on natural hazards in Australia. Canberra, May 1976. Cunnane, C. 1978. Unbiased plotting positions - a review. Journ. Hydr., vol. 37, p. 205-222. Davy, E. G.; Mattei, F.; Solomon, S. I. 1976. An evaluation of climate and water resources for development of agriculture in the Sudano-Sahelian zone of West Africa. Geneva, WMO. (WMO special environmental report no. 9), 289 p. Dhar, O. N.; Rakhecha, P. R.; Kulkarni, A. K. 1979. Rainfall study of severe drought years in India. Proc. Symp. hydrological aspects of drought, p. 363-372, Delhi, December 1979. Dyer, T. G. J. 1975. The assignment of rainfall stations into homogeneous groups: an application of principal components analysis. Quart. Journ. Royal Met. Soc., vol. 101, p. 1005-1013. Fiering, M. B. 1963. Use of correlation to improve estimates of the mean and variance. U.S. Geol. Survey. (Prof. paper no. 434C.) Foley, J. C. 1957. Drought in Australia. Melbourne, Bureau of Meteorology. (Bulletin no. 43). Gibbs, W. J. 19'75. Drought, its definition, delineation and effects. In: WO, Drought, special environmental report no. 5, p. 11-39. Geneva. (WMO no. 403). Gilroy, E. J. 1970. Reliability of a variance estimate obtained from a sample augmented by multiple regression. Water Resources Res., vol. 6, p. 1595-1600. Girard, G. 1975. Modèles mathématiques pour l'évaluation des lames écoulées en zone Sahélienne et leurs constraintes. Cahiers ORSTOM, Paris, vol. XII, no. 3, (série hydrol.). Gumbel, E. J. 1958. Statistics of extremes. New York, Columbia University Press, p. 299-305, 375 p. Hamlin, M. J.; Wright, C. E. 1978. The effects of drought on the river systems. Proc. Royal -Soc., vol. A.363. London. Hounam, C.; Burgos, J. J.; Kalik, M. S.; Palmer, W. C.; Rodda, J. C. 1978. Drought and agriculture. Geneva, WMO. WMO technical note no. 138, 127 p. 82 Ingram, M. J.; Underhill, D. J.; Wigley, T.M. L. 1978. Historical climatology. Nature, vol. 276, p. 329-334, November 1978. Institute of Hydrology. 1979. Low flow studies report. Available from Institute of Hydrology, Wallingford, UK. Joseph, E. S. 1970. Probability distribution of annual droughts. Journ. Am. Soc. Civ. Eng., Irrig. Drain. Div., vol. 96 (IR4), p. 461-474. KÜtzbach, J. E. 1975. Diagnostic studies of past climates. In: WMO, GARP Publication no. 16, Geneva, p. 119-126. La Marche, V. C. 1978. Tree ring evidence of past climatic variability. Nature, vol. 276, p. 334-338, Nov. 1978. Lamb, H. H. 1967. Climatic variations affecting the far south. WMO technical note 87 on polar meteoroïogy, WMO no. 211, (Spiil), p. 428-453. Lamb, H. H. 1977. Climate: present, past and future vol. 2. Climatic history and the future. London, Methuen and New York, Bomes and Noble, 835 p. Landsberg, H. E.; Yu, C. D.; Huang, L. 1963. Preliminary reconstruction of a long time series of climatic data for the eastern United States. Univ. Maryland Inst. Fluid Dynamics. (Appl. math., tech. note BN-571.) 30 p. Landsberg, H. E. 1975. Drought, a recurrent element of climate. In: WO, Drought, special environmental report no. 5, p. 41-90. Geneva. (WMO no. 43) 113 p. Le Roy, L. 1971. Times of feast, time of famine: A history of climate since the year 1000. London, Allen and Unwin. Maher, J. V.; Lee, D. A. 1974. World distribution of deciles of annual rainfall 1950 to 1970 with special reference to the Sahelian drought. Melbourne, Australian Commonwealth Bureau of Meteorology. (Unpublished paper.) Matalas, N. C. 1963. Probability distribution of low flows. U.S. Geol. Survey. (Prof. paper no. 434-A), 27 p. McMahon, T. A.; Mein, R. G. 1978. Reservoir capacity and yield. Amsterdam, Elsevier. 211 p. Millan, J. 1972. Statistical properties of runs as applied to hydrologic droughts. E. 2nd Int. symp. in hydrology, p. 627-636. Fort Collins, U.S.A., September 1972. Murota, A.; Eto, T. 1972. Theoretical studies on Gamma-type distribution and runs - their application to hydrology. Proc. 2nd Int. symp. in hydrology, p. 648-664. Fort Collins, U.S.A., September 1972. NERC. 1975. Flood studies report, vol. 1, Chap. 3, p. 253-260. London, Natural Environment Research Council. 5 vols. Nicholson, S. E. 1978. Climatic variations in the Sahel and other African regions during the past five centuries. Journ. Arid Environments, vol. 1, p. 3-24. Palmer, W. C. 1965. Meteorological drought. Washington D.C., U.S. Weather Bureau. (Research paper no. 451, 58 p. Plote, H. 1974. Essai sur la détérioration des conditions climatiques du Sahel Tropical. Rapport BRGM, 74, SGN 261 AME, Orléans, BRGM. Potter, H. R. 1978. The use of historic records for the augmentation of hydrological data. Institute of Hydrology, report no. 46, Jan. 1978, 59 p. Rodier, J. 1975.. Evaluation de l'écoulement annuel dans le Sahel tropical africain. Travaux et documents de 1'ORSTOM no. 46, Paris. Rodier, J. 1976. Evaluation de l'écoulement annuel dans les régions tropicales sèches d'Afrique occidentale. Cahiers ORSTOM, Paris, vol. XII, no. 3, (série hydrol.). Saarinen, T. F. 1966. Perception of the drought hazard on the Great Plains. University of Chicago, Dept. of Geography. Chicago (research paper no. 106), 183 p. Serra, L. 1960. Charactéristiques et causes météorologiques des sécheresses fréquences d'apparition. Proc. IAHS Symp., p. 48-59, Helsinki 1960 (IASH no. 51). Serra, L. 1964. Fluctuations de l'hydraulicité à l'échelle continentale. Ass. Gén. UGGI, Symposium AISH de Berkeley, Eaux de Surface, Louvain, Belgium (AISH, no. 63). 83 Servant, M. 1974. Les variations climatiques des régions intertropicales du continent depuis la fin du Pleistocène africain. Soc. Hydrotechnique de France XIIIème. Journées de l'Hydraulique. La Houille Blanche, Grenoble. Seth, S. K. 1963. A review of evidence concerning changes of climate in India during the protohistorical and historical period. Proc. Rome Symp. changes of climate, p. 443-454, Rome 1963. Paris, Unesco. (Arid zone research - XX). Sircoulon, J. 1976. La récente sécheresse des régions Sahéliennes. La Houille Blanche, no. 6/7, p. 537-548. Skees, P. M.; Shenton, I,. R. 1974. Comments on the statistical distribution of rainfall per period under various transformations. Proc. Symp. statistical hydrology, Tucson, U.S. Dept. of Agriculture. (Misc. publ. no. 1275). p. 172-196. Sneyers, R. 1960. Sur la -probabilité des sécheresses à Uccle (Belgique)~- et son influence dans la répartition statistique de la côte udometrique. Proc. IASH Symp., p. 72-80, Helsinki 1960, (IASH no. 51). Symons, G. J. 1888. Historic droughts, British rainfall, p. 23-35. London, 197 p. Szigyarto, 2. 1960. Length of periods without precipitation in Hungary. Proc. IASH. Symp., p. 95-108, Helsinki 1960, (IASH no. 51). Tabony, R. C. 1977. Drought classification and a study of droughts at Kew. Met. Mag., vol. 106, p. 1-10. Tase, N.; Yevjevich, V. 1978. Effects of size and shape of a region on drought coverage. Bull. of IASH, vol. 23, no. 2, p. 203-212. Thom, H. C. S. 1966. Some methods of climatological analysis. WMO technical note no. 81, WMO no. 199. Geneva. (TP103), 53 p. Tilho, J. 1947. Le Tchad et la capture du Logone par le Niger. Paris, Gauthier-Villars. Tixeront, J. 1963. Relations des fluctuations climatiques avec l'hydrologie, l'agriculture, et l'activité humaine en Afrique du Nord. Proc. Symp. changes of climate, p. 429-436, Rome 1963. Paris, Unesco. (Arid zone research - XX). Yevjevich, V. M. 1964. Fluctuations of wet and dry years. Part II - Analysis by serial correlation. Fort Collins, U.S.A., Colorado State University, (Hydrology paper no. 41, 50 p. Yevjevich, V. M. 1967. An objective approach to definitions and investigations of Continental hydrological droughts.. Fort Collins, U.S.A., Colorado State University, (Hydrology paper no. 23), 17 p. 5. The recent drought in tropical areas 5.1 INTRODUCTION Among those natural disasters which have afflicted our planet over recent decades the drought which has affected many areas within the intertropical belt, commencing at some sites as early as 1965 and others as late as 1968 and continuing through to the time of writing (1980) in some places, must rank as one of the most serious in the light of its consequential ravages and socio-economic upsets. It may not be, in hydrological or climatic terms, as severe as others at the beginning of the century but is more severe in terms of its socio-economic consequences because of the growth in population and economic activity during the ensuing sixty years. The following sections define the affected region of the globe, and the data and analyses that have been carried out to describe the severity of the drought. Sources are cited in the text; among others found useful were Davy (1974) , Dorize (1974) , Flohn (1964) , Koteswaram (1958) , Landsberg (1975) , Roche et al. (1973) and Winstanley (1973). 5.2 GENERAL CHARACTER OF THIS DROUGHT 5.2.1 Regional coverage In general terms, the region studied in this chapter is bounded by the two tropics. However, in India the summer monsoon spreads northwards as far as the Himalayan range so it is, in this case, necessary to consider the situation north of the Tropic of Cancer. On the other hand, a belt around the equator has been, relatively speaking, spared by the recent drought. In the study area the temperature and precipitation regimes are governed primarily by the trade winds and the various centres of action associated with the trade winds. The annual movement of these centres of action is responsible for creating the dry and the rainy seasons. Nevertheless, there are important regional differences from one part to another of this intertropical belt as illustrated by the following cases in tropical Africa west of the Congo and the Nile. a. In the Sahara, at the limit of the influence of the African monsoon, there is a short cold period in December and January, but over the greater part of the year the temperature is high; during the maximum temperature period - in mid-summer - several rainfall events may (or may not) occur which produce runoff in impervious and steep areas if the intensity exceeds about 10 mm per hour. This is the true desert region. b. On the southern border of the Sahara, throughout the summer, several rainfall events are observed during July, August or the beginning of September some of which produce runoff. The temperature decreases noticeably during this short rainy season. Runoff is experienced in the more established river networks with the exception of sand soil regions and flat clay plains in which swamps are sometimes formed. This is the sub-desert region. c. South of this belt (between the 300 mm and 750 mm annual isohyetal line) is the Sahel region. There is a three month rainy season July, August, September, with possible further precipitation in May, June or October. December and January are relatively cold but the temperature is also lower during the rainy season. Runoff occurs almost everywhere whether an organised river system exists or not. In very dry years the yearly 85 precipitation may, in the north of the Sahelian belt, fall below 50 mm but never below 30 mm. d. Southwards from the Sahelian region is the tropical dry area with an annual precipitation between 750 and 1200 mm. The rainy season lasts about four to five months. The dry season experiences no rainfall and also lasts about four to five months. There is a relatively cold period lasting one month from the end of December to the beginning of February. The temperature is very high at the end of the dry season. Runoff occurs throughout the region and the river network is well developed. Dry years give rise to significant deficits in runoff but not to its total disappearance as in many parts of the Sahel. Hydrographs of large rivers present a set of peaks between July and October with a long recession curve terminated by zero discharge. e. South of the tropical dry region is the tropical wet or tropical transition region with a short dry season and a very long rainy season. It is very unusual for a cold period in winter to occur. In this belt are the principal tributaries of the major rivers which, further downstream, cross or approach the Sahel: Senegal, Niger, Chari, and some major Nile tributaries. Further south still is the equatorial region with, in principle, two high flow periods and two low flow periods: a. The North transition equatorial region with the more important dry season in January, February and March. b. The equatorial region with two dry seasons similar on average. As a matter of fact these dry seasons are purely relative as they are wetter than the rainy season in the Sahel. c. The South transition equatorial region with the main dry season in July, August, September and October. d. South of the Congo river it is possible to find the same succession from tropical wet up through the desert regime. But even in Africa this very simple pattern may be perturbed by several factors. The existence of very high mountains such as Mount Cameroun and the mountains of East Africa may alter the simple characteristics through the changes they bring about in the circulation of air masses and by the direct influence of altitude which reduces the evaporation. For instance, the equatorial region around Mount Cameroun displays a very long rainy season with, in winter, a very short relatively dry season. In East Africa one may find, within a distance of 20 or 30 h, all the transitions between the Sahelian regime and what would correspond to the tropical wet regime. Close to the equator, eastern Kenya is desertic and it is difficult to identify the two dry and the two rainy seasons. The configuration of the coast can also perturb the general scheme. For instance, the northeast Ivory Coast, much of Ghana, southern Togo and Benin all should have a hydrological regime corresponding approximately to the tropical wet regime but, in much of this area, the yearly rainfall is equal to or below 1000 mm and the variation of this precipitation is much more critical than in the classic tropical wet regime causing a greater degree of irregularity in yearly runoff and droughts of a different character. Another coastal effect which is responsible for perturbations in the pattern of this climate is the cyclone and cyclonic heavy rainfall that is experienced in the Indian Ocean, part of the Pacific Ocean, and the Gulf of Mexico. The character of air masses in Asia is responsible for a situation which differs from that in Africa. While the summer monsoon is, in some countries, the major rainfall influence, a winter monsoon may also contribute to the annual total. Also, the existence of many mountain chains imposes a considerable variety of hydrological regimes. However, there is almost invariably a rainy season and a dry season with the only exception being some very humid areas where the dry season is as wet as the so-called 'dry season' of the coast of Gabon and the east coast of Madagascar. In tropical Australia, from the centre of the desert up to the extreme north of the continent, one may observe a succession of hydrological regimes similar to those of West Africa, but with more irregularity in the annual runoff from one year to another, and a cyclone influence near the coast. In tropical North and South America the succession of hydrological regimes from north to south displays some analogies with the hydrological regimes of Africa: for instance, the North 86 East (Nordeste) region of Brazil experiences a tropical regime similar to southern Benin although more irregular. In eastern Amazonia the transitional equatorial regime is similar to that of the corresponding region in northern Gabon although it is difficult to discern the second dry season in March. The configuration of the continent between the 9th and the 17th parallels with its narrow isthmus and numerous islands contrasts greatly with tropical West Africa, but there is invariably a rainy season in summer with the possibility of cyclones, and a dry season between January and March. The relief of the Andes and Mexican mountains, as well as the mountains on the numerous islands, is responsible for great regional differences in annual precipitation. Some small areas have hydrological regimes similar to the African Sahel and other areas, exposed to the east trade winds, have regimes quite similar to the equatorial humid African region. In Ecuador, as in other continental countries of South America, a mosaic of hydrological regimes may be found. Even in as massive a country as Brazil the succession of regimes from the Amazon up to Rio de Janeiro is not as straightforward as that in Africa. 5.2.2 Characteristics of tropical droughts generally and of the recent drought Drought in tropical countries is normally regarded as a deficiency of wet season runoff although it can also be viewed as a period of unusually low flow in the dry season. Very often the two are linked as the wet season rainfall is responsible for the low flows in the following dry season. In some countries the wet season rainfall may be almost negligible for two years or more. The widespread nature of the recent drought and the straightforward nature of the annual hydrograph in the affected countries permit a general analysis of this phenomenon. Such an analysis follows for the severe drought which, from 1965 or 1968 right up to the time of writing, affected practically all tropical countries of the world, with the exception of some occupying a narrow belt near the Equator. Of course in such a large area one finds differences in the pattern for reasons of natural heterogeneity (Section 2.3). Also, because both hemispheres were involved, the driest years differ from place to place. Nevertheless, in 1972 the drought was very general; there was some relief with, in some places, a very wet year in 1974 and 1975, but drought conditions were resumed in 1976 or 1977. 5.3 AVAILABLE DATA A sufficient quantity of good quality rainfall data is available for the 1965-1978 period in all the drought afflicted countries. Gaps are found in some of the runoff records of arid countries which is unfortunate as it is probable that zero annual runoff volumes were experienced in some of the gaps, and the records could well have been completed if local enquiries had been made shortly after the year in question. Runoff records which span only the most recent 10 or 15 years are of limited value because the average annual runoff deduced from such records is biassed downwards due to the influence of the low runoff years. So many of the methods of quantifying drought discussed in Chapter 4 involving frequency analysis of departures from the mean cannot be applied. Also, their brevity means that comparisons with past severe droughts cannot be made. Only the long term stations'records are useful. In addition, it is necessary to use some historical indications of the types given in Section 4.2. Of these, the most useful included local memory of a large shortage of natural water supply, a minimum level or drying up of lakes, and, the one which was most informative, the report that the river bed remained dry during the whole year. Generally, one finds very little information about the maximum or minimum yearly water level in the river during drought period, except those occasions when it was possible to ford a normally very large river. The most useful time series are those which include the drought which occurred at the beginning of the century. We have found only about twelve such series covering tropical parts of Africa, India and South America and furthermore it is unlikely that future discoveries of old records will increase this figure significantly. The very long 14 century Nile record deduced from the 'Rodda Nilometer' data does not provide as precise a drought record as one might have expected. There are serious suspicions of systematic overestimation of the maximum level in dry years, and the important distortion due to the operation of the Aswan Dam hinders validation of the very old data. It is therefore possible to deduce information of only a qualitative character from this record. Fortunately, it is possible to use series of forty to fifty years duration and even down to twenty years in arid countries if supplemented by careful enquiries on past droughts, as have been carried out in Australia, or if long rainfall records supplement or support the runoff observations. Perhaps fifty series lasting more than forty years are available between both tropics. 87 The runoff information used in the present chapter is selectedly taken from : a. 'Discharge of selected rivers of the world', vol. I,vol. II,and vol. III, parts 1 ana 2 (Unesco 1969, 1971 and 1974). b. The IAHS-WMO technical report presented to the International Conference on the results of the International Hydrological Decade (Paris, September 19741, 'Drought conditions in tropical and subtropical regions'. c. The documentation of ORSTOM, much of it published in Sircoulon (1976). d. Data sent to the Unesco rapporteur or to the IAHS General Secretary by many hydrologists of the world. Most of these records concern large and very large rivers. This is not a drawback because the spatial heterogeneity of drought is thereby strongly diminished and a more representative view of the drought is obtained. 5.4 ANALYSIS OF THE DATA 5.4.1 Numerical indices Drought severity is often expressed as the percentage departure of the average discharge in the dry year below the long term average annual discharge calculated from the entire period of record. This provides an index which, although readily interpreted, is not suitable for making comparisons between rivers in different regimes or with different runoff frequency curves. Thus for example a deficit of ten per cent below the long term mean may have a 0.1 probability of non-exceedance (ten year return period) on one river but a 0.033 probability (30 year return period) at a location where the flow regime is less variable. Therefore, drought severity has been expressed both in terms of percentage departure and in return period terms (see Chapter 4). A graph showing the successive values of either annual runoff or deficit gives a complex picture of drought because of time heterogeneity. A moving average applied to the same graph gives a much clearer picture of the successive periods of drought over several decades. But for a given drought, it is then difficult to associate the runoff characteristics of any year to the causative conditions of the atmosphere and oceans. In what follows, we shall use runoff deficit and frequencies for a limited number of benchmark stations, and present only some characteristic runoff sequences from the start of their observation periods. We shall also present tables of deficits below normal values, and return period for the maximum and minimum yearly values. The maximum is of interest because it is closely related to the fourth type of drought as enumerated in Section 1.2.3. The minimum corresponds to the second type of drought defined in that section. 