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lit Hydrological data UK b• • •e> -LF1/4- — ir 4 , / -;? - The 1988-92 Drought INSTITUTE OF HYDROLOGY • BRITISH GEOLOGICAL SURVEY á THE 1988-92 DROUGHT An occasional report in the Hydrological data: UK series which reviews the drought within a hydrological and water resources framework á THE 198842 DROUGHT bY T.J. Marsh, R.A. Monkhouse, N.W. Arisen, MS Lees fl MS. Reynard with additional contributions from S.C. Loader, S. Green & P. Doorgakant Financial assistance towards the preparation and production of this publication was provided by: The Department of the Environment The National Rivers Authority Institute of Hydrology British Geological Survey 1994 Natural Environment Research Council Published by the Institute of Hydrology, Wallingford, Oxon OX10 8BB ISBN 0948540 59 1 Editor Hydrological data UK series: T.J. Marsh Printed and bound by Bourne Press lid. on chlorine free environmentally friendly paper.. Graphics: J. J. Carr Cover: The River Kennet near Avebury Photograph: Neil Campbell-Sharp FOREWORD Theclimate of the United Kingdom is noted for its short-term variability; sustained periods of very wet or very dry weather are relatively rare. Human and animal communities tend to adjust to the climate's capricious nature and only when the normal range of variation is exceeded does any real threat to economic activity or the aquatic environment become a possibility. Droughts in the UK do not pose the very real threat to lives and livelihoods that persistent rainfall deficiencies do in many parts of the world. Nonetheless, fuelled in part by speculation concerning the effect of global warming on United Kingdom rainfall patterns, scientific, med ia and public interest in the 1988-92 drought and its effects remained at a high level in England for much of the recent past. Society now places a higher premium on the amenity and environmental benefits of rivers and wetlands whilst continuing to query the cost and justification for the range of water conservation measures developed to mitigate the impact of extended periods of rainfall deficiency. The recent drought prxMded a reminder of the conflicting demands on the water industry and the vulnerability of the UK to unusual climate conditions. It also demonstrated the ongoing need to develop improved water management practices to withstand the twin stresses imposed by increasing water demand and lengthy periods of low rainfall. The drought which, at one time or another, embraced much of Europe can be traced back to the spring of 1988 in much of the Eng lish lowlands. It was punctuated by a number of wet interludes but by early-1992 had become exceptionally protracted and, in groundwater terms, more severe than any this century. The 1988-92 period stimulated reviews of water management policies in a number of countries at a time when the search for practica I and scientifically-based sustainable development options is intensifying. Fortunately, the drought helped to provide many useful insights into both the scale and scope of the water resources and environmental problems caused by long-term rainfall deficiencies and the strategies needed to combat them. This, of course, wil I be of particular significance if the extra-ordinary weather patterns recently experienced become a more familiar feature of our climate in the future. This review of the 1988-92 drought is the latest publication in the Hydrological data UK series. A principal function of the series is to disseminate information relating to contemporary hydrological conditions and to provide a perspective within which to examine the effects of exceptional weather patterns. Publications in the Hydrological data UK series are prepared under the aegis of a steering committee which includes representatives of Government departments, the National Rivers Authority, the Met. Office and the water industry in England, Wales, Scotland and Northern Ireland. Professor W. B. Wilkinson Director, Institute of Hydrology 106 National Rivers Authority region or River Purification Board boundary Hydrometric area number and boundary 64 34 54 32 63 CONTENTS INTRODUCTION 1 The 1988-92 drought report 1 What is a drought? 1 Data and information sources 2 Administrative framework for hydrometry in the UK water industry 2 Acknowledgements 2 THE 1988-92 UNITED KINGDOM DROUGHT IN SUMMARY 3 HYDROLOGICAL BACKGROUND 5 Geology 5 Hydrology 5 The international context 6 A REVIEW OF WEATHER PATTERNS OVER WESTERN EUROPE 1988-92 9 Background 9 1988-92 9 Convectional rainfall in Great Britain 11 Snow 11 RAINFALL 13 Background 13 Overture to the drought - the 1980s 13 Development of the drought 14 Termination of the drought 15 The spatial extent of the drought 15 The severity of the drought 17 A comparison with historical droughts 21 Drought indices 22 EVAPORATION AND SOIL MOISTURE DEFICITS 27 MORECS 27 Potential evaporation 1988-92 27 Actual evaporation 1988-92 27 Soil moisture deficits 30 The severity of the drought in soil moisture terms 30 Effective precipitation and infiltration 30 RUNOFF 33 Problems of low flow measurement 33 Runoff in the 1980s 33 The last 100 years 33 Development of the drought 34 Termination of the drought 35 Shrinkage in the drainage network 35 Severity of the drought 38 The re-definition of low flow regimes since 1988 41 Low flow return periods 41 GROUNDWATER 49 Background 49 Groundwater levels 1976-88 49 Development of the drought 49 Recharge during the drought 53 How severe was the drought? 