ELEMENTS OF GEOGRAPHICAL HYDROLOGY Brian Knapp Leighton Park School, Reading London UNWIN HYMAN Boston Sydney Wellington © B.J.Knapp, 1979 This book is copyright under the Berne Convention. No reproduction without permission. All rights reserved. Published by the Academic Division of Unwin Hyman Ltd 15/17 Broadwick Street, London W1V 1FP, UK Unwin Hyman Inc., 8 Winchester Place, Winchester, Mass. 01890, USA Allen & Unwin (Australia) Ltd, 8 Napier Street, North Sydney, NSW 2060, Australia Allen & Unwin (New Zealand) Ltd in association with the Port Nicholson Press Ltd, Compusales Building, 75 Ghuznee Street, Wellington 1, New Zealand First published in 1979 Third impression 1989 This edition published in the Taylor & Francis e-Library, 2002. British Library Cataloguing in Publication Data Knapp, Brian John Elements of geographical hydrology 1. Hydrology I. Tit¢le 551.48 GB662.3 78–40373 ISBN 0-04-551030-X (Print Edition) ISBN 0-203-03894-0 Master e-book ISBN ISBN 0-203-19110-2 (Glassbook Format) PART TWO APPLICATIONS CONTENTS Chapter 3 SOIL DEVELOPMENT AND MANAGEMENT Preface page 4 Soils of temperate climates 43 Acknowledgements 4 The soil association 47 Soils of the humid tropics 49 PART ONE THEORY Soils of arid lands 50 Chapter 1 PROCESSES The role of hydrology in soil management 51 Introduction 5 Example problems 56 Precipitation 5 Chapter 4 SLOPE DEVELOPMENT Interception 7 Introduction 57 Evaporation and transpiration 8 Rapid mass movement processes 57 Infiltration, throughflow and overland flow 9 Surface-water erosion 62 Ground-water flow 12 Subsurface-water erosion 63 Streamflow 14 Hillslope models 64 The analysis of streamflow data 14 A detailed slope analysis 64 Chapter 2 CATCHMENT SYSTEMS An example for discussion 66 Introduction 17 Chapter 5 WATER RESOURCES Upland catchments 17 The scope of the problem 67 Lowland catchments 19 Water supply 67 The effect of permeable bedrock 21 Flood protection 70 Catchments of snow and ice 23 Water quality protection 71 Catchments of arid and semi-arid regions 27 A strategy for Britain 76 Humid tropical catchments 30 Conclusion 79 The influence of urban areas 31 Example problems 79 Larger catchments 34 Example problems 36 Glossary 81 Further reading 82 Index 83 TABLES 4.1 Denudation processes in the Kar- kevagge Valley, Sweden (1952– 1.1 Interception and evapotrans- 60) 63 piration losses for a mature spruce 4.2 Dissolved and solid transport of forest, Thetford, Norfolk (1975) page 7 major rivers 63 2.1 The effect of vegetation forms on 4.3 Material stability at Rockingham 65 snow accumulation 24 5.1 The quality of natural waters in 2.2 Hydrograph parameters for the England and Wales (1973) 73 Bray’s Bayou catchment, Hous- 5.2 River Tame water quality at Lea ton, Texas 34 Marston (in milligrams per litre) 74 2.3 Percentage contributions of the 5.3 River Tame water quality con- Thames and its tributaries at high sequent on a storm (4 November and low flow 38 1969) 74 2.4 Monthly average rainfall and 5.4 Public water demand in England runoff for the Thames catchment and Wales 76 above Teddington (1941–70) 38 5.5 Surface- and ground-water quan- 2.5 Unit response graphs for the River tities abstracted, 1974 (in millions Mole above Horley 40 of cubic metres) 76 PREFACE Although the science of hydrology has held a firm place the student will already have some basic knowledge and in higher education and industry for many years, it has will use the material included in this book to give a only recently found a formal place in the ‘A’ level hydrological slant to their further studies. syllabuses. With the upsurge of interest in water resources To help students consolidate their understanding of in particular, some aspects of hydrology have become more hydrology, there are selected problems at the end of most widely known. Nevertheless the central role that hydrology chapters. However, these problems are not repetitions of plays in a number of subjects, such as geomorphology examples in the chapters and are to be seen as integral and pedology, has been less effectively publicised. As a advances in each topic. The nature of the data is such result, the purpose of this book is not only to introduce that it may be analysed at a variety of levels. Students the theory of hydrology, but also to demonstrate its taking courses at more advanced levels can use the relevance in the real world by relying on detailed and examples with the confidence that they represent ‘the state wide-ranging specific examples. of the art’ in each topic. The book has been planned in the hope that it will I have been particularly conscious of the need to be available to students for the whole of their ‘A’ level provide up-to-date examples wherever possible and to courses. As such the first two chapters provide, as well reflect current