5.4.2 Some details of the analysis After a review of their quality, the series of average annual discharges were ranked in increasing order, the average value and the empirical frequencies were computed for the lowest values together with the percentage deficit below the average value for each year. Frequency analysis of some of the series was not straightforward, because consecutive values were dependent due to persistence. In such cases, we were obliged to use simple but approximate procedures. The frequency relationship was established using the period of record up to 1972 and the frequencies of the more recent yearly average discharges were interpolated from the frequency relationship established from that earlier period. A better procedure would be, for instance, to fit a Markov chain of first order to the statistical sample and to analyse the values of the stochastic terms (see Section 2.2 for information on the analysis of persistent data). But, the length of the series were often too short to justify the use of more rigorous procedures. In many cases, the frequency of the lowest value was evaluated in a rather qualitative manner. For instance, for the Chari river, frequencies were deduced by comparison with the level of Lake Chad whose minimum value was in turn, correlated with the average annual discharge of the Nile. For Australian rivers, we have used the comparison of runoff data with the results of the Foley (1957) study, brought up to date by Gibbs and Maher (1967) and presented in Coughlan et al. (1976). The use of rainfall data has aided the calculation of frequencies but in many cases, the start of the rainfall records was later than the beginning of runoff records. For some stations, a similar analysis was performed for the maximum yearly and the minimum yearly discharges. 88 5.5 RESULTS 5.5.1 Average annual discharges during the drought 5.5.1.1 Presentation of results The variation in the percentage deficiency of average annual discharge during the recent drought is presented in Tables 5.la and b corresponding to the northern and southern hemisphere. Use was made of 44 selected rivers from 1965 up to 1974 for some stations but up to 1977 for most. Table 5.2 presents the return periods for the four driest years. These are underlined in Table 5.1. In these tables, the results are shown for two stations north of the Tropic of Cancer, one in Tunisia showing the influence of the tropical drought north of the Sahara,. the other one for the Ganges river whose regime is linked with the Asian monsoon and consequently should be affected by variations similar to those of the other rivers of India. Data are also presented for three equatorial rivers, the Amazon, the ogooue and the Tana, in order to illustrate the fact that the drought had little effect close to the equator. The Ogooue was chosen rather than the Congo, because the Congo basin includes substantial areas, like the Ubangui catchment, which are in the drought affected belt. This is not the case for the Ogooue. The Sahelian tropical zone of Africa is represented by four rivers: Maggia, Batha, Goulbi of Maradi, and Gorouol. Unfortunately, the west and east parts of Sahel, which were the most affected, are not represented as there are no hydrometric stations with long observation series. However, it is possible qualitatively to estimate the severity of drought in these areas from which it is known that the Maggia,Goulbiof Maradi and Gorouol rivers were not as badly affected as the west or the east of Sahel. Concerning the drought in India, one must note that the isohyetal pattern does not have the essentially zonal nature as found in West and Central Africa, and so the latitude of the various basins cannot be used in the same way as for Africa to identify their climatic regime. To represent drought in Brazil's dry Nordeste region, we have used a regional mean of four rivers in order to reduce the errors in flow estimation from the early records, and also to smooth local peculiarities due to the spatial heterogeneity of drought in that continent. 5.5.1.2 Discussion of results From a general point of view the results presented in the preceding section verify that almost the total intertropical zone was affected by the drought with the exception of areas quite near to the Equator and part of Mexico. The return period of the driest year on the Ogooue between 1965 and 1977 was 5.5 years, which is quite normal to find in any period of 13 years. A similar observation could be made about the Amazon at Obidos and the Tana at Garissa. The drought did not affect thése basins. For all the other basins, the return period of the driest year generally exceeds 20 years. Only four examples present a return period below this figure: the Oueme 17 years, the Maggia 10 years, the Jaguarribe 10 years and the regional mean for the part of Nordeste of Brazil 10 years. However, inspection of Table 5.1 shows that for these four particular cases, several of the adjacent years also had return periods indicating significantly lower than average runoff, and for each case contiguous basins also suffered dry conditions. In more than 50 per cent of the basins the return period of the driest year is over 40 years. In more than 20 per cent it reaches 100 years and such values are encountered everywhere: in Africa, in India and in South America. In the tropical part of Australia the return periods are difficult to determine because of the relatively short records. At the beginning of the century it appears that some rather more severe droughts were experienced, so this would preclude 100 years return period low annual runoff for the period 1965-1978. Nevertheless, such is the scale of runoff deficiency for many of the Australian rivers that the difference between a 25 year and a 100 years drought is very small; the runoff deficiency dropping from 95 per cent to 99 per cent or even . 100 per cpt. Thus, as even the second driest years for the Australian rivers presented return periods of up to 20 years it is very apparent from these figures that the drought was very widespread and very severe. 5.5.1.3 Year by year account of the drought In order to give a quick picture of the severity of the drought for the various years of the period 1965-1977 in the various parts of the intertropical belt, we have used a type of display conceived by Bredenkamp (1974). On Figure 5.1, which shows this display, return periods of the four driest years for each basin are plotted. The overall severity of the event stands out very clearly on this graph. To supplement this summary form of presentation, 89 COMPARATIVE TABLE OF DROUGHTS - RIVERFLOW - (Mean annual discharges ) X 1964. 65 . 66 . 67 . O 1960. 69 - m 1970 . @ 1971 .+1972.01973. t 7 1974 .A l975.A I976 A1977. 200 . n I œ UEANGUI LW O NARMAOA KRISHNA CUNENE EWE CHARI SENW EAM loo-. O,-+ + +-+ -+-A- 60 OAMOOAR 7s SA0 FIIANUSCll- XU + 0 aN1GER.K SANAGA N1GER.N U> BUROEKHY FlnROy MARTINIQUE M ANORARE SASSANmA -0s -1 O O 6vp r9ûUEENS LAN0 .-O .z REVENTAZON GAHGAMAE~A'RANA SANGA ' NIûT.iY&F n + PENNER + GO YATE A 8 '2 n TAPTI 70 =Ia 60 OK AVA NGO MFDJEROPH -+ 7- 20 WEB1 SCEBELLI OUEME . - 8 + A- + X 65 67 A +A x X65 0. NORMSTE 66 69 x66 60 MPLIGIP, 1 + A -8-a X- 8-0 A-A-A- JAGUARAIEE +6F X 680 65 AG @A A+ A 64 65 - 7 A +a4 A ++ 65 ZA MBE 2 I 67 X - 8 X- A 56 @+ 69 65 66 6% x O+ 068 xx (BA 60 0 O 7 X. 66 Fig. 5.1 - Return periods of mean annual discharge for selected rivers worldwide during the recent drought 90 the following paragraphs detail the evolution of the drought over the entire period 1965 to 1977. -1965 appears dry for the majority of the stations: over west and central Africa, this is true in the Sahelian and subdesert parts but not for the tropical wet part; and this explains why the Senegal, Niger, Benue, Sanaqatand Sanga rivers were not affected by drought at this stage. India seems to have suffered widespread drought. In Australia and South America, some basins display considerable deficits in their annual runoff, but others do not. The situation over central America and Mexico seems to have been the same. But in none of the studied basins is 1965 the driest year. -1966 is also dry for much the same regions as in 1965 in west and central Africa, most of India and some parts of the tropical belt of Australia. For the Damodar basin in India this is the driest year of the period but the data from 1975 to 1976 were not available. 1967 is dry in some parts of India and for the Mekong river; but drought is not all-prevailing. Over some basins, for example the Niger river, 1967 is an exceptionally wet year. 1968 presents amore general and severe drought than 1965 for the northern hemisphere but much of the tropical wet area of west and central America is not drought affected. Australia is not affected but a substantial part of South America does suffer a severe deficiency. For the central part of the Sahel with perhaps the exception of the desert area, ie east of Upper-Volta and Niger Republic, 1968 is the driest year. Drought is general in India; for the Narmada in the north west of the peninsular it is the driest year. It is possible that the same can be said for the Mekong river but the records after 1974 were not available. It is difficult to determine the drought return period for the centre of the Sahel because the 20 to 24 year record lengths are too short for return period assessment. Also, most of the trees of the Maggia river valley had been cut down around 1961 with a consequent increase in runoff. If one can assume a homogeneous record, the return period of the 1968 runoff for this river would have probably been around 40 years similar to the Goulbi of Maradi and the Gorouol. For the Narmada the return period is perhaps 100 years. For many rivers the runoff of this year was similar to that experienced in 1971, 1972 or 1973. -1969 is a wet year for a substantial part of the intertropical belt; but it is dry in some parts of the Sahel, the rivers of the Benin Republic, the Damodar, the Krishna basin in India, the Mekong river, and the Santiago Grande river in Mexico. In the southern hemisphere 1969 is the driest year for the Parana river (return period 20 years); this river was not so badly affected by this drought as other large basins in the intertropical belt. 1969 is a dry year for the Sao Francisco river too. On the Burdekin and Fitzroy (Queensland) rivers in Australia it seems to be the driest year with a return period in the region of 50 years. It is arguable that these figures should be considered as part of the 1968 northern drought, taking into account the lag between the rainy seasons in the two hemispheres. -1970 for the northern hemisphere presents deficits in many parts of the tropical wet belt in Africa except for the Sahelian zone where this year is wet. In India some rivers present a significant deficit. But for the southern hemisphere 1970 is often very dry, particularly in the Nordeste of Brazil where 1970 is the driest year and also in the tropical part of Australia, where the Fitzroy river experiences the lowest annual runoff. However, for these two regions the return periods seem not to exceed 40 years: 10 years in the Nordeste and perhaps 25 years for the Fitzroy river. For this last named basin one might repeat the remark made at the outset of Section 5.3 about the difficulties in ascertaining return period with short records. -1971 marks the beginning of what seems to be the paroxysm (worst period) of the drought. With the important exceptions of the Ganges river, the Mekong river and some small parts of the Sahel and tropical parts OP Australia,deficits are almost universal. 1971 is very wet over the Ganges river. It is the driest year for the Pennar river in the south of India (return period 30-40 years) and for the Sao Francisco river in Brazil (return period 70-80 years). For some basins 1971 is the second ranking year. -1972 seems to be the worst year for most parts of the intertropical belt. It is the driest year of all for about 30 per cent of the studied rivers, all in the northern hemisphere with return period varying between 40 years and 100 years. And 1972 is the second driest year for over 20 per cent more of the studied rivers. Even in some relatively spared countries such as in Central America, 1972 was significantly dry or else very dry. 1973 is not dissimilar to 1972 but the area covered by the drought is slightly reduced and generally the deficits are not so great. Areas spared by the drought in 1973 include: some parts of Upper Volta; many parts of India, the Ganges, Damodar river, Pennar, Narmada and 91 Tapti rivers and the Sao Francisco river in Brazil. For the Narmada the year is very wet. But other areas present the driest year of the period: for instance the rivers of the Air massif in the south of Sahara (although 1968 data were unavailable). For basins at similar latitudes in Mauritania and Chad, the driest year is generally 1972. It is worthwhile mentioning that some desert basins in the 150 to 300 mm isohyetal band which have impermeable soils and steeper slopes experienced several hours of runoff. However, this runoff did not emerge much beyond the foot of the mountains. For Martinique, chosen as representative of the Antilles islands, the worst year is 1973 (return period 50 years), 1971 holds second place. For other islands of this group, the ranking is reversed, for instance in Cuba. For some important rivers: Niger (return period about 30 years), Chari (return period 70 years) and the Sanaga (return period about 15 years) 1973 holds second place. 1973 is also the driest year in the Ivory Coast. -1974 is for many areas in the intertropical belt a year of grace, in particular much of the African tropical Sahel where some rivers experience a substantial runoff excess. The deficiency for the Senegal, Niger, Chari, Benue and Ubangui rivers is significant but moderate. There is no drought for several tropical wet rivers such as the Sanaga and Sanga. In India the situation is complex; the deficit is only moderate for some rivers such as the Narmada and Damodar but quite large in the Krishna river (return period 15 years) and 1974 is the driest year for the Tapti river (return period 30 years) and the Godaveri river (return period 20 years) but for these rivers the records of 1975, 1976, 1977 were not available. It seems that there is no drought either in Brazil or in tropical parts of Australia; on the contrary for many rivers of these countries, the year is very wet. -1975 presents more or less the same aspect as 1974 but unfortunately the overview is restricted because we have few Indian records available. For some basins the year is wet, but for others a return to drought begins to appear as in central Chad, in some tropical wet areas in west and central Africa, and in some parts of Brazil and Australia. In the Sao Francisco basin, the 1975 drought holds third place (return period six years). -1976 is drier in general than 1975. For a great part of the Sahel, the drought is severe. 1976 is the driest year in the Batha river (return period 100 years?). For the Goulbi of Maradi, it is the third driest year (return period about 15 years); in the Senegal, Chari, Benue rivers and rivers of the Benin Republic the deficit is significant. The southern hemisphere situation can be judged from the Sao Francisco river (return period 35 years) and the Gascoyne river (Australia) which presents the driest year of this drought with a return period of about 25 years. But over the rivers of north-east Australia 1976 is wet or very wet. -1977 presents the same general character as 1976 although the spared and dry areas differ somewhat from previous years.. An exhaustive picture cannot be given because of the absence of many of the stations' data from the tables. Some parts of the African tropical Sahel experience dry conditions in 1977 but for the majority of the areas this year is either wet or very wet as in the Air Mountains. But for Senegal, Niger, Chari, Benue, Sanaga and Sanga rivers and the rivers of the Benin Republic, the deficit is the same or greater than in 1976.For the upper Niger, 1977 is the driest year (return period 40 years) and for the Sanga it is the second driest year (return period about 40 years). The western tropical part of Australia presents substantial runoff deficits (return period 10 or 15 years). Unfortunately, we have no data for India. It is possible that in general 1977-was drier than 1976. 5.5.1.4 Comparisons outside the tropical zone The tables show that it is possible to find an analogous chronological pattern outside of the tropical belt. Over the Mejerdah in Tunisia, for instance, 1974 was a dry year (return period 25 years), 1967, 1970 and 1975 also, but this set of dry years is broken by two exceptionally wet years : 1969 and 1972. The big floods in 1969 were the product of local atmospheric features. If these years had been normal, the average of the 1965-1977 period would have been significantly below the normal. 5.5.1.5 Summarising statement From the data of Figure 5.1 it will be found that 65 per cent of the 40 tropical stations presented at least one dry year with a return period of at least 40 years, and 87 per cent of the stations show a return period of 20 years or more. In general, the southern hemisphere seems less affected than the northern hemisphere by the recent drought: 75 per cent of northern hemisphere stations experienced at least one 40 year return period dry year but only 43 per cent of the southern hemisphere stations. Four basins, the Tana, Ogooue, Amazon and Mejerdah rivers, were included on Figure 5.1, but excluded from this evaluation because they are in equatorial or extratropical areas. 92 It appears from Table 5.1 that the drought was most pronounced in: -1968 in the African Sahel; in parts of India and Indochina; in southern Madagascar -1969 in some parts of southern hemisphere; Parana, Brazil; Queensland, Australia. -1972 was the worst year in many parts of Africa, north and south of the Equator; in parts of India and of central America; 25 per cent of the 40 selected basins presented the minimum mean annual discharge in 1972. In other basins, often quite close to those mentioned above, the minimum occurred in 1973 or -1971. In several cases the periods 1971-1972, 1972-1973, or 1971-1973 were very dry. 1974 was a year which gave some relief from the drought in most of the northern hemisphere basins affected and also a part of Brazil. Important exceptions were in India and in Madagascar where the minimum mean discharge of the entire period was observed. The situation was the same in 1975, although in some parts of the intertropical belt the drought began to set in again. 1976 was drier than 1975 in many parts of the studied areas: east of the African Sahel, West Africa, north western Australia and the Sao Francisco river. The pattern remained the same in 1977 although different basins were affected. Insufficient data were available for a full assessment of 1978 but indications are that at some stations (not among the list of selected ones) the annual runoff was the lowest of the entire period. Drought of very severe proportions is being experienced at the time of writing in parts of East Africa and Indochina. 5.5.2 Maximum and minimum yearly discharges during the drought 5.5.2.1 Maximum yearly discharge The maximum yearly discharge relates indirectly to the drought of the fourth type described in Section 1.2.3. An even more direct measure would be the duration spent by the river above a threshold defined as the operating level for run-of-river (ie no pumping) irrigation. Of course because such a level and consequently the duration would be specific to a particular irrigation system, it is not possible to use such a concept in a general analysis such as contained in this report. However, the yearly maximum for tropical rivers provides some numerical index of the difficulties produced by this fourth type of drought. We have mostly based analysis on the maximum daily discharges in each year although where the response is very rapid the instantaneous annual maximum has been preferred. Rivers omitted from this part of the analysis include those not affected by the drought, small mountain streams on which level is not an important factor in their exploitation for irrigation, and rivers with regulated flow regimes so the annual maxima do not represent natural conditions. Some of the remaining rivers suffered gaps in the annual maximum record. In such cases the deficiency has been remedied by making use of the correlation between the yearly maximum and the yearly average runoff. While this correlation is not high enough to permit a complete reconstitution of the flow record, it allows a better estimate to be made of the median value over the entire time span of the record especially for large tropical rivers. In Table'5.3 we present the annual maximal flows as percentage departures from this median figure. Table 5.4 gives the return periods of the four driest years in the drought period. An initial examination of the table of deficiencies shows some heterogeneity between stations, even between stations on neighbouring basins. This is mainly due to the difference between the various lengths of record, which vary from 13 years up to 75 years. Because of the short records, the median value is significantly too high or too low and consequently the deficiencies are systematically too high or too low. This was not so apparent with the annual average values on Table 5.1 because of the lower variability of that characteristic. The results are generally similar to those obtained in Section 5.5.1.2 with some discrepancies because on a large river it can happen that a large flood at the beginning of the rainy season is followed by several months with significantly below average runoff, or on the other hand a rainy season which is generally wet may not include any single large flood event. But for very dry years the maximum discharge is invariably low also. The outcome is that, although for average and wet years the rankings in tables of maximum and average values may well differ, the ranking are usually the same in the driest years. The following paragraphs show the chronology of the drought as expressed by annual maximum discharges. -1965 marks the beginning of the drought in many parts of the tropical belt: in the African Sahel, and in some parts of the tropical wet areas of west and central Africa, and in India. In South America and Australia the year is wet over some of the basins, dry over others. -1966 presents maximum flows which are generally above the median with important exceptions in India and Australia. 