54 RESOURCES AND WATER MANAGEMENT 59 Background 59 Reservoir contents 1988-92 59 Indexing drought severity 60 Water demand 60 Water management responses 61 EUROPEAN PERSPECTIVE 65 Introd uction 65 Rainfall 1988-92 65 River flows in continental Europe 1988-92 66 Development of the drought 67 Conclusion 69 REFERENCES 75 APPENDICES I Select bibliography of English droughts 77 II Publications in the Hydrological data UK series 79 INTRODUCTION The 1988-92 drought report associated with meteorological droughts - defined essentially on the basis of rainfall deficiency, The objective of this report is to provide hydrological droughts - where accumulated comprehensive documentation of the 1988-92 shortfalls in runoff and recharge are of primary drought within a hydrological framework and to importance, and agricultural droughts - where the establish a benchmark against which future periods availability of soil water through the growing season of severe rainfall deficiency may be compared. The is the critical factor. The impact on the community spatial and temporal variations in the drought's during most periods of large rainfall deficiency is intensity are examined and its severity assessed likely to be very unev en and dependent on a number within the perspective provid ed by long-term rainfall of features of the drought Hot weather and dry soils and hydrometric records. An introductory ma y g enerateheavy water demand in the spring, for hydrological overview of the United Kingdom is irrigation and garden watering in particular. This given to help pla cethe condi tions experiencedduring can overstretch the water distribution systems and 1988-92in a suitable context. The synoptic backcloth trigger hosepipe bans at a time when overall water to the drought's development is also reviewed and resources may be relatively healthy. Conversely a the European perspective is examined using selected wet summer, as in 1992,may suppress demand and rainfall and river flow records to index drought greatly moderate restrictions on water use when severity. Additionally, a short review of water resources are at historically very depressed levels. resource variability in Great Britain over the f eatured An additional factor is the public perception of five years - and the water industry's responseto the drought severity which may vary consid erably from actual and projected deficiencies - is included to help individual to individual. A very hot, dry summer, explain the often complex linkages between hydro- for instance, is likely to be viewed in a more relaxed logicalstressandwatersupplyirnpactsonthecommunity. manner by the holiday-maker than the farmer or For ref erencepurp osesa map is provid ed on page industrialist reliant on river abstractions, especially 73 to help locate the principal rivers, reservoirs and if reservoir or groundwater stocks are sufficient to monitoring sites mentioned in the report. provide continuity of domestic supplies. In such circumstances, the environmental stress resulting from drought conditions may be of considerably What is a drought? greater importance than the water supply impact. The water industry has developed a range of Droughts are multifaceted both in their character storage mechanisms and operational strategies, and range of impacts. Whilst in broad terms the linked to the probabilities of various drought concept of a drought is readily recognised by the intensities, in order to maintain water supplies. The public at large, translating this intuitive development of new gravity-fed or pumped storage understanding into an objective procedure for reservoirs together with the increased networking indexing or assessingdrought severity is far from of supply sources, often involving cross-basin straightforward. In part, this reflects the difficulties transfers or the further integration of surface and involved in quantifying a phenomenon which varies groundwater supply schemes in a regional grid, in its extent, duration and intensity both regionally provide a large measure of flexibility in combating and locally. Thus the 1975/ 76drought, for example, local or regional shortages.The complexity of water achieved a remarkable intensity in central southern resource systems and water utilisation patterns can England over a 16-month timespan13. By contrast, be such as to make the link between shortages of the 1984drought was largely restricted to the spring rainfall and water supply problems appear rather and summer and was most severe in the normally tenuous. This is particul arly true where su pply zones wetter northern and western parts of Britain3. Both relying on surface water sources and groundwater these droughts featured a sequenceof dry months sourcesare closely juxtaposed.