Report No. 107 Impact of Climatic Variability and Change on River Flow
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Report No. 107 Impact of climatic variability and change on river flow regimes in the UK N.W. Arnell, R.P.C. Brown and N.S. Reynard December 1990 Institute of Hydrology Maclean Building Crowmarsh Gifford Wallingford Oxfordshire OX108BB United Kingdom e Copyright Institute of Hydrology 1990 ISBN 0 948540 26 5 IH Report No. 107 Published by the Institute of Hydrology December 1990 British Library Cataloguing in Publication Data Arnell, N.W. (Nigel W) Impact of climatic variability and change on river flow regimes in the U.K. 1. Great Britain Rivers I. Title 11. Series III. Brown, R. P. C. (Robert P. C.) IV. Reynard, N. S. (Nick S.) 551.4830941 ISBN 0-948540 26 5 Abstract The objectives of this report are to examine past variability in river flow regimes, concentrating in particular on recent years, and to consider the possible consequences of future climate change for river flow regimes in the United Kingdom. The geological and climatic characteristics of a catchment determine how variations in rainfall from year to year impact upon river flow variability. In general terms, the drier the catchment (as indexed by the proportion of rainfall which runs off from a basin) and the lower the base flow component, the greater the variation in flow regime between years. There is some evidence that years containing similar hydrological characteristics tend to cluster: wet winters tend to follow wet winters, for example. There is no conclusive evidence, however, that 'recent' years (excluding 1989 and 1990) have seen an unusually large number of extreme events, although the test used is rather conservative and the period defined as 'recent' influences the results. Annual and seasonal runoff totals during the 1980s were generally higher than in previous decades, and there are some indications that year-to-year variability was also higher. Data from 1989 and 1990 were not included in the analysis. Future changes in UK flow regimes depend very significantly on assumed changes in evapotranspiration and, particularly, precipitation, which are currently very difficult to predict. The study therefore examined the sensitivity of river flow regimes to a range of feasible climate change scenarios, biased towards generally wetter conditions but assuming both wetter and drier summers. Simple regression-type relationships were considered, but most of the analyses used monthly water balance models applied at a range of representative sites. Changes in average annual runoff under a given climate change scenario were found to depend strongly on catchment dryness. If potential evapotranspiration is assumed to remain constant, for example, a 10% increase in annual rainfall could produce up to 30% extra runoff in south east England, whilst in more humid western regions it would result in only an additional 12 to 15%. Increases in average annual rainfall of between 8 and 10% would be required to offset the effects of 15% higher potential evapotranspiration. The effect of drier summers on summer river flows depends upon the current summer water balance and catchment geology: the greatest relative reductions are expected in responsive catchments which currently have a close balance between rainfall and potential evapotranspiration. In catchments with a large groundwater storage, delayed drainage of additional winter rainfall may mitigate the effects of drier, warmer summers. Acknowledgements This research project was jointly funded by the Department of the Environment Water Directorate, under contract PECD/717/210, and the Natural Environment Research Council. Contract PECD/7/7210 includes three reports: 'Impact of climatic variability and change on river flow regimes in the UK' - IH Report 107, 'Estimating low river flows in the UK' - IH Report 108, and 'Instream flow requirements of aquatic ecology in two British Rivers' - JH Report 115. The project has benefited directly from the involvement of the following current and past staff of the Institute of Hydrology: Max Beran, Dr Andrew Bullock, Janet Whiteley, Terry Marsh, Dr Graham Thomas, Stephen Heaney, David Kelly and Chris Nunwich. Dr P. D. Jones of the Climatic Research Unit, University of East Anglia, provided climate data and the reconstructed flow series used in some of the analyses. Contents Page 1. OBJECTIVES OF THE REPORT 1 1.1 The impact of climatic variability and change on 1 flow regimes 1.2 Project history 2 1.3 Structure of the report 2 2. RECENT VARIABILTY IN RIVER RUNOFF: 3 INTRODUCTION 2.1 Recent variability in flow regimes 3 2.2 River flow and climate data 3 2.2.1 River