93 -1967 is generally dry with important exceptions near the equator is Africa and for the Krishna river in India which suffered large floods in spite of its low average annual discharge. Tropical parts of Australia and some rivers of South America present above average flood maxima. -1968 presents a very mixed picture over the northern hemisphere with a few first ranking low maxima mainly in the Sahel but generally the year is not much represented among the driest values. The year was mostly wet in the southern hemisphere. -1969 presents low or very low maxima in the north east of Australia. -1970 presents the same pattern as 1967 with big floods in some rivers and low flows in others as determined by the latitude and the position in relation to mountain ranges. -1971 marks the beginning of the paroxysm of the drought although with some important exceptions. -1972 in most parts of the tropical belt shows below median maximum values, in some parts very low values with return periods of between 40 and 150 years. -1973 is the continuation of this paroxysm but drought is not so general. -1974 is often wet. -1975 also, but with important exceptions. -1976 and 1977 the drought re-establishes itself with annual maximum values which occasionally dip to the lowest of the entire drought period. However, the drought is not as general as in 1972 nor are there as many examples of overall minima. -1978 the drought continues. 5.5.2.2 Annual minimum discharges The analysis of the minimum discharge in a year is relevant to the study of the second type of drought as described in Section 1.2.3. Procedures used in such a study typically include: a. Comparison of the low flow hydrograph in months of interest with the average and minimum hydrographs for those same months. b. Comparison of the cumulative low flow hydrograph (mass curve) with average values and with other droughts; c. Frequency analysis of annual minimum flows over different durations, eg ten days, one month, two months, etc. In tropical countries two major categories of regime are found: one with a well defined rainy season and dry season, the other with two rainy and two dry seasons; both categories may be sub-divided as explained in Section 5.2.1. The character of the minimum flows differs according to which of the different regimes the river basin lies in. In most desert and subdesert regions river flow is very spasmodic; the period of river flow may total only a matter of a few days contained within a one or two month season. The intervals between such runoff spells vary around 11 or 12 months but the actual figure does not have much hydrological significance except in two cases where the duration of zero flow is a useful measure of the severity of hydrological drought. These are: a. Where aquifers are able to support river flow over several weeks or months, or indeed even permanently; and b. Where, in extremely arid regions, for example in the centre of the Sahara desert, the intermittent nature of flows is such that the interval between flows can be two, three or more years. In the Sahel region, where runoff lasts between three and five months, the duration of the zero flow period begins to become meaningful. It is more significant still in tropical dry or wet countries with very irregular regimes. Such rivers are found in Togo, Benin and some parts of the Ivory Coast and include the rivers of the Nordeste of Brazil, some Indian and many Australian rivers. For tropical or equatorial regimes which, on average, experience abundant runoff, large river flows usually remain above zero except during the most severe cases of drought. Given such an eventuality its analysis should encompass the values of mean discharges over several durations as mentioned above and in Section 4.4.3. But the annual minimum values nevertheless summarise quite well the severity of droughts because of the relative regularity of the recession curve of large rivers. This is because, during the rainy season, there is often a significant correlation between the mean and the minimum discharges. However, the correlation is limited because the minimum discharge results from the value of the discharge at the beginning of the recession and the duration of the recession which, in turn, depend on the previous wet season and the date of the onset of the following flood period. Although overall this means that the relationship between annual averages and minima is not perfect, for severe droughts the situation is more closely controlled and the minimum yearly value is a good 94 index of the runoff volumes over other durations. In a given year the rank of the minimum yearly discharge could be different from the rank of the mean yearly discharge. The many practical problems of making accurate observations of minimum flows within droughts include hydrometric difficulties and effects of man's influence such as large upstream regulating reservoirs. This topic is developed in Chapter 4. From the point of view of statistical analysis the main problem arises through the different effects these influences have from year to year. Moreover in many tropical rivers, no observations were made in past years during low flow periods. Because of these factors, the analysis of minimum yearly discharges in the intertropical belt has had to be conducted using a much reduced data set. All African Sahel stations are eliminated but the rivers of tropical arid and semi-arid Australia remain. In Table 5.5, we present for each of the two or three driest low flow periods of the 1965-1978 drought: the year of occurrence, the value of the discharge, the percentage deficiency from the median value (not the mean value, see Section 5.5.2.1) and the return period. If the median value of the annual minimum discharges is zero, the table presents for the two or three driest period of low flows: the year of occurrence, the duration of the period without flow and the return period corresponding to such a duration. The year shown is that of the causative rainy season. So when the minimum discharge Corresponding to the 1972 flow period occurs in 1973 it has been recorded as the 1972 minimum. For annual minima, just as for annual average discharges, 1972 is often in the first place, or else in second place after 1973 principally for the Senegal, Chari, Niger, Benue and Sanga rivers in west and central Africa. The years 1965, 1968, 1974, 1976 and 1977 also appear in the list of worst ranking droughts. In India, 1965 and 1968 are often the most severe years. It is difficult in India to establish the return period because of the brevity of available records, and data uncertainties due to regulating reservoirs. In South America, the years 1969, 1971 and 1972 appear in the highest ranking positions. In tropical parts of Australia typified by the Gascoyne river, the 1976-1977 year was the driest one followed by 1971-1972 and 1972-1973. It seems, however, that the return periods are not very long. The Fitzroy, whose regime is similar to that of the Sahelian rivers, presents in 1965, 1966, 1967, 1970, 1971, 1972, 1977 almost the same zero-flow duration; 180 days. In the north east of Australia the drought mainly affected the beginning of the 1965-1978 period: 1965, 1966, 1969 and occasionally 1972 presenting the longest zero-flow periods. In general, although with some slight differences, Table 5.5 indicates results that are similar to those for the mean and maximum discharges. Note particularly the extremely low values experienced in some large natural rivers during this drought. 5.6 COMPARISON WITH EARLIER DROUGHTS The earliest date for which a tentative systematic comparison of drought periods is possible is the beginning of the twentieth century because the number of river flow time series commencing before 1900 in the intertropical belt is extremely small, perhaps only four or five stations. The following subsections compare the recent droughts with events during this century region by region. 5.6.1 African Sahel There have been three major droughts during the twentieth century on the large rivers of Africa between Senegal and the Nile, corresponding to the Sahelian, tropical dry, and tropical wet regime. The first occurred near the beginning of the century but with incomplete knowledge of its duration, perhaps 1904-1915. Others occurred in the periods 1939-1945 and 1965-1978 or 1968-1978, the latter probably still incomplete. Within these dates some years experienced normal or even abundant runoff and it is not easy to present a clear picture of the droughts, especially over a very extensive area. A method of data presentation has been used based upon the Bredenkampf (1974) scheme. For 12 African stations (the Ba Tha, Gorouol, Niger at Koulikoro, Chari, Nile at Aswan, Senegal, Benue, Oueme, Sassandra, Sanga, Sanaga and Ubangui) years in which thé runoff exceeded the five years return period high runoff and in which the runoff was less than the five years return period low runoff were identified. These determined a 'wet' and 'dry' year at a station. During the total span covered by the 12 stations each station contributes +1 if a wet year and -1 if a dry year. An adjustment is made for years when not all 12 stations were operating. For instance, from 1911 up to 1921, only four of the stations were operating: Senegal, Niger, Ubangui and Nile; each dry or wet year is assigned a value of 12 4 = 3. A similar adjustment is made and the procedure is similar for the other periods. From 1956 onwards the value of a dry year for each station is -1. Summing the contributions of each station yields an index which is positive for a wet year, negative for a dry year and close to 95 zero for a year without abnormal flow, or with an equal preponderance of wet and dry stations. The chronological sequence of this index from 1902 is shown as Figure 5.2. In Africa north of the Equator, the close of the drought at the beginning of the century stands out clearly; however, at the beginning only two stations were recording: the Senegal and the Nile. In 1907, the northern part of Lake Chad was dry and in 1910, the Faguibine lake in the Niger system was also dry. For Lake Chad, this is possible only after a run of dry years over the Chari basin including at least one or two five-year return period low annual runoffs. Prior to 1907, there certainly were other rivers than the Senegal and the Nile on which dry years were experienced and this supports the figures given on Figure 5.2 for 1904 and 1905 despite the very small number of stations. The 1939-1945 drought shows clearly and likewise the 1965-1978 drought appears as being both very widespread and very severe perhaps more so than the 1902-1915 drought. Three isolated droughts are observed: 1921 as in West Europe, 1958 which was mainly limited to the equatorial belt. Also a short-lived drought appears between 1951 and 1953 which, as will be shown, was much more pronounced in India and South America. The procedure shown here is most useful for very extensive droughts covering the whole of the region being considered. If the drought coverage is only partial the relative abundance over the remainder will cloak the drought. Such was the case in 1960, 1965 and 1968 which were very dry in many parts of the Sahel and subdesert zones but wet in the South. Figure 5.2 shows the paroxysms of 1944 in the 1939-1945 drought and of 1972, and 1976 and 1977 in the recent drought. A more detailed examination of the three droughts taking into account the driest years of each period of drought and the occurrence of wet year contained within the outside dates shows that the 1939-1945 drought was more continuous than the two other drought periods but it was shorter and the return periods of the driest years was less than the return periods of the driest years of the 1902-1915 and 1965-1978 droughts. It is difficult to suggest whether the current drought period is more or less severe than that near the beginning of the century. Viewed from 1979 they appear rather similar overall with some rivers giving their minimum discharges in the earlier drought and others in the recent one. Both have been interrupted by relatively wet years; the earlier drought perhaps more so. In the recent drought significantly wetter years have been: 1969, 1970 (for some parts of the Sahel), and 1975 (slightly above normal). 5.6.2 India Figure 5.2 also shows a similar series for India based upon seven stations (the Ganges, Damodar, Godaveri, Krishna, Penner and Narmada rivers to which we have added the Mekong). For homogeneity with the African series the results have been multiplied by 12 f 7. The first and the third periods of drought mentioned above can be seen to have occurred in India also but no drought between 1940 and 1945 is apparent from Figure 5.2. A more detailed analysis does show, however, that 1940 and 1941 were dry in many basins but this drought was neither very severe nor general throughout India. On the other hand, the 1950-1952 drought is more evident. Just as in Africa, it is difficult to analyse the drought at the beginning of the century, only two stations being available from 1902. Moreover the processes which are responsible for the Indian rainy season are much more complex than in West and Central Africa: the influence of mountain ranges is much more important and therefore the pattern of dry and wet areas during a given year does not present the same tendency to uniformity in India as in the tropical region of Africa discussed above. 5.6.3 Central America and Mexico It has not proved possible to conduct the same type of comparison for central America and Mexico because of the unavailability of long records. 1972 was extremely dry in central America and the western part of Mexico although 1969 was the driest year on the Santiago river; 1974 was also dry but not to the same degree as 1972. In Cuba and the Antilles the driest years were 1970, 1971 or 1973, 1974 or 1975. 1959 is the only other notable drought year. No general drought has occurred across the American equatorial belt during the 1968-1978 period; the most recent severe drought in America is that of 1958-1959 as in Africa. In equatorial Africa, the return period of this drought was between 30 and 100 years. 5.6.4 South America In the southern hemisphere the comparison is not always easy. In South America the long cime series in Brazil's Nordeste and Sao Francisco show some similarities with West and Central Africa and India. A drought at the beginning of the century was observed only in the Nordeste of Brazil. It was significantly more severe than the 1969-1974 drought. The worst years were 1915 and 1919 but between those two dry years there was one exceptionally wet year, 1917, to be compared with 1916 and 1917 in Africa and India. As in India, there were practically no droughts between 1939 and 1949. Also as in India, the 1951-1955 period was dry, apparently of a similar order of severity as the 1969-1974 drought. A noteworthy isolated drought also 96 cc 4bi -4 -d asc HI Q On 0h a 4 n t I cv P -Pl E 97 occurred in 1958 or 1959. Unfortunately the Sao Francisco river flow record does not extend back before 1929. In this period the recent drought period was the most severe one; the drought period 1951-1955 being the second ranking event. No drought occurred between 1939 and 1945; 1958 was, like in the Nordeste, very dry. For the Parana the worst drought period is 1952-1955; the 1968-1974 drought was not exceptional. Other droughts occurred in 1939-49 although not severe, and two isolated dry periods: 1925 which was very dry, and 1934 and 1936. 5.6.5 Tropical South Africa Considering next tropical South Africa, Bredenkamp (1974) in a study of Zambesi river flows showed that a drought lasting from 1910 to 1920 was the most severe on record with return periods approaching 100 years in 1914-1915 (drought years also in West Africa). At its outset the recent drought was not very severe. The driest year, 1972/1973, experienced a five year return period drought only. Unfortunately it has not been possible up to the time of writing to analyse the second half of this drought, especially as in many countries outside the southern part of Africa, 1976 and 1977 have been very dry. On the other hand, a substantial part of the Zambesi basin is quite close to the equatorial belt which was not affected by the drought. For the Cunene and Okavango rivers, whose basins are further south than the Zambezi basin, the 1970-1977 drought is perhaps more severe than for the Zambesi with return periods of between 25 and 100 years for 1971-1972. On the Okavango, the 1954-1955 drought may be compared with the 1951-1954 drought in many parts of the tropical belt. Apparently the period between 1939 and 1945 was not dry in the Zambezi basin. In an unpublished report to WMO in 1974, Bredenkamp has compared droughts in South Africa with those in West Africa using a composite graph presenting the occurrences of the ten wettest and the ten driest years for sample rivers in those two parts of Africa (Figure 5.3). Three rivers were chosen to represent West Africa and four rivers for South Africa. The graph enabled the persistence through groups of wet and dry years, and also the concomitance between the two regions, to be plainly seen. These phenomena were particularly apparent in the wet period 1952 to 1958 although not so apparent in the two drought periods 1939-1949 and 1965-1972. However, the four southern rivers chosen were south of the tropics. 5.6.6 Australia and Oceania In tropical Australia and the islands of the Pacific Ocean the only available long series data are the observations on the Fitzroy river (Queensland) from 1914. Observations commenced on the Burdekin river in Queensland in 1950 and on the Yate river in New Caledonia in 1926. Even within the Australian tropical belt significant differences are seen between the southern and northern regions and also between the west and east. The use of calendar year instead of hydrological year introduces some confusion; on the Fitzroy, for instance, the water year 1972-1973 was very dry although individually neither 1972 nor 1973 were dry. From the enquiries of Foley (1957) extended by Gibbs et al. (1967) and Coughlan et al. (19761, it seems that the 1965-1977 drought has not been the most severe one; the period 1901-1919 experienced more severe conditions with particularly dry years in 1901-1902, 1904-1905, 1905-1906, 1914-1915, and 1918-1919. For the Fitzroy, the two driest years are 1915 and 1919 with return periods of 50-100 years. The year 1925 was very severe just as in some parts of South America. But between 1940 and 1949, it seems that only in Queensland was drought experienced. The drought of 1952-1954 is significant. In New Caledonia, the drought of 1940-1944 is probably the worst, the minimum yearly runoff in 1941 having a return period of perhaps 50-100 years. The 1977 runoff has a return period of 25-30 years. The 1968-1977 drought has been significantly less severe than that of 1940-1944, being relieved by two wet years 1975 and 1976. The period from 1952-1954 presents only a single dry year, 1953, with a return period of’four to five years. One must allow for the fact that in Australia, most islands in the Indian Pacific and West Atlantic Oceans, in Central America and parts of Africa south of the equator, the occurrence of cyclones during a drought period may disturb the pattern of succession of drought years. Unfortunately, the damage due to these events can exceed that which results from the shortage of water, but is can also happen that overall the effect is beneficial. 5.6.7 More extensive historical researches It is both interesting and important to have an idea of older drought periods than those observable through the hydrological record. Historical information of a qualitative sort together with various geological and geomorphological studies in Lake Chad and the central delta of the Niger has made it clear that more severe drought periods than those of 1902-1919 and 1965-1978 have occurred in the last ten centuries. Figure 5.4 shows the probable march of Lake Chad basin rainfall over the last two centuries using lake level information collected by General Tilho in 1907. Section 4.2 gives further details about such techniques. 98 Yi DDD.. O wmmmu L1 O, B.. ,I,,,- f P2 .m. 30 .. .-. z ... . I. 3 .~.. SOUTHERN AFRICA RIVERS : DORING - VAAL - ORANGE - KOMATI (for 1935 and 1936 : October-September hydrological year used) Fig. 5.3 - Frequency of variation with time of the ten highest and ten lowest flows based on river flow data for West and Southern Africa. (Based on Bredenkamp, 1974). 99 -1 -1 rainfall heavy Heavy Average Very weak Fig. 5.4 - Rough approximation of the variations of yearly rainfall in Central Africa from 1750 to 1978 based upon Lake Chad levels (dashed curve from 1860 to 1825 is based on sparse data). (Extended following Tilho, 1947). 5.7 SECOND PERIOD OF THE RECENT DROUGHT IN TROPICAL AREAS During the preparation of this report the drought continued and, in some parts of the world deficiencies were worse than before 1978. In order to avoid any extra delay for the printing of this report, it was, unfortunately, not possible to proceed with a systematical inquiry. However, it should be noted that some parts of Australia, the North-east part of Brazil, the Sahel, the tropical humid area south of the Sahel and even the equatorial belt (spared before 1978) were heavily struck by the drought. Some figures are given below (Sircoulon 1984) con- cerning large rivers crossing the Sahel and a wadi north of the Sahel : Teloua near Agadez (Niger). The deficiency is estimated on the basis of the mean computed up to 1972. For very long time series the difference between the two mean annual discharges up to 1972 and up to 1983 are not very large. Examples ( Senegal (1903-1972) : 753 m3s-’ (1903-1983) : 720 m3s-’ ( Niger (1907-1972) : 1500 m3s-’ (1907-1983) : 1470 m3s-’ Mean annual discharges : (surplus or deficiency) 1978 1979 1980 1981 1982 1983 Senegal at Bake1 - 34 -59 -47 -44 -60 -71 Niger at Koulikoro -13 O -42 -25 -40 -45 Chari at Ndjamena -44 (-50) Teloua at Azel O -58 +192 O -0.04 -75 Only for Senegal and Niger is it possible to compare the hydrological characteristics with those of the 1913 drought. The lowest mean annual discharges are ranked below in increasing order : Senegal : 1983 : 221 m3s-’ 1972 : 255 m3s-’ 1913 : 272 m3s-’ (r.p. 160 years) (r.p. 50-100 years) Niger : 1913 : 796 m3s-’ 1983 : 829 m3s-’ 1980 : 873 m3s-’ 1977 : fi85 m3s-’ (r.p. 160 years) (r.p. 50-100 years) Chari : 1972 : 537 m3s-’ 1973 : 572 m3s-’ 1983: (600 m3s-’) 1982 : 680 m3s-1 (r.p. 100 years) (1913 drought is missing) Teloua at Azel : 1972 : 0.03 m3s-l? 1976 : 0.06 m3s-’ 1983 : 0.9 m3s-’ (for this river, available for 27 years, firm data for 12 years) 100 The lowest maximum yearly discharges are as follows : Senegal : 1913 : 1040 m3s-’ 1983 : 1165 m3s-’ 1972 : 1430 m3s-’ 1944 : 1740 m3s-’ Niger : 1913 : 3560 m3s-I 1983 : 3640 m3s-’ 1982 : 3730 m3s-’ 1972 : 3820 m3s-I Minimum yearly discharges : The years given below are the years corresponding to the flood before the minimum. There is a lag of one year with the calendar year. The lowest minimum discharges follow : Senegal at Balcel: 1983 : O (?) 1973 : O 1981 : O (1913 drought is missing) Niger at Koulikoro: 1983 : (?) 