thoughts in the subject. As in many as hydrological theory, an adjunct to a more conventional rapidly advancing disciplines, not everyone will agree geomorphology text. Chapters 3, 4 and 5 build on the on each point of detail and the responsibility for particular foundation of the early part of the book but are largely viewpoints, omissions and the arrangement of the work independent of one another and can be used in conjunction is my own. with books on pedology, slope formation and resource B.J.K. development. For these later chapters it is expected that Woodley 1978 ACKNOWLEDGEMENTS A work of this kind involves the active cooperation of Survey for Figures 2.20, 2.21, 2.29; Institute of British a large number of people in the supply of data and to Geographers for Figures 2.25, 4.13, 4.14, Table 4.3; them I would like to tender my thanks. However, I am Bundesanstalt für Gewasserkunde for Figures 2.28, 5.15; particularly grateful to Denys Brunsden, Richard Chorley, Elsevier Publishing Company for Figures 1.12, 2.5, 2.32, Ian Fenwick, Roger Jones, Mike Kirkby, Pam Wilson, Peter 2.33; I.M.Fenwick for Figures 3.4, 3.7; Soil Survey of Worsley; the surface hydrology section of the Thames England and Wales for Figures 3.9, 3.26; Soil Survey Water Authority, especially Stewart Child; the Library staff of Scotland for Figure 3.11; C.W. Mitchell for Figure of The Central Water Planning Unit; and the Institute 3.22; Cambridge U.P. for Figures 4.2, 4.3; Institution of Hydrology at Plynlimon for their advice and encouragement. of Civil Engineers for Figures 4.5, 4.7; Geografiska Finally I would like to thank the following for help in Annaller for Figure 4.9, Table 4.1; Arctic and Alpine critically reviewing the typescript: Frank Button, Ray Research for Figure 4.10; Central Water Plannning Unit Jessop, Christopher Rogers, Darrell Weyman, Graham Agnew, for Figure 5.3; Institution of Water Engineers for Figure Mrs M.Walls, Mr K.Briggs, Richard Huggett, John Rolfe, 5.5; Severn-Trent Water Authority for Figure 5.8, Tables Mr J.A.Williamson and Nigel Bates. 5.2, 5.3; V.Sagua for Figure 5.9; Water Pollution Control I would like to join with the publishers in acknowledging for Figures 5.13, 5.14; Crown copyright is reserved on permission given by the following to reproduce copyright Figures 3.8, 5.17, Tables 5.1, 5.5; Water Data Unit for material: Table 5.6; National Water Council for the extract on Edward Arnold for Figure 1.17; Institute of Hydrology p. 78; Loyd Martin for the data in Example Problem for Figures 2.2, 2.7; Canadian IHD Committee for Figures (D) on page 40; US Department of Agriculture for Figure 2.11, 2.12; Norges Vassdrags og Elektrisitetsvesen for 2.3; Elsevier Publishing Company and the Institute of Figure 2.15; P.Worsley for Figures 2.13, 4.1; Int. Assn Hydrology for Table 1.1. Sci. Hydrol. Bull. for Table 4.2, Figure 2.18; US Geological PART ONE THEORY Chapter 1 PROCESSES Introduction Precipitation More than two-thirds of the Earth’s surface is The land-based part of the hydrological cycle begins covered by water, yet less than 3% of it is available with a study of precipitation which, although largely on land at any one time to provide the means of within the province of meteorology and climatology, landscape evolution and support life. It has long is introduced here in order to explain many runoff been clear that the water which flows down rivers processes. to the sea must somehow return to the land for All precipitation results from atmospheric cooling these processes to continue and from this has evolved and subsequent condensation of water vapour. Usually the concept of the hydrological cycle. such cooling happens either at the frontal zones We can begin with the oceans, the great storage of depressions, or because of convection, or because elements of the cycle. Evaporation from them provides an air mass is forced to rise over an area of the moisture for cloud formation and hence precipitation. high land. In addition the amount and distribution Then, from the moment water reaches the land as of precipitation depends on the global atmospheric rain to the moment it returns to the sea, it is circulation. Some regions, such as the Sahara and engaged in a sort of obstacle course in which only Antarctica, experience very little precipitation as a certain percentage reaches its goal. Much is they are permanently under the influence of the returned direct to the atmosphere by evaporation, high pressure zones of the global circulation (Fig.1.2). while some is held in various temporary stores such Continental interiors such as the south-west of as soil or rock. Because of this it is often convenient the USA, and central Europe, which are away from to consider the land-based part of the hydrological the main areas of high pressure, often