6k Institute of Hydrology Report No. 108 Low flow estimation in the United Kingdom .~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ l~~~~~~~~~~~~~~~aua E*rum.. U1c oni Report No. 108 Low flow estimation in the United Kingdom A. Gustard, A. Bullock and J. M. Dixon December 1992 Institute of Hydrology Crowmarsh Gifford Wallingford Oxfordshire OXIO 8BB United Kingdom © Copyright Institute of Hydrology 1992 ISBN 0 948540 45 1 Printed in the United Kingdom by Hobbs the Printers Ltd. Totton. Hampshire 5040 3WX IH Report No. 108 Published by the Institute of Hydrology December 1992 A catalogue record for this book is available from the British Library Preface This report describes the techniques for calculating low flow statistics and catchment characteristics at gauging stations, and the development of methodologies for estimating low flow statistics at ungauged sites. The primary objective of the report is to provide national design techniques for estimating low river flow statistics at ungauged sites. The commissioned programme of work contained three principal areas of development: 1. To update low flow estimation procedures to benefit from extended data series and to include recent extreme events; 2. To analyse flow records in a manner that produces results which can be used by the water industry of the United Kingdom; 3. To investigate the feasibility of estimating low flow measures for a standard long period. The work was commissioned by the Department of the Environment through Mr. C.E. Wright of the Water Directorate under research contract No. PE.CD/I7I210, which also includes IH Report No. 107, 'Impact of climatic variability and change on river flow regimes in the UK', IH Report No. 115, 'Instream flow requirements of aquatic ecology in two British rivers' and reports to DoE describing the development of the 1989 drought. The opinions expressed are those of the authors and are not necessarily those of the Department. In parallel with the development of methodologies for estimating low flow statistics funded by the Department of the Environment, the Institute of Hydrology has developed the MICRO LOW FLOWS software package for the automatic calculation of catchment characteristics and application of the low flow estimation methodologies. Acknowledgements The authors are grateful for the considerable support given to the project by the National Rivers Authority in England and Wales, the Purification Boards in Scotland, and the Department of the Environment (Northern Ireland), both for the advice on the quality of the records and for providing the flow data without which this study would not have been possible. Additional material has been provided by various research bodies and public undertakings. We would also like to thank the National Water Archive based at the Institute for providing access to the streamflow data. The HOST (Hydrology of Soil Types) project provided a key data base for regionalising low flows and the authors would like to acknowledge the project group of David Boorman (Institute of Hydrology), John Hollis (Soil Survey and Land Resources Centre), Alan Lilly (Macaulay Land Use Research Institute) and Jim Cruickshank (Department of Agriculture Northern Ireland). Rob Brown who assisted with the data assessment and Ros Gross, who assisted with regionalising low flow frequency curves, also provided an important contribution to the project. Finally, we would like to thank Colin Wright from the Department of Environment whose interest and advice at various stages of the project is much appreciated. Executive Summary This report contains procedures for the calculation of low flow measures from observed data and for their estimation at ungauged locations throughout the United Kingdom and is a successor to the Low Flow Studies report (Institute of Hydrology, 1980). There are four principal improvements in the hydrological data base available to this report which benefit the updated estimation procedures; (1) more gauging stations; (2) most gauging stations possess extended data series; (3) the extended data series include recent extreme drought events, in particular 1975/76, 1984 and 1989; (4) data from Northern Ireland are analysed. All time series of daily river flow data on the National River Flow Archive have been graded in relation to their hydrometric quality and degree of artificial influence at low flows (Chapter 2). All grades have been verified by measuring authorities in the United Kingdom. Low flow grades and periods of record usable for low flow analysis are presented in Appendix 1. Methods for the calculation of low flow measures from gauged flow data are described, particularly the mean flow, Base Flow Index, flow duration curve and low flow frequency. The methods described have been applied to all gauging stations with usable periods of data and calculated values of