1976 : 16 m3s-’ 1979 : 16 m3s-’ 1915 : 17.5 m3s-l Chari : 1983 < 20 m3.s-’ 1973 : 38.6 m3s-’ (1913 drought is missing) For West Africa, the second period is generally drier than the first one. For many characteristics, 1983 is worse than 1913. Tables 5,2, 5.4, 5.5 should be reviewed in the future after an exhaustive inquiry. The period 1875-1895 was very wet. Consequently, the minimum values for 1913 or 1983 are the minima for at least one century. However, at this time it is possible to say that the 1965-1983 drought was by far the longest. In some areas the drought has already spanned 16 years. 5.8 CONCLUSION One of the main lessons of this survey is to confirm that a definitive zonality of the drought phenomenon exists, with an extensive band around the tropics, north and south, similarly af- fected at the same time. The recent drought was extremely severe, even devastating, in some regions. The minimum observed values of mean annual discharges, maximum and minimum yearly discharges are not unprecedented, and events of similar order of magnitude can be expected on the average once every century. But the duration of the drought in several parts of the world suggests a return period far exceeding 100 years. The durations of droughts show very significant examples of persistence. With such examples at hand, it would be unwise to attempt to reconstitute long time series by simulation techniques without taking this persistence into account. Another consequence is the necessity, in the estimation of averages of hydrological char- acteristics, to take into account the series of wet and dry years included in the total records in order to make the nec.essary corrections (Rodier 1983) to avoid calculated averages too far from the true mean, computed for instance with time series of 100 years or more. For record lengths near or exceeding 100 years the correction is negligible. It has been necessary for the preparation of this chapter to make use of specially de- signed techniques of presenting drought information in order to smooth out effects that are purely local and transient. We have also shown how to construct measures of drought severity which focus on special facets of the phenomenon which affect different water users. As a post script, it may be wondered if the fact that drought appears to straddle the equator and create dry conditions in the south with a six month lag might not represent some kind of forecast ability. While not impossible in the view of the authors, the confirmation or rejection of this supposition will have to await more detailed and directed research combined with a fuller understanding of air mass and ocean processes. 101 - r P PYI m pi rl P IDP N mI m YI+ 3 d N m PYI O3 m IDm YIrl NO YII PN m Y++ I rl - W m YI W 0 IDO YI Prl rl rl YI I 3 NI NI m N+ -3 - - - P N m rl mI Wd P 30 NI + +I +I I m I rl m N YI WN PN OP P In YI TI m I l -d - - - - W rl n Pd mI Ina mN 3N d mI ID 3I NI m I l -rl - - m W N W WN m W mI OP m OP3 I N rlI I 2 I rl I m O d m N ID m m -3 - W ID mI ID ID PI rlI -O>3 - NI PN NI p.ID d P m -3 - - - N ID O WIn Nrl lnI rl3 YII 2n m I rl - - W p. W O0 IDYI ON PO N 3 ;I m rl rl a SO eoCU N3 W a a- CW P rl O m ON OIn c U\ W a mm YI m O '4 E 2 O: Ia d 3 N rl In O O O O e- * InN m *I mN ? 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WMO report (Low flow and related aspects of droughts). Unesco-WMO. International Hydrological Decade. Coughlan, M. J.; Hounam, C. E.; Maher, J. V. 1976. Drought a natural hazard. Symp. on Natural Hazards in Australia, Canberra, Australian Academy of Science. Davy, E. G. 1974. La sécheresse en Afrique Occidentale. WMO Bulletin, vol. XXIII, no. 1, p. 19-25. Dorize, C. 1974. L'oscillation pluviométrique sur le bassin du lac Tchad et la circulation atmosphérique générale. Geog. Phys. Geol. Dyn. Review, 2, vol. XVI, fscs. 4, p. 393-420. Foiey, J. C. 1957. Drought in Australia. Melbourne, Bureau of Meteorology. (Bulletin no. 43). Flohn, H. 1964. Investigations on the tropical easterly jet. Bonn Met. Abh., vol. 4. Gibbs, W. J.; Maher, J. V. 1967. Rainfall Deciles as Drought Indicators. Bureau of Meteorology, Melbourne. (Bulletin no. 45). Koteswaram, P. 1958. The easterly jet stream in the tropics. Tellus, vol. 10, p. 24-42. Landsberg, H. E. 1975. Sahel drought: change of climate or part of climate. Arch. Geoph. -Biol., Ser. B, vol. 23, p. 193-200. Roche, M.; Rodier, J. 1973. La sécheresse actuelle en Afrique tropicale. AISH Bulletin, vol. 18, no. 4, p. 411-418. Rodier, J. 1983. Hydrological computations for water resources development with inadequate data, in Hydrology of humid tropical regions. Proc. Hamburg Symp. Aug. 1983, IAHS Publ. no. 140, p. 447-458. Sircoulon, J. 1976. Les données hydropluviométriques de la sécheresse récente en Afrique intertropicale. Comparaison avec les sécheresses "1913" et "1940". Cahiers ORSTOM, série Hydrologie, vol. XIII, no. 2, Paris. Sircoulon, J. 1984. La sécheresse en Afrique de l'Ouest. Comparaison des annhes 1982 et 1983 avec les années 1972-1973. Cahiers ORSTOM, série Hydrologie, vol. WI, no. 1, Paris. Unesco. 1969. Discharge of selected rivers of the world: Volume I,General and regime characteristics of stations selected. Unesco, Paris. Unesco. 1971. Discharge of selected rivers of the world: Volume II,Monthly and annual discharges recorded at various selected stations (from start of observations up to 1964). Unesco , Paris. Unesco. 1971. Discharge of selected rivers of the world: Volume III (part I),Mean monthly and extreme discharges (1965-1969). Unesco, Paris. Unesco. 1974. Discharge of selected rivers of the world: Volume III (part II),Mean monthly and extreme discharges (1969-1912). Unesco, Paris. Winstanley, D. 1973. Rainfall patterns and the general atmospheric circulation. Nature, vol. 245, p. 190-194. 115 6. Temperate zone drought 6.1 INTRODUCTION 6.1.1 Regional coverage This chapter describes droughts in three geographical regions of the world: a. West maritime: including, in Europe, the United Kingdom, the low countries and parts of France and Spain; and in America, western Canada and north western United States. Winters are cool and summers are moderate. Rain can occur at any time of the year. b. Semi continental: including the remainder of western Europe as far east as the Black Sea and the western states of the USSR, but excluding the' Mediterranean countries and north Scandinavia. Much of the eastern seaboard of the USA and southern Canada falls into this category. Winters are cold, summers are hot and precipitation can occur at any time of the year. c. Prairie-steppe: consideration is given mainly to the prairie region of the United States and Canada where winters are cool and dry and summers are warm with most precipitation occurring in early summer, or as winter snow. In no part of these areas is the Budyko-Lettau dryness ratio (i.e. number of times net radiation could evaporate annual rainfall) in excess of two (Budyko, 1974; Henning, 1970). Attention is concentrated on the northern hemisphere. There are regions in the southern hemisphere which fall into these categories although much more restricted in extent because of the smaller land mass south of 30's. 6.1.2 Characteristics of temperate zone drought Of the three regions listed in the previous section two do not experience a marked seasonality of rainfall incidence within the year; only in the prairie-steppe region is precipitation highly seasonal. However, the examples studied in this chapter concern regions of highly organised agricultural and water resource development which is tailored to suit the seasonal pattern that does exist. Many countries have adopted purely administrative definitions of drought. For instance, the United Kingdom Meteorological Office designate a drought as a rainless 15 day period. Although some very sensitive water supplies may suffer during such an abbreviated event, the types of drought described throughout this chapter are not of this kind. As in Chapter 5, drought is analysed wherever practicable using river flow data - annual and seasonal total or minimum discharge. The administrative definition does serve to emphasise that in the study areas, drought is to be viewed as a departure 'in degree' from climate normals, and not a totally destructive phenomenon. However, events have been experienced in fie 1970s, especially in the west maritime region, that border on differences in 'kind' rather than ' degree ' . Between May 1975 and August 1976 western Europe suffered a drought unlike any in living memory. This event occurring as it did in a region of normally reliable rain served as a reminder that no place on the planet surface is immune to drought. Although the 1975/1976 European drought was the most notable event in respect of its contrast with the usual climate pattern, other droughts have occurred through the '70s which have had as severe consequences. Table 6.1 summarises these events based upon data from two periodicals: the WMO Bulletin, and the Climate Bulletin of the University of East Anglia, UK. 117 Table 6.1 - Summary of recent temperate zone droughts Year West maritime Semi-continentai Prairie-Steppe 1968 West and north west Hungary from February to early July only 10%of average rainfall 1969 Unusually dry Summer in Sweden. July to October dry in Denmark and UK with water rationing 1971 Winter in Spain driest for Year exceptionally dry in Serious drought in SW USA as 30 years most countries of Europe. far north as Oklahoma in Intensive drought.in first half of year Poland in July and August. Lowest levels on Rhine since 1818 1972 Precipitation deficit in Rivers of European UK territory of USSR had lowest flows of 50-80 years 1973 Winter 1972 to Spring 1973 Low snowpack in Austria driest in 200 years in reduced power supplies. parts of eastern UK Low Winter and June to September rain in German DR. Second driest year on record in Czechoslovakia 1974 Dry Spring in Norway with Very dry March and April in Continuation and extension rainless April at some Austria. 25 to 30% average north of 1973 drought during stations. Unprecedented precipitation in western first half of 1974 9 week drought in part of USSR Sweden; Denmark and Holland also had unusually dry Spring. France dry from April to August 1975 March to August in ‘Ireland Dry winter in Poland, Urals area had dry Summer, driest for over a century. Hungary and German DR. less than half average rain. February to August driest Low water levels in many July dry in prairie states of 20th century in UK. basins of European USSR. of USA Parts of southern Sweden Drought during Spring had driest Summer on and early summer in record. Record dry eastern maritime Canada. October in Belgium. 21% Persisted into August to of normal June rainfall the north in N. Germany 1976 Lack of rainfall in first NW Europe drought affected Drought in wheat states and half of year from France this zone. Austria elsewhere during January and to Scandinavia. Belgium persistently dry from February. Manitoba, Canada, driest since 1921. March to May. suffered drought period from Denmark driest since Czechoslovakia dry in July with longest dry period records began in 1874. February, June and July, and rainfall deficit records Northern France rainfall 33 day drought is longest broken. Drought persisted of order of 100 year on record. German DR in bordering US states return period low. rainfall from February to Ireland suffered unusual August nowhere above 70% soil moisture deficits. of normal. Lowest Rainfall in Netherlands rainfall in German FR during February to August since 1891, 39 consecutive driest for 125 years with dry days unprecedented. rainfall below half February and July normal. South east unusually dry in Hungary. 118 Table 6.1 (contd.) Year West maritime Semi-continental Prairie-Steppe Norway rainfall in March In Switzerland from to September period December 1975 until June lowest on record. 1976 rain only 55% of Sweden February to normal, previously September rainfall a experienced once since third below normal. 1864. Lowest water levels Drought in UK of 16-month for 100 years on R. Neman duration and unprecedented in Byelorussia, USSR. since 1727. Scotland had Eastern seaboard of USA driest Summer for 108 suffered drought years. California, USA, suffered continuing drought 1977 W. seaboard of USA Saskatchawen and Manitoba, suffered worst drought in Canada, drought of 1976 its history. Dry spell persisted until May 1977. from May to August in UK Snowpack at record low with but followed very wet consequent record low river Winter. Mid-Scotland had flows. Similar situation driest summer since 1868. in US corn belt and Great N. Ireland suffered 7th Plains successive below normal summer rainfall 1978 September to November Hungary experienced rainfall in SE England between 25% and 50% of driest since 1752. normal rainfall in the Driest October and Autumn. Summer drought in November on record in Southern Ontario, Canada parts of western France 6.1.3 Contrast between Sahel and temperate zone drought 6.1.3.1 Climatic contrasts As in the subtropical zone, atmospheric subsidence is responsible for dryness in temperate areas too. The factors responsible for subsidence may differ though. The major contrast in temperate regions between wet and dry conditions is associated with strong westerly circulation patterns against a meridional or 'blocking' type of circulation. Cross hemisphere influences do not enter as a significant factor as in the tropics. This topic is covered more fully in Chapter 3; The apparent persistence of circulation type from year to year is just as evident in both zones. This is less evident from the climate record because of the much subdued range of variability of the common climate variables. 6.1.3.2 Vegetative and societal contrasts The differences'between the average value and the interannual variability of climate variables are at the root of the profound contrast between the vegetative and societal features of the two zones. The climax vegetation in the temperate zone typically occupies a shallower depth within the soil horizon and thus drying does not extend to the very great depths that are found in sub-tropical areas. On the other hand, the density of vegetation is much higher and as a consequence it is possible for actual evaporation to exceed the potential rate from an open water surface given favourable circumstances. Soils that are established in the arid areas are not as moisture retentive. The lower interannual variability of temperate zone climate has meant that societal structures to cope explicitly with the drought problem have not commonly been developed. The regular institutions have shown or have acquired the resilience necessary to cope with the disruptions to normal life that deficit years bring. In fact society at large remembers drought years for the high temperatures and increase in sunshine rather than for the water shortage and high price agricultural produce. 119 6.1.3.3 Hydrological contrasts High natural soil storage and a rainfall pattern that is less starkly seasonal in the temperate zone results in a reduction in the amplification factor between climatic and hydrological variability by comparison with the subtropical zone. The six types of drought discussed in Section 1.2.3, nevertheless, apply equally to the temperate zone. However, the material presented in this chapter deals mostly with types 2 and 3, minimum discharge and total annual runoff. Some remarks on extended drought, type__ 5, are included in the next section which sets recent drought in historical context. 1.0 0.9 O. 8 O. 7 ...... 0.6 0.5 0.4 -0.2 -0.3 Il -0.4. i -05 -06 -0.7 ...... -0.8 O O0 O r- a a -0.9 50 $$ c c -1.0 Fig. 6.h - Drought index for northern Europe. (The index is the proportion of studied rivers experiencing conditions drier or wetter than the 5-year return period level). 120 6.2 DROUGHTS OF THE RECENT PAST 6.2.1 Western Europe A review of the usefulness of historical data is given in Section 4.2. In this section the hydrological dataofthe past century andahalf has been usedtodescribe the zonaldroughtbehav- iourofthe Europeanwest maritimeandsemi-continental region. The procedure and data source is similar to that used in Sections 5.5 and 5.6 for the Sahel and Indian areas. Summer seasonal Elowshavebeen used rather than annual totals. Figures 6.la and b have been prepared for a 1 -0 0.9 0.8 0.7 O O . 0.6 0.5 0.4 0-3 0.2 0.1 0.0 H -0.1 - 0-2 I II - 0.3 - 0.4 - 0.5 - 0.6 i . . O . - 0.7 - 0.8 -0.9 O O0 jr Os!! - 1.0 Fig. 6.lb - Drought index for west and central Europe. (The index is the proportion of studied rivers experiencing conditions drier or wetter than the 5-year return period level). 121 group of stations representing (a) north and (b) west and central areas. Some tendency to cluster is apparent; see, for example, the period around 1850, after 1921, a very dry year in northern Europe; around 1940; and again the period from 1959 until the 1970s. The southerly group shows fewer examples, the series appearing quite random until the mid OS, except for a period in the third quarter of the 19th century. The combined series (not illustrated) shows persistence primarily in wet summers and highlights 1857, 1863, 1911, 1921, 1947, 1959, 1964 and 1973 as individual summers of continent-wide drought. The year 1876, although not shown, would of course also rank similarly. Serra (1963) shows that the hydraulicity was between 75 per cent and 100 per cent for 1959 over much of north west Europe. Namias (1964) studied this drought from the synoptic viewpoint. Scandinavia was worst struck in that the following year was similarly dry. The use of annual data to index floods is severely deficient for analysing temperate zone drought because the runoff is not the product of a single and clearly recognisable hydrograph period. The year 1959 illustrates this point in that the spring was quite wet and so annual totals were maintained. The correlation between annual totals and absolute minima is low and the correlation with the minima of the absolute maxima (drought of the fourth type) is even lower. The root of the problem is, of course, the very complex transfer function between climate and river flow; the storage properties of various soil and rock types, the distribution of rainfall between intense and continuous types of storm events, the seasonal behaviour of consumption by vegetation, and the spatial integration of possibly disparate subcatchment contributions all are partly responsible. As a result of these randomisation and smoothing effects, on single basins the listed years of widespread drought do not appear more frequently in a list of years in which recorded minima occurred than may be expected from a random disposition of minimum years. 6.2.2 Past droughts in North America 6.2.2.1 General This study has focused on three regions within the temperate zone of the United States and Canada: the West Maritime region consisting of the northern part of California, and coastal rivers of Oregon, Washington and southern British Columbia (6.2.2.2); the northern Prairie region between the Missouri and Mississippi rivers taking in Dakota, Minnesota and southern Saskatchawen and Manitoba (6.2.2.3); the Northeast including the United States eastern seaboard north of Washington D.C. and neighbouring rivers in Quebec and New Brunswick in Canada (6.2.2.4). Figures 6.2a, b and c show the past drought record as observable in the annual runoff record from several rivers in each region. The years which indicate some measure of drought in all three regions are 1911, 1929-1931, 1939-1941, 1944 and 1963. Apart from 1931 this list of years does not include first ranking events so it would appear to be more useful to consider the regions separately. Unlike Europe there are few records extending back into the early 1800s although there have'been some notable attempts to reconstruct long climatic records by piecing together historical information and palaeoclimatic indicators. Examples are Landsberg et al (1968) annual temperature and precipitation record for the Philadelphia area from 1738, Tannehill's (1955) investigation into drought regularities in the U.S. Great Plains, and Stockton's (1977) reconstructions of the Colorado River (Arizona) and Green River (Utah) hydrographs from the 16th century. Thomson (1963) also refers to drought incidences back to the late 18th century in the U.S. Great Plains. Matthai (1979) provides a useful summary of previous droughts in a region by region account of the 1976-77 drought, and this is the major source for subsequent detail. 6.2.2.2 West maritime region The classic drought with which the recent California drought is commonly compared occurred between 1922 and 1925 reaching its lowest point in 1924 but although this event is clearly visible on Figure 6.2a the years closing that decade appear worse at least in terms of river flow. In the Pacific northwest the deficiency gave rise to the drying of lakes in Oregon and the lowest depletions of other lakes in Washington in 60 years. No previous drought on this scale had occurred in California since the 1860s, during which there was a particularly serious drought in the south of the state. Other years which appear on Figure 6.2a as being similarly severe and widespread are 1931, and 1943/44. The familiar tendency for above and below average periods to occur in lengthy runs is quite apparent. Shelton (1977) gives a detailed comparison of Californian rainfall totals for 1924 and 1976/77. Percentages of-average were remarkably similar for the two events; in the northern part of the state annual totals from 40 per cent of average to 50 per cent being common. The recent event appeared to be very marginally longer and more intense than the earlier one at 122 1.0- 0.9- 0.8- 0.7- 0.6- 0.5- 0.4- o. 3- x o. 2- w z0- 0.1- I- o*o-- 1 1 I 1 3 -0.1- O p -0.2- - 0.3- - 0.4- -0.5- - 0.6- - 0.7- -0 8- - 0.9- 0 -1 o-ig E 0 5: c 5 E 5 Fig. 6.2a - Drought index for west coast of North America. (The index is the proportion of studied rivers experiencing conditions drier or wetter than the 5-year return period level). many sites although the 1924 event seemed more extensive at least within the state of California. 6.2.2.3 Northern prairies Strictly the area included in this temperate zone study would include areas north of 40' latitude but because Dakota is included and because the corn belt extending south through Nebraska, Colorado and Kansas have historically been treated together, we shall not exclude 123 this more southern region in this summary. The main thrust of the 1976/77 drought was in the north, and most intensely felt towards the east so this is the part of the region which is considered in detail in Section 6.5 and displayed in Figure 6.2b. A lengthy historical perspective on drought is available in this area dating back to the time of early settlement during the 19th century. It is interesting to record that early surveyors emphasised the aridity of the area (Saarinen, 1966; Hili, 1979) suggesting that such conditions nowadays regarded as damaging droughts were then endemic to the region. Tannehill (1955) has listed droughts in this region around 1800, 1830, 1840, 1860, 1890 and 1910 prior to 1.0- 0.9- 0.8- 0.7- 0.6- o. 5- t ‘4- 0.3- x 0.2- w n - 0.1- t O O (Y 5: Flg. 6.2b - Drought index for northern prairie states in USA and Canada. (The index is the proportion of studied rivers experiencing conditions drier or wetter than the 5-year return period level). 