become cycle as a linked series of cascading reservoirs, very hot in summer and produce conditions just each with a limited storage capacity (Fig. 1.1). right for convectional instability. The resultant It is the role of the hydrologist to attempt to storms are important hydrologically because of understand each part of the hydrological cycle, but their very high intensity, localised occurrence and with an emphasis on those parts that are most short duration. More persistent areas of instability directly connected with the land. occur under the influence of the Intertropical Convergence 6 PROCESSES Figure 1.1 The land-based part of the hydrological cycle can be thought of as a series of storage units. As each store fills after rainfall, water cascades to the next unit, eventually reaching the sea. Figure 1.2 The general global circulation determines where rain will fall and its dominant form. INTERCEPTION 7 Zone (ITCZ) and convective storms become a daily phenomenon. In the middle latitudes precipitation mainly results from the passage of fronts. These provide a much more uniform regime than do convectional storms, with precipitation generally of low intensity and frequent occurrence. Nevertheless within frontal systems there are usually convective cells giving higher precipitation intensities and it is such variations that often make frontal storms so complex. Further precipitation in such regions is provided by small convective showers which form in unstable polar airstreams. The main effect of upland is to force an air mass upwards thereby promoting cooling and condensation, even without frontal or convective uplift, which increases the frequency of precipitation. But whether precipitation is caused by fronts, Figure 1.3 Rainfall interception on leaves. convection or relief, there is often a marked seasonal pattern in the regime and this gives river flow a are more or less immediately available. Rainfall is corresponding seasonality. Such effects are primarily measured in millimetres depth of accumulation in caused by a shift of atmospheric circulation patterns a specified time period. Snowfall is converted to on a global scale with the attitude of the sun as rainfall equivalent. it moves between the Tropics. Such seasonality is particularly pronounced in tropical areas where the Interception ‘wet’ season corresponds to the influence of the ITCZ and the ‘dry’ season to the subtropical high When precipitation reaches the ground, it is intercepted pressure belt. by a great variety of dry surfaces which need to Ice crystals (snowflakes) fall from most clouds be thoroughly wetted before they will transmit water in middle and higher latitudes even in summer. (Fig. 1.3). The importance of interception varies However the form they take by the time they reach considerably depending on factors such as surface the ground is determined by the temperature of the roughness. In the case of a forest cover, there are air through which they fall. Thus in summer, high also many levels of surface to be wetted because air temperatures cause melting and rain falls, but drips from the upper wetted leaves will largely be in winter there is a greater likelihood of snow, intercepted in turn by lower leaves (Table 1.1). This especially in continental interiors where temperatures is, of course, why rainfall takes such a long time are very low. to reach the ground beneath trees. By contrast in It is important to understand that there is a urban areas there is much less loss as the surfaces fundamental hydrological difference between snowfall are smooth (roofs, roads, etc.). However in all and all other forms of precipitation. This is because circumstances it is the first part of the precipitation snow usually remains on the land surface for a long that will be lost to interception and this is a fixed time after it has fallen; it frequently drifts; and amount for the surface concerned. After this initial it is only available to the hydrological cycle during loss all further precipitation will be shed so that spring melting. By contrast all other types of precipitation it is relatively insignificant in a long storm but very Table 1.1. Interception and evapotranspiration losses for a mature spruce forest, Thetford, Norfolk (1975) 8 PROCESSES important in a short shower. Again, because of high of water from the land surface. Transpiration is evaporation rates in summer, surfaces dry very quickly the process whereby water vapour escapes from so that there is a full interception loss for each living plants mainly by way of leaves. This loss new shower. In winter, with lower evaporation rates, of water through plant surfaces is replenished by surfaces dry only slowly and a full interception loss water being drawn up through the plant tissues may not occur with each new