low flow statistics and catchment characteristics (Chapter 3) are presented in Appendices 2 and 3 respectively. Relationships are developed (Chapter 4) between catchment characteristics and flow statistics, specifically the mean flow and two key low flow statistics, Q95(1), the 95 percentile from the 1 day flow duration curve, and MAM(7), the mean annual 7 day minimum, representing the flow duration and low flow frequency measures respectively. This work has been significantly advanced by improvements in digital cartography enabling automatic derivation of catchment characteristics by overlaying digitised catchment boundaries on thematic data bases. The use of a provisional HOST (Hydrology of Soil Types) classification is a principal development within this report, replacing the prior requirement to define local relationships between catchment geology and the Base Flow Index. Twelve different Low Flow HOST Groups have been identified from the 29 HOST units with the addition of URBAN and LAKE. The proportion of the catchment area in each of these Groups has been derived for 865 gauged catchments in the United Kingdom and related to Q95(1) and MAM(7). This has enabled parameter estimates to be derived for each statistic for the twelve Low Flow HOST Groups. The following table summarises this analysis. Low Flow Q95(1) MAM(7) HOST Gmups Parameter Parameter LFHGI 40.8 50.8 LFHG2 31.9 40.3 LFHG3 65.7 71.3 LFHG4 25.0 27.5 LFHGS 49.0 53.4 LFHG6 6.5 1.4 LFHG7 10.7 12.4 LFHGS 1.1 0.1 LFHG9 15.0 14.4 LFHG10 6.8 5.9 LFHG1I 29.4 33.8 LFHG12 65.1 49.6 The two key statistics can be calculated at ungauged sites by calculating the fractions of Low Flow Groups from the different soil association units shown on published maps and using weighted parameter estimates (Appendix 4). The development of intemal relationships between Q95(1) and other flow duration percentiles and between MAM(7) and other frequency statistics (Chapter 5) enable the construction of flow duration and low flow frequency curves at ungauged sites. Other internal relationships allow the construction of frequency curves of annual minima of different durations (Appendix 5). Procedures for the incorporation of local data are presented (Chapter 6) and the sequence of steps in the estimation procedure is summarised, with guidance according to data availability at, or adjacent to, the ungauged site (Chapter 7). The primary achievements of the project have been the assessment of hydrometric quality and degree of artificial influence at low flows of the UK flow archive, the application of advances in digital cartography to regional hydrology and the improvements in the ease and accuracy of low flow estimation at the ungauged site. This has been underpinned by the provisional HOST database which is a key variable in estimating low flows at ungauged sites. A number of recommendations for future research activities and hydrological data bases arising from this report are presented in Chapter 8. The most important of these is to update the estimation procedure when the final HOST data set is available, to explore the opportunities that further analysis at a finer spatial resolution of HOST will provide, to derive national standard period low flow statistics (Appendix 6), and to identify the most appropriate distribution and fitting techniques for single station and regional annual minimum distributions. Contents Page 1. INTRODUCTION 1 2. GAUGING STATION SELECTION 6 2.1 National low flow classification procedure 6 2.1.1 Hydrometric quality classification 6 2.1.2 Degree of artificial influences classification 7 2.1.3 Multiple grades at a gauging station 9 2.1.4 Results of national classification of low flow 10 data quality 2.1.5 Short-term anomalies at a gauging station 12 2.2 Comparison of stations used in 1980 and 1992 Low Flow 16 Studies 3. CALCULATION OF FLOW STATISTICS AND CATCHMENT 19 CHARACTERISTICS FOR GAUGING STATIONS 3.1 Flow statistics 19 3.1.1 Mean flow 20 3.1.2 Base Flow Index 20 3.1.3 Flow duration curve 25 3.1.4 Low flow frequency curve 27 3.2 Method of calculation of catchment characteristics 29 3.2.1 Catchment area 29 3.2.2 Standard period average annual rainfall 29 3.2.3 Average annual potential evaporation 30 3.2.4 Hydrology Of Soil Types (HOST) classification 30 3.2.5 FALAKE 34 3.3 Low Flow Study database 35 3.3.1 Number of stations and record length 35 3.3.2 Low flow statistics 35 3.3.3 Catchment characteristics 37 4. ESTIMATION OF MEAN FLOW, Q95(1) AND MAM(7) AT 39 UNGAUGED SITES 4.1 Mean flow 39 4.1.1 Catchment water balance 39 4.1.2 Regional regression equations 42 4.2 Flow duration curve: 95 percentile and mean annual 43 7 day minimum Page 5. ESTIMATION OF FLOW DURATION AND LOW FLOW 57 FREQUENCY CURVES AT UNGAUGED SITES 5.1 Flow duration curve 57 5.2 Flow frequency curve 58 5.2.1 Duration relationship 58 5.2.2 Frequency relationship 62 6.