124 1.0 0.9 0.8 0.7 . . . . . 0.6 0.5 0.4 0.3 0.2 O1 0.0 I H I / - 0.3 - 0.4 B 2 - 0-5 L 5 - 0.6 . . - 0.7 - 0.8 - 0.9 O O 5 E r - 1.0 Fig. 6.2c - Drought index for north east coast of USA and adjacent area in Canada. (The index is the proportion of studied rivers experiencing conditions drier or wetter than the 5-year return period level). the famous 'dustbowl' era of the 1930s. The apparent cyclicity of these recurrences has been referredto inSection 2.4. The reconstituted Green River hydrograph (q.v.1 shows clearly that the opening and closing years of the 19th century and'also a period between 1840 and 1850 were periods of runoff deficiency. Palmer (1965) has computed his index for Kansas from 1887 and a severe drought is indicated in 1894 which was singled out also by Tannehill as being of particularly disastrous proportions. This same tabulation shows 1911, 1917 and many years during the 1930s and mid 1950s as drought years for Kansas, all visible on Figure 6.2b. A summary table for North 125 Dakota adds 1946, 1949 and 1960 to 1962. In the period illustrated by Figure 6.2b it can be seen how well the picture given by river flows conforms to this set of years, only the North Dakota droughts of the 1940s are missing from that diagram. The extraordinary degree of bunching of deficit and surplus years stands out more clearly with this data set than with any other (see Figure 6.2b). The measure of the disaster of the 1930, is also highlighted. Laycock (1960) has shown that in the Canadian prairies 1936/37 was the most intense part of that drought while further south the earlier part of the decade provided the more severe deficits. An earlier major drought in the late 1880s and early 1890s affected few settlers. The more northerly region discussed here escaped the severe drought of 1952-1954 which affected not only the southern Great Plains but also a very large proportion of southern and eastern USA as well (Namias, 1955). However, drought did assert itself in this northern area in 1961, that year being embedded within a seven year run of consecutively below average years. The Palmer index for North Dakota supports this observation. Agricultural losses were experienced during this period but not on the scale of the 1930s. Apart from the slightly lower severity many lessons had been learned in the interim about dryland farming practices. There has been no return of drought on this scale since then. Considering next past droughts in the more easterly portion: Minnesota, Wisconsin and Iowa adjacent to the upper Mississippi. Iowa, like its more westerly neighbours, was severely struck by the 'dustbowl era' drought of the 1930s and again in the 1955-58 period. Minnesota was affected by drought in 1891, 1934 and 1936 prior to 1976. Wisconsin also has a record of severe precipitation shortfall; in six years, 1895, 1910, 1939, 1948, 1958 and 1976, the deficits were statewide in extent. 6.2.2.4 The northeast region The incidence of drought in the rivers of northeastern United States and adjacent parts of Canada appears from Figure 6.2~to be fairly random. The most severe drought periods occur in 1910, 1922-24, 1930-31, 1941, 1944 and in several years in the mid 1960s culminating in 1965. The Philadelphia climate record (q.v.) supports this finding and highlights 1930 and 1965 as having extremely low precipitation; only 1821 shows a lower value in the two century record. Schwarz (1977) refers to the stringent emergency measures that were necessary to overcome water shortages during the 1960s. Namias (1966) in an analysis of the rainfall data and synoptic and global circulation aspects of the drought maps seasonal rainfall in terciles from which the shortfall is seen to be greatest in the northeast. New York received only 60 per cent of its normal rainfall in the four year period 1962-65. Temperatures were uncharacteristically below normal during this particular drought. In runoff terms the observed totals in 1965 were among the lowest three ranking events on every river. 6.3 THE 1972 DROUGHT IN THE EUROPEAN TERRITORY OF USSR Buchinsky (1963a and b) has presented accounts of droughts in the recently affected region from the earliest t.imes aswell as tabulations of relative drought frequency from the middle ages. The years 1891, 1892, 1897, 1924, 1931, 1938 and 1946 are cited as years of drought over the east European plain judged largely from deficits in agricultural production. Vladimirov (1974) has described the low flows in the European territory of the USSR during 1972. However, a run of dry years in this area commenced earlier, very low rainfall having been experienced on the Polish plain (Prus-Chacinski, 1976) between 1969 and 1973. Kritsky and Menkel (1960) have referred to the eight year dry period in many rivers in European and Asian USSR between 1933 and 1940. The point is made that such a lengthy period is almost irreconcilable with the serial correlation coefficient and a Normal Markovian model. In widely separated parts of the region June 1972 air temperatures approached or exceeded the highest values previously recorded and precipitation was much lower than normal. The largest rainfall deficits occurred at Leningrad in the far north, Rostov in the far south and Saratov on the Volga at the eastern limit of the territory. In July the drought intensified with temperature records broken by a wide margin. The hot weather continued into August 1972 in the north west, central and south eastern portions of the territory. Conditions were closer to normal in the west and south west regions. The rivers and streams of the territory are usually aquifer fed during at least part of the year; in the north and north-west (Dvina and Neman rivers) in August or September, in the centre (Dnieper, Volga and Don basins) in July and August; and in the south (Ukraine and Black Sea) the low flow period extends from June to August. Thus, the behaviour of rivers in this period is controlled by the strength of the aquifers and the preceding recharge season, especially the spring. In fact the spring preceding the 1972 dry summer was not unusual and what was responsible 126 for the extreme low flows that were experienced was the unusually long season and the high evaporative losses within it. Vladimirov categorises rivers according to the 'dependability' of their minimum monthly runoff which is related to the exceedance probability. The driest category includes rivers with 95 to 99 per cent 'dependability' and this describes a meridional band extending from Leningrad through Moscow to the eastern Black Sea. Parallel bands of decreasing 'dependability' run either side of this worst affected 1000 km broad strip. As expected smaller basins were worst affected. High storage basins, such as those containing karst or lake areas, maintained their runoff even in the driest band. An analysis of maximum catchment areas for which rivers dried up showed a variation of from 300 km2 in the north-west up to 8000 km2 in the Caspian Sea area. Rivers in the south and south-east dry up in most years but what distinguished 1972 was the northward extent of the phenomenon. No frequency analysis is presented by Vladimirov although a comparison with other years is given for the driest zone. This shows 1972 to have been the driest year for 30 years,but over a 40 year span worse conditions were observed on some rivers, especially in the north-west, in 1938. What distinguishes 1972 from all other years on record is the widespread lack of water. 6.4 DETAILS OF DROUGHT IN WESTERN EUROPE DURING THE 1970s 6.4.1 General This most recent drought period has been thoroughly documented due to the advanced state of hydrometric networks (WMO, 1977) and hydrological organisations. Reports and scientific papers have been obtained which encompass the following countries; Czechoslovakia, Belgium, France, Germany (Bavaria), Netherlands and United Kingdom. The meteorological conditions giving rise to the drought have been described by Brochet (1977) ; Perry (1976) ; Ratcliffe (1976 and 197733) ; Miles (1977) ; Kamamitsu et al. (1978) and Namias (1978) among others. The particular conditions are set in the context of a run of mild winters through the early 1970s by Ratcliffe (1977a) and Wright (1976). While the 1975/76 drought over north west Europe is the most severe culmination of events others are observed throughout the decade as is visible from Table 6.1. Maps of pluviosity for the period from December 1975 until July 1976 (Brochet (1977); Doornkamp et al.(1980)) show an area of south east and south central England plus Brittany and Normandy experiencing less than 40 per cent of average rainfall. All of northern and western France, England south of the Wash and a separate region covering the Upper Rhine and Rhone basins experienced less than half the average rain. Almost the whole of the rest of north-west Europe experienced less than 75 per cent average rainfall. Temperatures were correspondingly high throughout this period so evaporation and soil moisture deficits were both considerably above average. The following subsections summarise the conditions within individual countries against this general background. 6.4.2 The 1976 drought in Belgium and its consequences The water years 1920/21 and 1975/76 have been the two driest years of the century in Belgium (Belgian IHP, 1976). Four regions were selected for special study, the Escaut (Schelde) basin to Antwerp, the Meuse basin, the coastal zone and the Semois. Comparisons of catchment rainfall in 1976 with the average and with accumulations of different return periods show return periods exceeding 40 years in all regions, very considerably so over the Semois. In this latter case a comparison with 1920/21 conditions is given showing a very similar pattern of accumulation (Table 6.2) . River discharges were the lowest of the century. Table 6.3 shows that, despite the higher flows in the Meuse at the beginning of the year, by late summer the river discharge was less than in 1921. Rainfall after 10 July rapidly filled the surface layer of the unsaturated zone of the soil at least in the Brussels region although this was too late to improve agricultural productivity and in any case other parts of Belgium were less well favoured. A water balance of the Dyle catchment, a small basin close to Brussels, indicated larger and more sustained soil moisture deficits in 1921. Three consecutive dry winters from 1970/71 until1972/73had left aquifers at a low level but the winter of 1974/75 had filled most to a more normal level. The two years of depletion, 1975/76 being a year of poor recharge, reduced levels to as low a value as in 1973. Nevertheless, in aquifers of small capacity or of high exploitation there were severe supply problems. Agricultural production was very severely affected. Water supplies were maintained although not without imposition of restrictions and emergency expedients. Industrial life was also affected through water shortage and navigation restrictions. 127 ~~ ~~~ 1975/76 1920/21 Depth Departure Depth Departure Month mm from normal mm from normal % % Ott 25.7 - 69.6 34.1 - 59.7 Nov 126.6 10.4 51.2 - 55.4 Dec 44.1 - 67.6 83.6 - 38.7 Jan 1976 110.0 + 4.2 114.2 + 25.6 Feb 56.6 - 44.9 17.7 - 82.8 Mar 37.9 - 53.7 30.7 - 62.5 APr 19. a* - 73.4 41.5 - 44.4 May 72.8 - 6.3 48.9 - 37.1 Jun 7.9*** - 90.7 60.4 - 28.7 Jul 47.8 - 47.9 18. 3** - 80.0 Au9 15. a*** - 85.1 88.2 - 17.1 Sep 76.0 - 18.7 34.0 - 63.6 Note: *, **, *** indicate that the monthly rainfall is rarer than 10, 20 and 40 year return period Table 6.2 Rainfall over catchment of Semois (Belgium) during the 1920/21 and 1975/76 droughts Rainfall 1975/76 Average monthly river flows Departure from 'Onth Depth 1976 1921 normal mm m3/s m3/s % Oct 16.5 - 76.9 Nov 106.2 20.2 Dee 35.7 - 63.8 Jan 97.4 12.2 Feb 43.5 - 46.2 Mar 35.4 - 46.1 145 97 APr 20.4 - 68.6 93 76 May 52.2 - 29.5 59 70 Jun 11.4 - 85.9 38 65 Jul 81.4 - 7.6 28 30 Aug 17.3 - 82.8 20 30 Sep 63.7 - 17.7 25 30 Table 6.3 Rainfall on the Meuse basin near Liege (Belgium) for 1975/76 drought and comparison of flows with the 1921 droughts 6.4.3 The 1971-74 and 1976 droughts in Czechoslovakia The rainfall and runoff features of the drought are studied with reference to the catchment of 2 the Elbe at Decin, a 51,100 km area covering most of Bohemia (Anon, 1977~). A flow record 128 extending back to 1851 and a rainfall record from 1876 has enabled the four dry years 1971-1974 and the 'agricultural' drought of 1976 to be placed statistically in historical context. It was observed that when viewed as a four-year event, rainfall and runoff shared similar statistical properties, the five lowest ranking rainfall periods being represented, but with slightly altered ordering, in the five lowest ranking runoffs (Table 6.4). The period 1971-74 was the third lowest in the 100 year period of record common to both rainfall and river flow records, but viewed over the total 125 years of Decin record it was at least fifth ranking. In rainfall terms 1973 was the driest year of the recent drought, in fact there were only three drier, 1947 (the driest ever), 1943 and 1959, in the 101 year series. In discharge terms only one case since 1876 experienced lower flow, 1934; but five in all back to the start of the record; 1864, 1865, 1866 and 1874. Average Average Rank in 1876-1976 period Four year precipitation discharge period mm m3/s precipitation Discharge 1971-1974 602 220 1961-1964 613 232 1949-1952 604 206 1933-1936 598 2 08 1 2 1884-1887 609 248 4 5 Table 6.4 Most severe four year droughts in Bohemia (Czechoslovakia) After a year's respite with average precipitation and runoff slightly above average, drought conditions were re-established in 1976. This 1976 drought was notable for an exceptionally dry April and June but particularly for a rainless and very hot period 18 June to 12 July. A study of the April-September growing season rainfall deficits and temperature surpluses shows 1976 as fourth ranking in lo1 years with 1947 clearly the most severe. In streamflow terms 1976 was not too severe with only a few small tributaries giving discharge rarer than a ten year return period. Over an 11 month period most stream flows remained higher than the five year return period level. Only the Cidlina, a right bank Elbe tributary, gave a 20 year return period event over this longer duration. Preceding conditions were favourable, heavy but uniform January rains with relatively high temperatures leading to good recharge conditions and a gradual snow-thaw maintaining mountain springs until July. Releases from reservoirs on the River Vltava maintained flows of 50 cumecs and without these the Elbe at Decin would probably have dropped briefly to 40 cumecs, which is among the ten lowest recorded instantaneous flows in 125 years. The average flow over the six months growing season, 186 cumecs, was not exceptional having a six year return period. 6.4.4 The drought in France from December 1975 to July 1976 Much of the available documentation about the drought in France (Brochet, 1977; Anon, 1977d) has focused on the climatic aspect (but see Josa et al. (1977); Castany et al.(1977) and bibliographic list). Previous work (Serra, 1960 and 1963) had pointed the way to the description of drought analysis in river flows and shown the 1920/21 period and the closing years of 1940s.and 1958/59 as times of drought in French rivers. Chaumeau (1959) and Hallaire (1960) study this latter drought. Long term records from six raingauges in northern France; Lille, Beauvais, Caen and Rennes, show that the period, Dec. 1975 to Aug. 1976, is the driest such period since at least 1873. The previous records were broken by close to 50 mm except at Beauvais where the margin was 87 mm drier than the previous (1874) record. Further south, at Paris and Nancy, the 1976 rainfall was among the three driest totals. Temperatures generally were higher than average in June, very much so during the period 22 June to 13 July when temperatures exceeded 35OC, at least 5OC higher than normal. Deficits can be placed in a statistical context by noting that in the north west a 60 per cent deficit is typically of 100 year return period, and a 40 per cent deficit is about 30 year return period. Dealing with the northern area, the rivers of Artois-Picardy are strongly groundwater fed by the chalk aquifer and their response is a function of winter season rainfall accumulated over several seasons. Deficits of 30 per cent to 40 per cent below normal were experienced. Smaller watercourses may provide a better indication of drought effect. Many of these, in the 129 Brittany region, came close to drying and 90 to 100 per cent deficits were common. The Loire experienced deficits of 50 to 70 per cent ln different months, amounts which were typical of rivers in other regions such as the Meuse, Moselle and Rhône. Larger deficits were experienced in the Seine and Saône flowing north and south out of Burgundy. The month of lowest flow in the Seine at Paris Suresne, August 1976, averaged 13.4 cumecs. The previous lowest, in 1947, was more than double this figure. Over a six month period that earlier drought experienced the lower runoff. However, over a range of durations 1976 ranks among the two most serious in a record extending back to 1928. Low flows in the Seine are supported by reservoirs in the headwaters whose total capacity has increased over the period of the record. While flows in the Loire at Blois were low during 1976 they were not extraordinarily so viewed over the total record which commenced in 1863. Flows lower than the lowest calendar month flow of 45.2 cumecs in August 1976 had been experienced in ten years and similar numbers of non-exceedance have been experienced for events up to three months duration. The 1976 drought was less rare over longer durations with some 20 five month droughts of greater severity. Over the shorter and apparently more severe durations there has been no similar event since 1952 although in the short period from 1945 to 1952 as many as five years experienced lower flows. Over short durations 1949 was the driest year while over longer durations 1870 and 1947 were drier. On the Garonne, draining the Pyrenees and Massif Central, the flow record commences in 1921 (not a dry year over this basin) and lower flows than those experienced in 1976 have been reported in eight years over one month, and in 11 years over two months. Over longer durations the 1976 flows are less significant. However, almost all the previous non-exceedances occurred in the period 1945 to 1953 with 1949 showing clearly as the most severe. On the Rhône measured at Beaucaire, which derives a considerable proportion of its runoff from snowmelt in the Alpine region (but also rainfall derived runoff from the Saône, the major tributary mentioned above) the picture was very different. Since the start of the record in 1920 there has been no year in which flows have been as low as 1976 over all durations up to six months. Drought conditions returned to France in Autumn 1978 and bulletins were issued by the Climatology Division describing the water shortage. Between September and December rainfall over the whole country was deficient; a few locations received more than half the average and much of the north west and south east received below 30 per cent. Soil moisture deficits had not been made good by the end of the year (Anon, 1979). The impact of climatic drought on glacier fed streams is discussed by Vivian (1979) and Loriferne et al. (1977). Vivian based her analysis on the unusually low rainfall in spring 1976 and autumn 1978. As elsewhere in France, the seven months from December 1975 to June 1976 were extraordinarily dry in the alpine headwaters of the Drôme and Drac, tributaries of the Rhône. Pluviosities of below 40 per cent to 50 per cent were widespread and the snowpack was 50 per cent of normal. The hydrological consequences of the rainfall deficit and high May and June temperatures and insolation were very varied. Severe discharge deficits built up in streams within the rain and snow fed regimes and some measure of deficit was apparent in all streams irrespective of altitude and glacial component until April. However, from May onwards a sharp distinction became apparent between streams in the glacial and pluvial regimes. In June the hydraulicity of glacial streams exceeded unity, one approaching 1.5; many pluvial and pluvial-snow regime streams experienced hydraulicities of less than 0.3. The dry autumn, September to November 1978, was most severe in the more southerly part of the study area and as in 1976 many records were broken for the particular season. This condition accelerated the recession rate in the lower pluvial regime so that values of hydraulicity of .4 and less were felt in September. The streams with a snow and glacial component were not affected until October and even then did not reach the low values of the pluvial regime rivers. These results highlight the fact that a river flow drought in a mountainous region is not a necessary consequence of a climatic drought and adds to the difficulty of drought analysis in the temperate zone. Despite these effects in the headwaters the deficient snowpack in the Alps and its premature disappearance led to the lower flows than normal that have already been referred to in the Rhône and Garonne in the south of the country. 6.4.5 The 1976 drought in the Federal Republic of Germany (Bavaria) The effect of the drought was quite widespread in Germany (Doornkamp, 1980) but a report has been received only from the Bavarian State Water Management Authority (Anon, 197733). In rainfall terms the drought was most severe in Northern Bavaria, north of the Danube. Between February and June a rainfall deficiency of 50 per cent and locally up to 65 per cent was experienced setting new records dating from 1881, the commencement of rainfall measurement. South of the Danube deficits of 20 to 25 per cent were more characteristic and such values are 130 experienced at five to ten year recurrence. At some sites rainfall was consistently below the 1931-1960 normal for every month from November 1975 until August 1976. The consequences to riverflow and water supply were amplified further by the hard-rock nature of much of the northern area. Lacking the gravel aquifers and valley storage of the south runoff performance in the north was much reduced. Extractions for irrigation and water supply exerted a strong influence on measured flows but the typical picture shows runoff over the five month period April to August as 30 or 40 per cent of the mean. The drought appears more severe than an earlier one in 1964 although instantaneous minimum low flows did not set new records. Water shortages were experienced by the majority of residents although only about 50,000 people had significant supply problems which entailed importation of drinking water. 