flow data 3 2.2.2 Climatological data 11 2.3 Flow indices 11 2.3.1 Daily data 12 2.3.2 Monthly data 12 2.4 Final comments 12 3. VARIATIONS OVER TIME IN RIVER RUNOFF 14 3.1 Introduction: objectives of chapter 14 3.2 Variability between years 14 3.3 Variations in variability between catchments 16 3.4 Distributions of events over time and space 25 3.5 Why does seasonal climate vary from year to year? 36 3.6 Summary 38 4. RIVER FLOW REGIMES IN RECENT YEARS 39 4.1 Introduction 39 4.2 Some notable recent events 40 4.3 The recent rate of occurrence of extreme events 43 4.3.1 Introduction 43 4.3.2 Reconstructed flow series 45 4.3.3 Recorded river flow data 45 4.3.4 Conclusions 49 4.4 The characteristics of flow regimes in recent years 49 4.4.1 Introduction 49 4.4.2 Changes in average seasonal and annual 53 runoff volumes 4.4.3 Changes in the variability in runoff between 55 years 4.5 Conclusions and implications 58 5. VARIABILITY IN RIVER FLOWS OVER TIME: A 60 SUMMARY 5.1 Patterns of variability 60 5.2 Implications of patterns in variability 61 6. FUTURE CHANGES IN RIVER FLOW REGIMES: 62 INTRODUCTION 6.1 The context 62 6.2 Some qualifications 63 6.3 Implications for hydrological processes and water 65 resources 7. CLIMATE CHANGE SCENARIOS 68 7.1 Scenarios for climate change studies 68 7.2 The development of climate change scenarios 68 7.2.1 Scenarios from climate models 69 7.2.2 Scenarios from past instrumental data 71 7.2.3 Palaeoclimatic analogues 71 7.2.4 Scenarios based on physical and statistical 72 reasoning 7.2.5 Spatial analogues 72 7.2.6 Arbitrary change 73 7.2.7 Combination or 'committee' scenarios 73 7.3 Climate change scenarios for the UK 73 7.3.1 Methods employed 73 7.3.2 Potential evapotranspiration 74 7.3.3 Precipitation 76 7.4 Summary and conclusions 79 8. ESTIMATING THE EFFECTS OF CLIMATE CHANGE 80 USING SIMPLE EMPIRICAL METHODS 8.1 Introduction and objectives 80 8.2 A review of other studies 80 8.3 Estimating average annual runoff 83 8.3.1 Introduction and methods 83 8.3.2 The models used 83 8.3.3. Sensitivity to changes in rainfall and potential 85 evapotranspiration 8.3.4 Spatial implications of climate change 86 8.4 Measures of flow regime 89 8.4.1 Introduction 89 8.4.2 Regional regressions 89 8.4.3 Spatial transfer of information 92 8.5 Summary and conclusions 96 9. IMPACT ASSESSMENT USING HYDROLOGICAL MODELS 99 9.1 Introduction 99 9.2 Background 99 9.3 Methods and models 100 9.4 Changes in average annual runoff 105 9.4.1 Effect of changes in total annual precipitation: 106 potential evapotranspiration constant 9.4.2 Effect of changes in potential evapotranspiration: 109 rainfall constant 9.4.3 The effect of the seasonal distribution of 110 changes in rainfall 9.4.4 Changes in annual runoff: a summary 113 9.5 Changes in flow regimes 116 9.5.1 Changes in monthly flow regimes 117 9.5.2 Changes in flow seasonality 120 9.5.3 Changes in the frequency of low flows 123 9.5.4 Changes in the rate of occurrence of extreme 125 low flows 9.5.5 Reservoir storage-yield relationships 125 9.6 Summary and conclusions: possible changes in flow 128 regimes in a warmer UK 9.6.1 Average annual runoff 129 9.6.2 Average monthly runoff 130 9.6.3 The frequency of low flows 133 9.6.4 Reservoir reliability 133 10. ESTIMATING FUTURE RIVER FLOWS FROM PAST 135 EXPERIENCE 10.1 Introduction 135 10.2 Using the past to define a possible future 135 10.3 Inferences from past events 139 10.4 Conclusion 39 11. CLIMATE CHANGE AND FLOW REGIMES IN THE UK: 141 A SUMMARY 11.1 Introduction 141 11.2 Methods used 141 11.3 Changes in flow regimes 142 11.4 Some tentative implications for water resources 143 12. SOME SUGGESTIONS FOR FUTURE RESEARCH 145 12.1 Introduction 145 12.2 Climatological inputs into hydrological studies 145 12.3 Studies of past and recent hydrological processes and 145 regimes 12.4 The impact of climate change on hydrological processes 146 and regimes 12.5 The impact of climate change on water resources 147 management REFERENCES 149 1. Objectives of the report 1.1 TILE IMPACT OF CLIMATIC VARIABIITY AND CHIANGE ON FLOW REGIMES The late 1980s and early 1990s have again demonstrated the notorious variability in the climate of the United Kingdom. The winter of 1988/89, for example, was exceptionally mild and the period from November to January was the driest over England and Wales for over 100 years. Only heavy rainfall in late spring prevented the summer of 1989 beginning with large-scale water deficits. However, this was followed by prolonged dry weather and drought, peaking in severity first in one part of the UK and then another. Water supply difficulties arose in the south east, south west, Yorkshire and some other parts of 'the UK.