6.4.6 The 1976 drouaht in the Netherlands A comparison of monthly precipitation over Holland in 1976 with that over the long term shows that the drought commenced in February and terminated in September, with April and August being especially dry. By extrapolation from the Hoofddorp record (starting in 1735) it appears that such a spring/summer precipitation is only to be expected once in 300 years. An analysis of areal precipitation yields a similar result. Fortunately for water resources in Holland some rainfall did occur in the upper region of the Rhine basin, from which river many of the country's major water resources are found. Nevertheless, discharges in that river were considerably below the average. Table 6.5 compares 1976 mean monthly discharge at the Lobith gauge with the 1901-1975 long term average. ~~ ~ ~~ J F M A M J J A S O N D ~ 1901-1975: 2683 2606 2453 2423 2188 2181 2146 1894 1737 1649 1947 2289 1976: 2281 1832 1329 1130 1167 1272 953 1141 1021 1068 994 1811 Table 6.5 Monthly average discharge (m3/s) during 1976 drought compared with long term average for Rhine at Lobith Table 6.5 shows a deficit during every month of the year with the most severe shortfalls in the March to July period. The minimum discharge of 782 cumecs in July was the lowest for this month since 1901. In terms of annual total runoff, 1976 ranks third with 1333 cumecs after 1921 with 1096 cumecs and 1949 with 1193 cumecs. A similar pattern is observed for the River Meuse at Lith gauge (Table 6.6). There were several occasions during July, August and September when the flow stopped entirely. The year's average runoff of 120 cumecs ranked lowest in the period of record starting 1911. Second lowest was 1921 with 128 cumecs average and third, 155 cumecs in 1934. ~ ~~ J F M A M J J A S O N D 1911-1960: 641 547 477 372 256 174 136 139 152 193 360 386 1976: 325 350 182 113 69 37 20 13 21 24 72 208 Table 6.6 Monthly average discharge (m3/s) during 1976 drought compared with long term average for River Meuse at Lith 6.4.7 Drought in the 1970s in the United Kingdom A run of dry winters throughout the 1970s had given rise to occasional and local water problems and left aquifers depleted. Even before 1976 the period beginning 1972 included rainfall records over a three year accumulation that had not been experienced since 1871 (Tabony, 1977). Such records were soon to be broken by the 1976 dry period which closed at the end of August 1976. A rainfall record starting in the early 18th century has been reconstructed for England and Wales and this provides a useful backcloth against which to judge the recent period. Table 6.7 taken from CWPU (1976) shows that rainfall accumulations up to the end of August 1976 are comparable with, or exceed in severity, extremes which date back to before 1800. Other sources which set the drought in longer term context are Thom et al. (1976) and Wigley et al. (1977). 131 Period in Provisional % of average Frequency of Previous lowest on record months ending rainfall (1916-50) occurrence (I) (1727-1975) August 1976 (mm) (1 in X years) Ending August Ending any month - -~-(Year) (nun) (m) 3 77 36 300 1800 74 45 6 205 52 400 1741 184 155 9 355 (2) 55 1000 1731 400 343 12 57i(2) 63 650 1750 608 556 16 757 (3) 64 1000 + 1750 809 779 18 909 (2) 70 5O0 1742 914 885 24 1497 83 30 1741 1259 1259 36 2314 85 45 1742 1987 1938 ~~~ ~~ ~ ~~ ~~ ~ Notes: (1) Starting in a given month (2) New lowest for period ending in August (3) New lowest for period ending in any month Table 6.7 Rainfall over England and Wales to end of August 1976 Fig. 6.3 - Rainfall as percentage of long-term average during the 1921 and 1976 drouqhts in the U.K. (Prepared from data qiven in WU, 1976). 132 Over the 16 month period from May 1975 to August 1976 the rainfall map shows a broad band through the south and centre of England and South Wales where less than 60 per cent of the average rain was experienced. The ten month period from November 1975 to August 1976 shows a similar location for the northward extension of the 60 per cent line but the deficits in the south, west and east had intensified and over a very extensive area in south central England rainfall totals over the ten month period were less than half the average. Figure 6.3 shows this and a pocket on the Isle of Wight where the rainfall was less than 30 per cent of the average. There are many ways of assigning a frequency to such an event: point or areal, fixed start months or moveable period; but all ways confirm the extreme rarity of such low rainfalls as experienced in this period. Statistical analysis of previous rainfall accumulations would have suggested very long return periods (greater than 1,000 years) for rainfall totals over this duration of 50 per cent of long term average. Figure 6.3 shows that the 1921 drought was of a similar extent at least as far as the position of the 70 per cent line is concerned. But in 1976 more of the country suffered 50 per cent shortfalls and pockets down to 30 per cent were unknown during the earlier event. Looking at the five month period, May to August 1976, much of central southern England, mid Wales and the Midlands experienced below 40 per cent, the contours extend below 30 per cent along the entire south coast and mid-Wales and down to 20 per cent and 25 per cent along parts of the south coast. Although the deficits are greater than for the longer durations, a statistical analysis does not suggest that these deficits are any rarer although the return periods are still measured in hundreds of years. The drought, viewed in terms of river discharge, presents a variable picture according to the degree of base flow support from aquifers and also according to the length of record. As an annual total 1976 was not an unusual year because the heavy rains which closed the drought (the September and October rainfall of 313 mm, had never before been exceeded (Murray, 19771, gave rise to compensating high flows late on in the year (Morris and Ratcïiffe, 1976; Ratcliffe, 197733; Richards, 1976). Considering runoff totals ending in August and September 1976 the 1976 drought usually produced record low flows on medium term and recently established gauging stations. However, for the longer flow records the 1975/76 totals do not appear necessarily as the lowest values. For example, the Vyrnwy, a small upland stream in mid-Wales, 1933-34 presents lower runoff totals over all consecutive accumulation periods up to 17 months. That same drought gave lower flows for the Severn and the Thames, the United Kingdom's two largest rivers in area terms. Again the answer and the rankings were according to the details of the method of analysis: method of accumulation, fixed or variable starting date; but the picture remains that the 1975/76 river flows in accumulation were not unprecedented. Instantaneous and short period flow values were commonly lower than any previously measured. Measurement problems resulting from abstractions and the disturbing influence of point effluent discharges mean that recorded flows are not necessarily comparable with those of the past. Over most shorter gauging station records 1959 was the usual previous record low flow at least over short periods. Other prominent dates are 1949, 1934 and 1921 but very few flow records extend back this far. Groundwater sources were able to maintain their yield in most cases. In the chalk of southern England and the Triassic sandstones of the Midlands, the two major aquifers of the UK, response is to winter rainfall. The 1975/76 winter was drier than average and infiltration was calculated to be between 13 per cent and 61 per cent of average, mostly nearer the former figure. Difficulties were experienced with supplies from some shallow wells and wells in upland areas where the groundwater fluctuates over a large range. The level at the 140 year record length Chilgrove well in the chalk of the South Downs reached its all-time low level (Figure 6.4). The previous lowest level was achieved in 1973 and before than in 1934. The next two levels occurred in the mid 19th century (Speight, 1977). There was considerable disruption to economic and domestic life which-touched al?ost- everyone although very few suffered to the extent of standpipe water supply or water rationing. The unusual weather conditions moved the Government to appoint a Minister with Special Responsibilities for the Drought to administer the special powers of an Act of Parliament that had been rushed through in early August. By relaxing consents and prescribed flow conditions, actual inconvenience was limited, as far as householders were concerned, for the most part to garden hosepipe bans and strictures to use water sparingly (Lillicrap, 1977). The drought was later estimated to have cost the Water Industry €34.3 million but this does not include loss of agricultural production which would greatly exceed that figure (Freeman, 1977; IWES, 1977; Jeffers, 1976). 133 I' i s 0 fo Q A U 45 a z Z U t 1 E Y ül 35 > Pb I O llll F: (0O H O c $!! Fig. 6.4 - Long-term level records (metres above sea level) in chalk aquifer at Chilgrove, U.K. (Prepared from data in Speight, 1977) . 6.5 DROUGHTS IN THE TEMPERATE ZONE OF THE UNITED STATES DURING THE 1970s 6.5.1 Areas affected during the 1976-77 drought Two of the three regions discussed in this chapter: the west maritime and the northern prairies, suffered widespread and severe drought. Small areas in the Washington D.C. area and neighbouring areas of Virginia also experienced low flows but with few exceptions these did not approach the lowest flows on record. Figures 6.5a and b trace the progress of the drought through the march of the Palmer index -4 contour which is taken as the threshold of extreme drought. These maps have been prepared from'more detailed ones in Matthai (1979) which shows the much greater extent of areas where the index dropped to below -2, the threshold of moderate drought. This whieved its maximum extent in July 1977 (Figure 6.6) showing extensive areas of the south east and almost the entire Canadian border under stress. Individual values of -7 and -8 were experienced in 1977 in both centres of the drought with minima hitting -9 in the western area. Such values are lower than those calculated for the great Kansas droughts of the 1930s and 1950s. 6.5.2 Chronology of the drought in western USA Given the very low Palmer indices it is not surprising to find record low river flows and aquifer levels throughout the affected area which, as can be seen from Figures 6.5 and 6.6, extended over the whole of California in 1976 and spread to the more northern states of Oregon and Washington in 1977. In the summer of 1975 the flows in the region were generally above or near to average and remained so for the remainder of the year although reductions were noted between November and December. During the rainy season, October 1975 to April 1976, California had experienced severe deficits; pluviosities of between 30 per cent and 90 per cent with one of the worst affected regions being the agricultural central valley which increased irrigation demand. Snowpacks at one third of the snow courses in the Sierra Nevada were at their record lowest values. Groundwater conditions declined particularly in the Sacramento Valley where wells are a major source and in one valley overpumping to the extent of three times the normal abstraction took place. 134 A 1976 -.-..- JulyMayA 31 - August 28 ...... October 30 B -19777 February 1 - July 2 ...-...October 29 Fig. 6.5 - USA areas affected by extreme drought (Palmer index less than -4) during 1976 and 1977 drought. (Prepared from data in Matthai, 1979). Fig. 6.6 -USA areas sufferingdrought (Palmer index less than -2) as at July 1977. (Taken from Fig. 5 of Matthai, 1979). 135 Rains in later summer cancelled the deficit by October (Figure 6.5a) but only in the northern part of the area were river flows normal; those on the Williamette, Fraser and Columbi, were well above their median levels. A rapid reversal of trend in late autumn left rivers in this area with discharges below their median values, the Williamette especially contained only 12 per cent of normal. Conditions in the United States appeared worse than in Canada and the situation seemed set to worsen as snowpacks far below normal were reported by January. This situation was aggravated , ' by the unseasonably warm weather which caused precipitation to occur as rain rather than as snow and hence further reducing the already deficient snowpack and contributing to runoff rather than to storage. The Water Resources Review for February 1977 published by the United States Geological Survey issued the following report: 'Serious drought conditions persisted in large areas of the United States. Critical seasonal water shortages were occurring in northern California and Oregon and parts of adjacent States. Snowpack was far below normal throughout the Western United States. Some water-supply reservoirs in the Far West were lowest of record. Monthly and daily mean flows were lowest of record in parts of California, Oregon, Washington, Idaho, Colorado, Utah, South Dakota, Wisconsin, Michigan and also Hawaii. ' Prognoses issued by the Departments of Agriculture and Commerce for the Western United States and Canada showed flows of between 10 per cent and 40 per cent on a majority of streams with even lower values for California streams fed from the Sierra Nevada. The snowpack there had been declared as the lowest on record and many reservoirs were already down to a third of active capacity. Some relief came to the northern parts of the western region in March but the drought persisted in California with critical water supply shortages. Precipitation over the state of California in April 1977 was only 10 per cent of normal, the seventh consecutive month of below average conditions. River flows in consequence were below 20 per cent of normal for the following season for coastal rivers and below 50 per cent on rivers with mountain headwaters. The lowest monthly totals were of similar magnitudes to others in 1924 and 1931 but the year overall was much more serious than those droughts. The lowest calendar monthly flows on the Sacramento River appear to have occurred in Autumn 1977 when flows of about 180 cumecs were experienced at Verona gauge. This compares wit minima at Sacramento gauge slightly downstream during 1949 (179 cumecs) and 1971 (200 cumecs). The lowest flows on the Columbia at the Dalles gauge were between 1500 and 1700 cumecs which occurred in early 1977. Flows of a similar and lower magnitude were experienced on several occasions during the 1930s, although after adjustment for storage the runoff was the lowest since 1879. New record low flows continued to be established in California in August 1977 and again in September although by that month a noticeable contraction in the affected area appeared. The trend continued and a large input in November helped to raise reservoir levels and river discharges although they still remained well below normal. By the winter, rivers in the region were flowing at or near their normal rates. The accumulated runoff totals for the period October 1977 to March 1978 were everywhere near to or above normal although many reservoirs had not refilled by March. Pockets of below normal flows were experienced in Oregon and British Columbia in spring and summer 1978. Some reference has already been made to the rainfall comparisons with 1924 in California. Rainfall in the Upper Sacramento valley was only 260 mm in 1976 and 330 mm in 1977 compared with a long term average of 580 mm and 310 mm in 1924. A number of authors, in particular Namias (1978) and Ratcliffe (1976),have drawn attention to the stationary atmospheric waves that gave rise to this drought and simultaneously the West European drought. 6.5.3 Chronology of the drought in the northern prairie region Figures 6.5a and b indicate that the drought was both more extensive and continuous in the region to the west of the Great Lakes than it was in the Pacific coast area considered in the previous subsection. However, it ended sooner, rains in late summer 1977 having cancelled the Palmer index deficit before October. Some sign of drought had already been felt in 1974 in western parts of Iowa and in South Dakota and Nebraska. In the earlier period the development of drought was very patchy with bands of above average precipitation being found in most states except Iowa where the rainfall deficit was worse than in the 1930s and 1950s. Generally the drought conditions did not reach critical proportions until 1977. Southern South Dakota was an exception where the driest year this century was suffered with pluviosities of 30 to 65 per cent. 136 The below average snowpacks in the mountains in the 1976/77 winter kept flows in the major streams ipw; and variable, but generally adequate rainfall in the January to May period retarded drought development until the early summer. The James River, which was in the 'eye' of the 1976 drought, had ceased flowing in July 1976, but responded to March 1977 rainfalls. The ten months no-flow duration was the longest since 1959-60. Flow ceased again in May 1977 and did not start again until December 1977. East flowing streams from the divide peaked early because of the deficient snowpack and remained in the lower quartile of historic values month by month for eight or nine months. The eye of the 1977 drought appears somewhat east of the 1976 position (Figure 6.5b) in the headwaters of the Mississippi. Peak discharges on that river were one-quarter to one-third of their 1976 values in this area. Another major stream in this area, Red River of the North, experienced the fifth lowest flow in over 70 years of record with all the first four ranking values in the 1931-36 drought. Storages in this area were already severely depleted in October 1976, 21 per cent of capacity, and the expected recharge did not materialise so dropped to 6 per cent of capacity by February 1977. 6.6 REFERENCES TO CHAPTER 6 Anon. 1977a. The drought of 1976. 2nd Conf. on Hydrological problems in Europe, Brussels, September 1977. (Netherlands national report to UNESCO/WMO). Anon. 1977b. Die Trockenperiode des Jahres 1976; Wasserwirtschaftliche Auswirkingen und Folgerungen. Report of Bayerisches Landesamt fÜr Wasserwirtschaft. (Private communication to WO). Anon. 1977c. Dry periods in Czechoslovak SSR in the years 1971, 1974 and 1976. (Private communication to m0). Anon. 1977d. La sécheresse en 1976: aspects climatologiques et incidences sur la ressource en eau. Ministère de la Culture et de l'Environnement, Paris. (Private communication to WMO) . Anon. 1979. Note d'information sur la sécheresse automnale 1978. Service météorologique métropolitain, Direction de la Météorologie, Division de Climatologie. (Private communication to WO). Belgian IHP Committee. 1976. Conséquences de la sécheresse en Belgique. Report to OECD. Bernier, J. 1965. Sur les probabilités d'occurence des sécheresses et des étiages. Bulletin du Centre de Recherches et d'Essais de Chatou. Brochet, P. 1977. La sécheresse, 1976 en France: aspects climatoiogiques et conséquences. Hydrol. Sci. Bull., vol. XXII, no. 3, IASH. Buchinsky, I.E. 1963a. Climatic fluctuations in the arid zone of the Ukraine. Arid zone research - XX, p. 91-95, Paris, Unesco. Buckinsky, I.E. 1963b. Droughts in the East European plain during the past thousand years. In: Dzerdzeevskii, B. L. (ea.), 1954. Sukoveis and drought control, p. 23-29. Jerusalem, Israel Programme for Scientific Translations, 366 p. Budyko, M. I. 1974. Climate and life. D. H. Miller (ea.), Academic Press, New York. 508 p. Castany, A.; Margat, J.; Rossier, J. 1977. La sécheresse de 1976 (suite). Techniques et Sciences Municipales, p. 409-427. Chameau, R. 1959. Sécheresse 1959. L'eau, vol. 46, no. 12, p. 287-294. Climate Monitor (formerly CRUMB). Quarterly publication of Climatic Research Unit, University of East Anglia, Norwich, UK. (Each issue contains a summary of climatic events during the previous quarterly period and a country by country list of exceptional climatic events. CWPU. 1976. The 1975-1976 drought: a hydrological review. Tech. note no. 17. Reading, UK, Central Water Planning Unit, 36 p. Dacharry, M. 1975. Sur le fléchissement des débits estivaux de deux rivières du Massif Centrai francais, la Sénouire et l'Allanche, affluent et sous-affluent de l'Ailier. Revue géographique de l'Est, no. 1-2, Nancy, France. Davy, L. 1975. Une nouvelle approche de la sécheresse dans le bassin de 1'Ebre. Etude des épisodes secs. Revue géographique de l'Est, no. 1-2, Nancy, France. 137 Doornkamp, J. C.; Gregory, K. J. 1980. Atlas of drought in Britain, 1975-1976. London, UK, Publ. Inst. British Geographers. Frecaut, R. 1975. Contribution à l'étude statistique des étiages. Application au domaine tempéré océanique. Revue géographique de l'Est, no. 1-2, Nancy, France. Freeman, L. 1977. The 1976 drought and all that. Water, no. 14, May 1977, p. 17-19. Grosrey, A. 1961. Periodes séches et périodes humides. La Météorologie, série 4, no. 61, p. 71-77. Paris. Hailaire, M. 1960. La sécheresse de l'année 1959 dans la moitié nord de la France. & Météorologie, série 4, no. 58, p. 289-301. Paris. Henning, D. 1970. Composition heat balance calculations. Proc. Symp. world water balance, (IASH Bu1,l. no. 911, p. 361-375, Reading, July 1970. Hill, H. M. 1979. Drought mitigation in Canada's prairie provinces. Proc. Symp. Hydrological Aspects of Drought, p. 570-576. New Delhi, December 1979. I.W.E.S. 1977. Operational aspects of the drought of 1975-1976. Proc. one-day seminar, Inst. Water Eng. and Sci. and Inst. Civ. Eng., London, March 1977. Jeffers, J. N. R. 1976. NERC Research in relation to drought conditions: Sumer drought - effects and implications for terrestrial ecology. (Natural Research Council internally- circulated document, 76/89.) Josa, F.; Descroix, P. 1977. La sécheresse de 1976 (fin). Techniques et Sciences Municipales, p. 465-474. Kanamitsu, M.; Krishnamurti, T. N. 1978. Northern summer tropical circulations during drought and normal rainfall months. Mon. Weath. Rev., vol. 106, no. 3, p. 331-347. Kritsky, S. N.; Menkel, M. F. 1960. Methods of quantitative estimation of lingering droughts, on rivers. Symp. on droughts (IAHS Publ. no. 51), p. 130-131. Helsinki, 1960. Landsberg, H. E.; Yu, C. S.; Huang, L. 1968. Preliminary reconstruction of a long time series of climatic data for the Eastern United Statas. Univ. Maryland Inst. Fluid Dynamics, Appl. Math. Tech. (Note BN-5711, 30 p. Laycock, A. H. 1960. Drought patterns in the Canadian Prairies. Helsinki Symp. on Drought, (IAHS Publ. no. 51). Helsinki, 1960. Lillicrap, R. J. 1977. Meeting the problems. Proc. Seminar operational aspects of the drought of 1975-76. Inst. Water Engr. and Scientists and Inst. Civ. Eng., London, UK. March 1977. Loriferne, H.; Bremond, R.; Rapinat, M. 1977. La sécheresse dans les Alpes du nord en 1976. Revue de géographie alpine, no. 3, p. 241-256. Lvovich, M. I. 1972. Hydrologic budget of continents and estimate of the balance of global fresh water resources. Soviet hydrology, no. 4, p. 349-360. Matthai, H. F. 1979. Hydrological and human aspects of the 1976-77 drought. US Geological Survey, prof. paper 1130. Washington, D.C., 84 p. Miles, M. K. 1977. Atmospheric circulation during the severe drought of 1975/76. Met. Mag., vol. 106, p. 154-164. Morris, R. M.; Ratcliffe, R. A. S. 1976. Under the weather. Nature, vol. 264, p. 4-5, 4 November 1976. Murray, R. 1977. The 1975/76 drought over the United Kingdom - hydrometeorological aspects. Met. Mag., vol. 106, p. 129-145. Namias, J. 1955. Some meteorological aspects of drought with special reference to the summers of 1952-54 over the United States. Mon. Weath. Rev., vol. 83, no. 9, p. 199-205. Namias, J. 1964. Seasonal persistence and recurrence of European blocking during 1958-1960. Tellus, vol. XV1, no. 3, p. 394-407. Namias, J. 1966. Nature and possible causes of the north-eastern USA drought 1962-65. E. Weath. Rev., vol. 94, p. 543-554. Namias, J. 1978. Recent drought in California and Western Europe. Rev. Geophys. and Space Physics, vol. 16, p. 435-458. 138 Palmer, W. C. 1965. Meteorological droughts. Research paper no. 45. Washington, D.C., US Dept. of Commerce, Weather Bureau, 58 p. Perry, A. H. 1976. The long drought of 1975-1976. Weather, vol. 31, p. 328-334. Prus-Chacinski, T. M. 1976. Comments on the assumptions applied in hydrological forecasting and some remarks on the droughts in Europe. Journ. Inst. Water Eng. and Sci., vol. 30, p. 381-391. Ratcliffe, R. A. S. 1976. The recent dry period. Weather, vol. 31, p. 271-273. Ratcliffe, R. A. S. 1977a. A synoptic climatologist's viewpoint of the 1975/76 drought. Met. Mag., vol. 106, p. 145-154. Ratcliffe, R. A. S. 1977b. The wet spell of September-October 1976. Weather, vol. 32, p. 36-37. Richards, H. 1976. Water everywhere. Nature, vol. 264, p. 5-6, 4 November 1976. Saarinen, T. F. 1966. Perception of the drought hazard on the Great Plains. Research paper no. 106, Chicago, University of Chicago, Dept. of Geography, 183 p. Schwarz, H. E. 1977. Climatic change and water supply: how sensitive is the northeast? In: National Academy of Sciences, Climate, Climatic Change and Water Supply, chapter 7. Washington, D.C., (Studies in Geophysics), 132 p. Serra, L. 1960. ' Caractéristiques et causes meteorologiques des sécheresses. Frequences d'apparition. Proc. Conf. on surface water (IAHS Publ. no. 511, p. 48-59, Helsinki 1960. Serra, L. 1963. Fluctuations de l'hydraulicité à l'échelle continentale. Proc. Symp. Eaux de Surface, AISH Publ. no. 63, Berkeley, p. 269-280. Shelton, M. L. 1977. The 1976 and 1977 drought in California: extent and severity. Weatherwise, vol. 30, part 4, p. 139-146. Speight, H. 1977. Setting the scene. Proc. Seminar operational aspects of the drought of 1975-76. Inst. Water Eng. and Sci. and Inst. Civ. Eng., London, UK, March 1977. Stockton, C. W. 1977. Interpretation of past climatic variability from palaeoenvironmental indicators. In: National-Academy of Sciences, Climate, Climatic Change and Water Supply, Chapter 2. Washington, D.C., (Studies in geophysics), 132 p. Tabony, R. C. 1977. Drought classification and a study of droughts at Kew. Met. Mag., vol. 106, p. 1-10. Thom, A. S.; Ledger, D. 1976. Rainfall, runoff and climatic change. Proc. Inst. Civ. Eng., vol. 61, part 2, p. 633-652. Thompson, L. 1963. Cyclical weather patterns in the middle latitudes. Journ. Soil Water Conservation, vol. 28, p. 87. Tannehill, I.R. 1955. Is weather subject to cycles? Water, Yearbook of Agricule, Washington, D.C., p. 84-90. Tschirhart, G. 1969. Causes et caractéristiques méteorologiques des sécheresses. Bulletin technique d'information, no. 237, p. 79-92. Paris, Ministère de l'Agriculture. Vivian, H. 1979. Climatic and hydrologic droughts in northern Alps. Proc. Symp. HydrologicaL Aspects of,Droughts, p. 18-26. New Delhi, December 1979. Vladimirov, A. M. 1974. The extreme drought in the rivers of the European territory of the USSR during summer 1972. Meteorology and hydrology, no. 10, 1974. (Translation provided by WMO). Wigley, T. M. L.; Atkinson, T. C. 1977. Dry years in south-east England since 1698. Nature, vol. 265, p. 431-434. WMO (1969-1979). WMO Bulletin, quarterly publication. July and October issues include a review of exceptional weather events of the previous year. WMO. 1977. Statistical information on activities in operational hydrology. Operational hydrology report no. 10, WMO no. 464. Geneva. Wright, P. B. 1976. Four mild winters in Europe. Weather, vol. 30, p. 125-126. Wright, P. B. 1977. Persistent weather patterns. Weather, vol. 32, p. 280-285. 139 7. Prospects for the limitation of the consequences of hydrological drought 7.1 GENERAL Wherever they occur droughts have catastrophic consequences: they may even cause a breakdown of normal commerce and social practices and will certainly produce definite changes in the socio- economic organisation of affected populations. Even though it is no easy matter, everything that might obviate these consequences should be attempted. The first set of measures concerns the improvement of the availability of water during the drought by artificial modification of surface water flows (Section 7.21, by a greater or a better utilisation of groundwater (Section 7.31, or by a combination of the two. The reduction of losses is also important: reduction of losses in the irrigation system or reticulation network of domestic water supply; reduction of evaporation from free water surfaces (Section 7.4); and lastly the artificial inducement of precipitation (Section 7.5). Many of these measures require international effort: for instance the integrated management of large Basins, or aquifers which straddle national boundaries. Of equal importance is the international effort needed for the selection and marketing of agricultural products. One should also stress the international role of transmitting experience and technology from one country to another; from developed country to developing country and also between developing countries facing similar problems. Unhappily it has been found impossible to meet water needs during the more severe droughts by the above technical means. For instance, the use of arid zone pastures by some nomadic peoples had reached some sort of perfection, balancing precisely needs and availability. But even with the assistance of the best modern pasture management and with adequate permanent wells or small farm dams (impossible in many cases), most parts of the Sahel are not capable of supporting as many cattle in a drought as during wet years. For many rivers, even allowing for the maximum degree of regulation, it would be impossible to provide enough water for- irrigated agriculture at the scale required to feed the growing population numbers. Measures other than the purely technical should be taken involving: food storage during wet years, insurance, exchange of products between countries, agistment of stock, and lastly - but not a desirable solution - employment of the affected manpower outside the region or country. This last-named solution is best avoided as it raises even more difficult problems of international co-operation than the technical measures, but nevertheless is practised in Senegal, Mali and Upper Volta. 7.2 SURFACE WATER MANAGEMENT Seed germination and crop growth are affected by rainfall deficit, but the needs of cattle and other stock and the sprinkler or drip irrigation of crops are often dependent upon surface water. Furthermore, during a drought irrigation takes on an importance in countries where in more normal times it is not a generally applied practice. Hydrological drought also affects urban and rural water supply, energy production (directly in the case of hydropower but indirectly by restricting cooling water supplies for other types of power stations) and navigation. Even in normal conditions some regulation of river flows is needed using reservoirs. Such regulation is even more important when coping with droughts, for which even larger reservoirs would be required in order to store the excess flows from wet years. But this would require very large reservoirs, which in the arid and semi-arid zones lose a great deal of their stored water through evaporation. Section 7.4 discusses this particular problem. An important problem in the arid and semi-arid regions is the provision of drinking water 141 for stock. A large number of drinking points are needed; too low a density is instrumental in creating desertification by overgrazing, soil compression and other effects when too many cattle are concentrated around the same pond or well. It is also undesirable for watering points to be far removed from the good pasture. In practice the natural variability of geological conditions force a combined use of groundwater and surface water sources for stock watering needs. Natural depressions suitable for drinking ponds are typically very shallow in these regions and 95 per cent of the water is lost by evaporation. One solution lies in increasing the depth of water by creating small deep artificial reservoirs within the depression: this has been done very cheaply in the Sudan where they are called 'hafir'. An extension of this practice would be required in order to conserve water in depressions, concentrating first on those which receive runoff from their surrounding catchment even during drought periods. A second solution for these same arid and semi-arid areas is to construct small reservoirs in the upper reaches of intermittent streams. But for these reservoirs to operate successfully during droughts they have to be established in areas where the rainfall runoff ratio is high enough, say 15 per cent to 20 per cent. This presupposes a good knowledge of the hydrology of the area. A variation on this scheme is to use covered cisterns supplied by natural or artificial impermeable catchments (including their own rooves). It is very understandable how, in many cases, the use of surface water supplies is not possible during severe drought so it is necessary to make use of such groundwater resources that are available. Section 7.3 discusses the joint use of water from surface and groundwater sources. Because of the larger quantities involved, the needs of irrigation are much more difficult to satisfy than stock drinking water. In tropical dry, semi-arid and arid regions the large rivers which derive their water from wetter parts deliver an important volume of runoff to the affected area even when that runoff has itself been depleted by the drought. In any strategy for combating the drought, first priority ought to be given to upgrading the management and operation of these large rivers without, at the same time, neglecting the smaller scale solutions mentioned above. But in reality, it may often not be possible to provide the irrigation water that the minor water-courses normally support at a sufficient level. So in this case also it seems necessary to introduce groundwater in association with the traditional surface supply. It is always necessary to bear in mind that an invariable consequence of reservoir or pond development is an increase of loss by free water evaporation. The general strategy for regional or basin water management must allow for this fact and this should lead to the realisation of an integrated management and operation of all water resources. These comments have largely been directed to solutions in drier parts of the world although the more general strictures apply equally to the temperate zone. Water authorities, in the recent droughts that occurred in Europe and USA, were able to cope by restricting access to the water supplies by users either through legislation or by encouraging voluntary water , conservation practices at the domestic or industry level. A ban on the use of hosepipes for garden watering seems to be a universal early move in the struggle against drought. Such a hierarchy of strategies has been fully documented for the 1975/76 U.K. drought in IWES/ICE (1977). 7.3 GROUN'DWATER MANAGEMENT 7.3.1 The need to avoid overexploitation Groundwater is less sensitive to evaporative loss than surface water and in the case of deep groundwater is entirely unaffected. In arid countries it is often the sole large-scale source of water. During hydrological drought the use of groundwater is general in all countries, arid or not. Too large a depletion of the aquifer, or its total exhaustion, before the end of the drought must be avoided. In countries at all stages of development aquifer levels are being lowered to dangerous levels causing land subsidence, saline intrusion and degradation of the fissure network. Many arid countries exhaust their small aquifers during an event as frequent as the five or ten year return period drought. If the volume of available fresh groundwater is larger than the volume used during a very dry year, or if the recharge is considerable even during a drought, then there is no problem with the use of groundwater and it is permissible to extract in a single year more water than is recharged. But care should be taken not to extract the same volume in the following or subsequent years. This sort of restriction is difficult to obtain because farmers are tempted to proceed in the same manner every year once the precedent has been set. The struggle against over-exploitation is part of the struggle against drought. 142 7.3.2 Groundwater source development The further development or optimal managenient of the subsurface water resources requires two priority and complementary action points: a. quantification of existing water resources. b. optimisation of water use. Some specific aspects of the problem of evaluating the size of the water resource follow. The input to the aquifer is randomly distributed in space and time and is tied to the variation in effective rainfall and the permeability of river beds. The replenishment of aquifers in the arid and semi-arid zones occurs largely as percolation through the beds of oasis, intermittent streams, alluyial fans, shallow depressions and spreading in endorheic basins. In consequence, the calculation of effective rainfall as normally understood does not have any significance. One method of calculation is to deconvolve the piezometric level from the input, either effective rainfall or flood water quantities. The study of the weak replenishment of deep aquifers is based on the use of distributed mathematical models. The exploitation policy rests on the prediction of the decline of the piezometric surface as a function of abstracted quantity. It is not possible to separate the management of surface and soil water from that of groundwater as it is the former which goes to recharge shallow and deep groundwater. Possibilities exist for the artificial augmentation of this recharge (see Section 7.3.3). The second prerequisite of resource development is the optimisation of water use for agriculture. Some pitfalls have already been mentioned in Section 7.3.1. Other aspects which relate to irrigated agriculture are: a. correct choice of irrigation technique, e.g. sprinkler or drip feed, based upon availability of equipment, suitability of soil and water economy. b. correct determination of applied quantities to maintain plant growth, achieve water economy and to avoid any increase of soil or groundwater salinity. Studies are carried out in pilot areas to determine the above criteria and the plant water relationships for given crops. Other studies that need to be carried out concern agro-economics and are aimed at producing planning maps of water resources. The agro-economic studies are to compare species for their adaptability to local conditions, to study crop rotation, to investigate marketing aspects and national needs, and to investigate dry and wet period agricultural practices. Planning maps are needed which show the productivity of wells, the cost of obtaining underground water and the position of exploitable underground reserves. The water resource maps are used in conjunction with the agro-economic results to form rational management policies. 7.3.3 Possibilities 'for augmenting aquifer recharge and yield The artificial augmentation of aquifer recharge can take two forms: a. increasing the area covered by the floods in arid countries and also increasing the duration of the submersion using a small dam and diversion channel ; b. directing the water to wells, or to a locality of very high infiltration capacity. The second method carries the danger of clogging the infiltration zone or the well so that solid material has to be filtered or allowed to settle out before use. Sometimes these artificial recharge methods can be combined with a medium sized reservoir scheme which is used at the same time for regulating stream flow and producing clean water for infiltration. Aquifer recharge and pumping efficiency can be increased by controlled underground explosion to shatter the rock material and so increase transmissivity and storativity. Another method is the driving of adits from the main well, again with the object of increasing the yield. 7.3.4 Groundwater quality problems Up to this point the water has been supposed to be of acceptable quality for man, stock and irrigation. The chemical composition of dissolved matter in ground (or surface) waters may be such that their use for one or more of these purposes is difficult or impossible. Slightly saline water may be used for irrigation but only for certain resistant crops and if the nature of the soil permits it. If a variety of water qualities are found then they may be mixed together to form an 143 acceptable mixture for all purposes. Typically, this is done where the groundwater salinity is not too high and there is a plentiful supply of fresh surface water to dilute the salt. Irrigated agriculture can itself be responsible for flushing out salts and alkaline substances which are latent within the soils and rocks and these then find their way to the groundwater as well as causing structural change within the soil and crop damage. 7.3.5 General concluding remarks The lesson of this and the previous section on surface water management must be that the mitigation of drought impact is not a matter of finding an individual source and exploiting that. Indeed, the problem is broader even than hydrology. It may be that, even after taking all the measures suggested in the two sections and co-ordinating them all perfectly, the total amount of water available might still not be sufficient to meet the urgent need within a severe drought. In such ca'ses, which are the rule in semi-arid and arid countries, an integrated approach embracing food, social, political, land and agricultural management is necessary (Tixeront, 1979). 7.4 REDUCTION OF EVAPORATION The loss of stored water by evaporation from the water surface is an important consideration in the design of drought alleviation projects, and can indeed make the difference between a viable and an unworkable scheme. Methods of inhibiting or retarding evaporation have been under investigation for many years but there are few examples of successful operational systems. The ideas that have been promoted are windbreaks, monomolecular films, and floating covers of various types. The details of the properties of each method are taken from an article which appeared in the WMO Bulletin (1974) which was in turn based on a report of C. E. Hounam. Experience with floating particles has not been satisfactory because,when the wind velocity exceeds 10 km/h,the reduction of evaporation is sharply reduced. Monomolecular layers are formed on the surface of water by certain materials, in particular long-chain fatty alcohols. Any such material must be non-toxic, relatively impermeable to water vapour, and be 'self repairing' when broken by wind action or other cause. Cetyl alcohol and octadecanol are the most frequently used alcohols because they fulfil the named conditions and are relatively cheap. High winds disturb the monolayer and may carry it off. The reduction in efficiency is in approximate inverse proportion to the square of the windspeed. Floating granules suffer from the same defect. It is advantageous to use windbreaks in conjunction with the monolayer. The shelter that is provided reduces evaporation and increases the effective life of the monolayer. Air temperature affects the stability of the layer through the spreading rate and biological decay rate. It should be remembered that evaporation acts to cool the remaining water and the rise in temperature that is consequential to evaporation reduction may have adverse effects. While it is relatively straightforward to monitor the effectiveness of evaporation retardants in enclosed tanks, the field evaluation over natural or artificial reservoirs can be very difficult. This is because the evaporation reduction is of the same order of accuracy as the measurement of some of the elements of the heat and water budget. However, it is found that a 30 per cent reduction in evaporation can be achieved under good conditions. The state of the art of evaporation reduction is that knowledge is still incomplete, especially of the efficacy of the various measures in the arid regions of the worlds. 7.5 ARTIFICIAL ENHANCEMENT OF PRECIPITATION Ice crystals can be formed within clouds under suitable conditions if they are seeded with a nucleant. The cbmmonest nucleant is silver iodide but dry ice and sodium chloride are also used, the latter over a period of several years in West Africa and India. However, the next stage in the process of rainfall formation is very complicated although an encouraging beginning has been made to the understanding of the dynamic and microphysical processes within a cloud. Much the greater research effort has been put into field trials of cloud seeding using ground measurement to substantiate (or otherwise) the modification that is achieved. Despite the many positive claims that have been made for the efficacy of cloud seeding, it must be said that it still remains in the research stage. The difficulty of arranging sufficiently sensitive statistical tests, the effects of geographical location, cloud droplet size distributions, ice crystal properties and concentrations, all contribute to important remaining questions. It is indeed a possibility that even if it is proved that rainfall is enhanced by seeding clouds over an area (and to hit the target area with some precision) other areas downwind may have less chance of precipitation than if there had been no cloud seeding. With regard to the possibility that cloud seeding may assist in drought alleviation 144 Herschfield (1966) makes the following points. Low rainfall areas are also areas of high variability and low frequency of conditions favourable to rainfall production as indexed by the number of rain days. This is an unfortunate circumstance for rain stimulation because the success of cloud seeding must inevitably depend on the presence of suitable clouds. During major dry spells such clouds are generally absent and so it is concluded that cloud seeding, even if it works, does not hold out much hope as a drought alleviation measure. Nevertheless, cloud seeding continues to be employed as a routine in many parts of the world and an important industry is employed in it. It is therefore important that legitimate doubts about its true efficacy are resolved and so basic physical research at the process level must continue hand in hand with extensive and fully controlled field trials. An example of the latter is the PEP experiment (precipitation enhancement project) of WMO (Kahan, 1977). This experiment is planned to last five years and cover areas of the order of 10,000 km2. One feature of this experiment is the proposal to use streamflow as an indicator of the effect of seeding. 7.6 LAND MANAGEMENT, LOGISTICAL AND SOCIAL MEASURES FOR MITIGATING DROUGHT CONSEQUENCES In Section 7.3.5 it was mentioned that it was desirable that an integrated approach was necessary to counter the effect of drought; throughout this report mention has been made of the social disruption caused by historic droughts and the modus vivendi achieved by certain communities faced with a variable and drought prone climate. In this section we bring these points together in a brief overview of non-hydraulic response to hydrological drought. Nomadic pastoralism has often been regarded as a maximal response to the problem of climatic variability and recurrent drought in particular. However, there is room for argument on this point and a case has been made that such practices have contributed to the degradation of arid and semi-arid areas. Whatever the truth may be, their way of life cannot be held up as a viable model for the future: land has been enclosed and pressures exist from mining, industry, tourism, the need for individual fulfilment of the population, but most importantly, the imperative need for food in increasing quantities; all cannot be reversed. In the developed world an important objective of drought planning is to diversify activities to include drought tolerant ones such as industry and tourism (Hill, 1979). In the less developed world the theory of integrated development of drought response fails to take into account the existing cultural and social environment. Inappropriate technology, human and stock disease, cultural rivalries, and inadequate infrastructure of administration and transport all stand as obstacles to the actual implementation of a sophisticated drought plan. Glantz (1977) in a wide-ranging paper makes the point that even if a six month forecast of weather were available for the Sahel few of the areas could have responded in any different way to that which actually happened. 7.7 REFERENCES TO CHAPTER 7 Glantz, M. 1977. The value of a long-range weather forecast for the West African Sahel. Bull. Amer. Met. Soc., vol. 58, no. 2, p. 150-158. Herschfield, D. M. 1966. A note on the variability of annual precipitation. Journ. Appl. -Met., vol. 1, p. 575-578. Hill, H. M. 1979. Drought mitigation in Canada's prairie provinces. Proc. Symp. Hydrological Aspects of Drought, New Delhi, 3-7 December 1979, p. 570-576. Institution of Water Engineers and Scientists/Institution of Civil Engineers. 1977. Proceedings of the one-day seminar on the operational aspects of the drought of 1975-76. London, 24 March 1977. Kahan, M. A. 1977. A review of the hydrological aspects of evaluation of precipitation enhancement. WMO PEP Report no. 44, Geneva, 15 p. La Documentation Francaise. 1975. La sécheresse en zone sahélienne. Causes. Conséquences. Etudes des mesures à prendre. Notes et études documentaires no. 4216-4217, Secrétariat Général du Gouvernement, Académie des Sciences d'outre-Mer. Tixeront, J. 1979. Necessity of treatment of drought problems on integrated basis. -Proc. Symp. Hydrological Aspects of Drought, New Delhi, 3-7 December 1979. WMO. 1974. Techniques for reducing evaporation from lakes and reservoirs. WMO Bulletin, vol. XXIII, no. 2, p. 119-123. 145 8. Recommendations 8.1 GENERAL RECOMMENDATIONS FOR RESEARCH 8.1.1 Introduction Research needs can be reviewed from three points of view; the scientific hydrologist is likely to subdivide aspects of drought along lines of scientific disciplines to identify gaps in knowledge in, say, mathematical descriptions of hydrological variables or inadequacies in understanding of aquifer behaviour. A second viewpoint is that of the water manager who is likely to see problems in terms of his own activities so the gaps that are identified concern the 'output' of the hydrological system rather than in processes. The final viewpoint is that of the water user who sees problems in terms of outputs from the total system, for example land management, industrial output, irrigation policy. A researcher may claim that there is a large gap in ow understanding of the spatial coverage of droughts and wish to institute research to improve prediction of drought probability in different parts of a region. However, the water manager will be more sympathetic to research which addresses explicitly the joint operation of sources and supplies and discusses, perhaps, the probability of failure of the water supply system. The water user will focus on institutional and market mechanisms which will allow affected individuals to cope with an extensive drought with as little disruption as possible. In practice, the manager and the user do not perform the research so it is up to the scientific hydrologist to translate their needs into his own terms and represent the results in a comprehensible manner. More importantly, he sets the results in the context of the manager and users' perceptions of the particular problem. The following section follows the traditional discipline oriented approach but endeavours to relate the suggestions to user needs. 8.1.2 Research into drought indices The latter part of Chapter 1 and also Sections 4.1 and 4.3 in essence address the problem of how drought may be summqrised and quantified in order to make it amenable to analysis. This is a central problem to all drought forecasting and mitigation measures. Numerical indexing of drought may be totally hydrograph or aquifer level based such as the listed types of Section 1.2.3 and these ideas provide a relatively simple avenue to understanding drought of great appeal to hydrological analysts. However, water managers and users see a more complex problem and would prefer to make decisions on a broader based definition of drought which incorporates features of their own operations. There has been no theoretical work done on index construction for this purpose but like all indices it would be either a simple 0,l switch indicating whether at any given time a given set of desirable conditions apply, or else a value on a scale which encapsulates hydrological and other pertinent factors. When the index falls below some particular threshold then a particular action is called for. In other fields such indices tend to be economic as only when money values are assigned can different factors be reduced to a common scale. In any event, there is a requirement for several types of index corresponding tp the various categories of users and climatic zones. 8.1.3 Droughts in time and space A drought would not be perceived as such unless the deficit was maintained over some considerable duration, and cover a considerable area. Despite much research into the temporal aspects of drought there remain many unanswered questions and conflicting observations. Section 2.2 points to the large scope for further work in time series modelling of 147 hydrological series. High priority needs to be given to models of time series behaviour which reflect the tendency for dry to follow dry with relatively higher probability than for other transitions. The spatial aspects of drought, especially its quantification, have been little studied (Section 2.3) although information on coverage and shape of afflicted areas would be of enormous benefit in planning. The possibility of time trends, fluctuations, and cyclicities in the mechanisms giving rise to drought are of obvious interest to drought studies. Despite the enormous literature (see Section 2) much is of indifferent quality, but nevertheless it is important for hydrologists to remain conversant with the climate change literature. Research into the response of the hydrological cycle to hypothetical changes in climate inputs is of obvious importance. An area that is missing in most studies of change is the effect on variability as opposed to the effect of the change on the mean value. The 'reason why this gap should be filled is that drought is, and will remain, an outlier from the mean value. Further historical surveys should be very illuminating. The World Climate Programme provides an excellent focus for research in this area and all effort should be made to ensure that due attention is given to the hydrological impacts. 8.1.4 Drought mechanisms The cause of drought in climate terms (Section 3.1) is a high priority area for climatological work. The hydrological researcher should acquaint himself with advances in this field because of the avenues they suggest for the better forecasting of drought onset and continuation. Many of the suggested approaches of Section 3.3 are in need of further study. The teleconnection approach (Section 3.3.1.3) seems an attractive line of attack-as it is underpinned by a reasonable (if not yet proven) supposition about drought mechanism. Another possibility that was mentioned in Section 5.7 concesned the use of the hemispheric phase shift. Man's activities have the potential, possibly to alter the meteorological process, but can certainly have a drastic impact on the land phase of the hydrological cycle. The 1977 United Nations Nairobi Desertification Conference in Nairobi suggested many areas where our knowledge is deficient and field experimentation is highly desirable. Land management practices, although largely aimed at stabilising the land surface in time of flood, yield benefit also in time of drought (Section 7.6). The history of drought in a region is embodied in the landform, river and lake morphology, and climate characteristics. Studies of the type described in Section 4.6 will enable statistical statements to be made about frequency of low flows based upon mappable characteristics of the basin. Other techniques of climate reconstruction referred to in Section 4.2.2 are capable of much wider use than hitherto. 8.1.5 Drought surveys The preparation of this report has highlighted the difficulty of finding material on various aspects of a given historical drought. All planning is, to a large extent, based upon the experience of the past so the collation of information into a single set of documents would be of enormous benefit locally as well as to those making international comparisons. The report should include hydrological and climate data and analysis, plus information on the drought's impact on economic and social life in the affected region. The starting point of any drought survey must be the routinely collected data from the hydrometric networks supplemented by special purpose measurements and field surveys instituted during the drought. It is absolutely essential to continue to improve the basic networks and to build into the organisations the capacity to mount special surveys of river flows, aquifer levels, lake levels and soil and crop condition. Even in the droughts of the 1970s there were very many cases where gaps in the records have impeded the correct assessment of the drought severity. 8.1.6 Drought consequences The normal rules of water supply and use will not apply during times of drought. The response of the soil profile, aquifer, streams and human systems to the stress are all fruitful areas for future research. Much past effort has been put.into methods dealing with the physical aspects of the problem although much development work remains in applying those methods to specific locations. Human response to drought, the institutional needs and ways of presenting data and strictures to encourage the population to act so as to minimise its adverse affect, is a much less well explored area. Much of Part II of Yevjevich et al. (1977) is devoted to topics in this area and lists research needs. 148 8.2 SUûGESTIONS FOR INTERNATIONAL CO-OPERATION The construction of new and relevant drought indices is a new area (Section 8.1.2) and one where international agencies can offer a lead. The linkage between economist and hydrologist is not readily available but the appointment of a rapporteur who would review the related water resource literature and draw from experience outside hydrology if necessary should be considered. Many droughts affect regions which cross national boundaries and, like major aquifers and river basins, can only be successfully studied if an international view is adopted. The free availability of data is a prerequisite and an international agency can serve a vital role as a data holding organisation even if it does not conduct the analysis itself. It would be valuable in this connection and in the context of drought surveys (Section 8.1.5) for a model drought report to be constructed and all countries should be encouraged to adopt this as a standard. This would ensure that comparable data is collected and published. Both Unesco and the World Meteorological Organisation are well placed to ensure that hydrologists are made aware of relevant investigations in climate change and drought mechanism research. The World Climate Programme will hopefully influence the thrust of that research in order to provide answers that are relevant to drought hydrology. The possibilities of using sea surface temperature and other teleconnections for hydrological forecasting is one obvious area (Section 8.1.4), the need for information on variability trends and not just trends in the mean is another (Section 8.1.3). The international organisations are already well represented in co-ordinating efforts into the problems of land use change and practices on the hydrological response, although more effort is expended on average conditions rather than for the more defensive requirements of what occurs during time of drought. The hydrologist needs to be made more aware of the human aspect of drought management. There is perhaps insufficient linkage between the organisation concerned with scientific hydrology: IAHS, Unesco and WMO, and those concerned with specific water uses such as the International Water Supply Association, FAO, WHO, the International Commission for Irrigation and Drainage, UNEP, UNDRO and UNICEF. Joint working groups, judiciously led, may be very helpful in answering questions concerned with drought response which in turn feed back into the strict hydrological issues of drought definition and description. 8.3 REFERENCE TO CHAPTER 8 Yevjevich, V.; Hall, W. A.; Salas, J. D. 1977. Drought research needs. Water Resources Pubi., Fort Collins, USA, 276 p. 149 Titles in this series I. The use of analog and digital computers in hydrology. Proceedings of the Tucson Symposium, June 1966 /L’utilisation des calculatrices analogiques et des ordinateurs en hydrologie : Actes du colloque de Tucson, juin 1966. Vol. 1 et 2. Co-edition IASH-Unesco JCoédition AIHS-Unesco. 2. Water in the unsaturated zone. Proceedings of the Wageningen Symposium, August 1967 / L‘eau dans la zone non saturée: Actes dy symposium de Wageningen, août 1967. Edited by / Edité par P. E. Rijtema & H. Wassink. Vol. 1 et 2. Co-edition IASH-Unesco J Coédition AIHS-Unesco. 3. Floods and their computation. Proceedings of the Leningrad Symposium, August 1967 / Les crues et leur évaluation : Actes du colloque de Leningrad, août 1967. Vol. I et 2. Co-edition IASH-UnesccFWMO/Coédition AIHS-Unesco-OM. 4. Representative and experimental basins. An international guide for research and practice. Edited by C. Toebes and V. Ouryvaev. Published by Unesco. (Will also appear in Russian and Spanish) / bsbassins représentatifs et expérimentaux: Guide international des pratiques en matière de recherche. Publié sous la direction de C. Toebes et V. Ouryvaev. Publié pur l’Unesco. (A paraître également en espagnol et en russe). 5. Discharge of selected rivers of the world / Débit de certains cours d’eau du monde /Caudal de algunos rios del mundo / Pacxopbi BO&[ u36pan~bixpeK Mupa. hblished by Unescolfiblié par i’ünesco. Vol. I : General and regime characteristics of stations selected / Vol. I : Caractéristiques générales et caractéristiques du régime des stations choisies / Vol. I : Caracteristicas generales y caracteristicas del régimen de las estaciones seleccionadas / TOM I : 06uiue H pemmsie XapaKTepucruKH ~36pa1i~bixcmlmii. Vol. II : Monthly and annual discharges recorded at various selected stations (from start of observations up to 1964) /Vol. II : Débits mensuels et annuels enregistrés en diverses stations sélectionnées (de l’origine des obser- vations à l’année 1964) / Vol. II : Caudales mensuales y anuales registrados en diversas estaciones seleccionadas (desde el comienzo de las observaciones hasta el aRo 1964) / TOM11: MecnIimie H ronoebre pacxomi Bonbl, 3apernc~p~po~amsiepa3nHwaim U36paHHbIMU cTmlmnm (c uaqana ~a6moneeno 1964 rom). Vol. 111 : Mean monthly and extreme discharges (1965-1969)/ Vol. III : Débits mensuels moyens et débits extrémes (1965-1969) /Vol. 111 : Caudales mensuales medianos y caudales extremos (1965-1969) /TOM 111: Cpervie- MeCRWbIe U 3KCrpeMaJibHbie paCXOnb1 (1965- 1969 ri.). Vol. III (part II) : Mean monthly and extreme discharges (1969-1972) /Vol. 111 (partie II) : Débits mensuels moyens et débits extrémes (1969-1972) /Vol. 111 (parte II):Caudales mensuales medianos y caudales extremos (1969-1972)/ TOM111 (qacn il):Cpeme-Mecmsie H 3~c~pe~ansmiepacxow (1969-1972 rr.). Vol.lll (part Ill) : Mean monthly and extreme discharges (1972-1975) (English, French, Spanish, Russian). 6. list of International Hydrological Decade Stations of the world / Liste des stations de la Décennie hydrologique inter- nationale existant dans le monde / Lista de las estaciones del Decenio Hidrologico Intemacional del mundo / CmcOK CTUIIJJ~~ MemqyHapomoro rwponormecKoro necnmnemn ~~MHOTOuiapa. Published by Unesco /hblié prl’Unesco. 7. Ground-water studies. An international guide for practice. Edited by R. Brown, J. Ineson. V. Konoplyantzev and V. Kovalevski. (4 supplements published). 8. Land subsidence. Roceedings ot the Tokyo Sy~iiposium,September 1969 /Affaissement du sol : Actes du colloque de Tokyo, septembre 1969. Vol. 1 et 2. Co-edition IASH-Unesco f Coédition AIHS-Unesco. 9. Hydrology of deltas. Proceedings of the Bucharest Symposium, May 1969 / Hydrologie des deltas : Actes du colloque de Bucarest, mai 1969. Vol. 1 et 2. Co-edition IASH-Unescol Coédition AIHS-Unesco. Io. Status and trendS.of research in hydrology / Bilan et tendances de la recherche en hydrologie. Published by Unesco/ Publié prl’Unesco. II. World water balance. Proceedings of the Reading Symposium, July 1970 /Bilan hydrique mondial : Actes du colloque de Reading, juillet 1970. Vol. 1-3. Co-edition IAHS-Unesco- WMO J Coédition AIHS-Unesco-OMM. 12. Research on representative and experimental basins. Proceedings of the Wellington (New Zealand) Symposium, December 1970/ Recherches sur les bassins représentatifs et expérimentaux : Actes du colloque de Wellington (N.Z.), décembre 1970. Co-edition IASH-Unesco f Coédition A IHS-Unesco. 13. Hydrometry : Proceedings of the Koblenz Symposium, September 1970/ Hydrométrie : Actes du colloque de Coblence, septembre 1970. Co-edition IAHS-Unesco- WMO J Coédition AIHS-Unesco-OMM, 14. Hydrologic information systems. Co-edition Unesco-WMO. 15. Mathematical models in hydrology : Proceedings of the Warsaw Symposium, July 1971 / Les modèles mathématiques en hydrologie : Actes du colloque de Varsovie, juillet 1971. Vol. 1-3. Co-edition IAHS-Unesco-WMO. 16. Design of water resources projects with inadequate data : Proceedings of the Madrid Symposium, June 1973 / Elaboration dës projets d’utilisation des ressources en eau sans données suffisantes : Actes du colloque de Madrid, juin 1973. Vol. 1-3. Co-edition IAHS- Unesco- WMO J Coédition AIHS-UnescdM. 17. Methods for water balance computations. An international guide for research and practice. 18. Hydrological effects of urbanization. Report of the Sub-group on the Effects of Urbanization on the Hydrological Environment. 19. Hydrology of marsh-ridden areas. Proceedings of the Minsk Symposium, June 1972. 20. Hydrological maps. Co-edition Unesco- WMO. 21. World catalogue of very large floods / Répertoire mondial des très fortes crues / Catilogo mundial de grandes crecidas / BCeMHpHbIfi KaïaJïOr 6onsm HaBOJlKOB. 22. Floodflow computation. Methods compiled from world experience. 23. Guidebook on water quality surveys. 24. Effects of urbanization and industrialization on the hydrological regime and on water quaky. Proceedings of the Amsterdam Symposium, October 1977, convened by Unesco and organized by Unesco and the Netherlands National Committee for the IHP in co-opration with iAHS/ Effets de l’urbanisation et de l’industrialisation sur le régime hydrologique et sur fa qualité de l’eau. Actes du colloque d’Amsterdam, octobre 1977. convoqué par l’Unesco et orpnisé par l’Unesco et le Comité national des Pays-Bas pour le PHI en coopération avec I’AISH. 25. World water balance and water resources of the earth. 26. Impact of urbanization and industrialiZation on water resources planning and management. 27. Socieeconomic aspects of urban hydrology. 28. Casebook of methods of computation of quantitative changes in the hydrological regime of river basins due to human activities. 29. Surface water and groundwater interaction. 30. Aquifer contamination and protection. 31. Methods of computation of the water balance of large lakes and reservoirs. Vol. I : Methodology. Vol. II : Case studies. 32. Application of results from representative and experimental basins. 33. Groundwater in hard rocks. 1 34. Groundwater Models. Vol. I : Concepts, problems and methods of analysis with examples of their application. 35. Sedimentation Roblems in River Basins. 36. Methods of computation of low stream flow. 37. Proceedings of the Leningrad Symposium on specific aspects of hydrological computations for water projecis(Kussian). 38. Methods of hydrological computations for water projects. 39. Hydrological aspects of drought. 40. Guidebook to studies of land subsidence due to groundwater withdrawal. 41. Guide to the hydrology of carbonate rocks. SC.84/XV-39/ A