precipitation event. from the root hairs which are in contact with soil Interception loss is measured in millimetres per water. Transpiration is a very powerful process storm event. because it draws upon the reserves of water stored in the soil pores at depth and is in direct contrast to evaporation which simply dries out the soil Evaporation and transpiration surface. Whenever unsaturated air comes into contact with The factors controlling the rate of transpiration a wet surface, a diffusion (or sharing) process are similar to those for evaporation. In addition, operates. This process, evaporation, happens at however, there are controls exerted by the soil and the surfaces of lakes, rivers, wet roads, and even the plant itself. There is still no general agreement raindrops as they fall from clouds. When the water about the exact nature of the factors controlling body is not large, as in the case of water intercepted plant transpiration, but it seems that water can move by road surfaces, it may be evaporated completely freely to leaves only if there is an unrestricted and quickly, leaving the air still unsaturated. supply available to the roots. This does not mean Precipitation leaving the cloud base also loses that the soil has to be saturated, but clearly as mass but it usually falls too quickly to be entirely moisture is lost from soil pores fewer roots will evaporated. Evaporation is measured in millimetres be in contact with water and supply to the plant per time period in the same way as precipitation will be restricted. Transpiration is really a ‘leakage’ (e.g. mm/h). from leaves over which many plants have very little The rate of evaporation is dependent on several control. As a result it continues at near the potential factors, the most important being a source of maximum rate until a stage of soil moisture deficiency energy for vaporisation. This is largely supplied is reached when the uptake of water no longer by solar radiation which is at a maximum in balances transpiration. After this the difference has summer, making evaporation take place more quickly to be obtained from within the plant itself and in summer than in winter. In addition, warm air this causes wilting. The wilting point corresponds will hold more moisture than cold air, and dry to a very low soil moisture value which is not air will take up moisture faster than wet air. often reached in humid regions except on very However, if air remains completely still over a shallow or permeable soils. However in arid and wet surface, the air soon becomes saturated and semi-arid regions the wilting point would regularly no further evaporation takes place. Wind is therefore be reached in plants unadapted to the prevailing needed to bring fresh, unsaturated air into contact conditions and so indigenous plants have special with the wet surface. leaf configurations (e.g. cacti). Special leaf forms Evaporation takes place continually from a lake and the general scarcity of vegetation make the or a river and this is the maximum continual loss role of transpiration very much less important in of water possible—the potential evaporation. However arid and semi-arid regions. in most cases the surface area covered by water Clearly it is very difficult to separate evaporation is small and most evaporation takes place from from transpiration, so the term evapotranspiration soil and plant surfaces. Water is not easily transferred is used to describe the combined effect of the two from the soil body to the surface, so the surface as they influence storage of precipitation in the quickly dries out. After this, although some evaporative soil. This total loss by evapotranspiration is of loss does occur, it is well below the potential rate major importance and may account for about three- and is of minimal significance (except in arid quarters of all precipitation in a year, even in humid environments). regions. In most humid regions the proportion of the The role of evapotranspiration in the terrestrial land surface subject only to evaporation is small; part of the hydrological cycle may thus be summarised for most of the year soil surfaces are covered by as the progressive loss of soil moisture to a degree vegetation and in these circumstances transpiration far beyond that which would occur through gravity will be the major factor determining total losses drainage alone. A moisture deficit is thus created INFILTRATION, THROUGHFLOW AND OVERLAND FLOW 9 which has to be replenished before any fresh rain 1.4 shows the soil saturated for a depth H above water is available for the runoff process. This a plane XY. Under these conditions the speed of replenishment takes time and is partly responsible water movement varies directly with H (the hydraulic for the lag between rainfall and runoff. Finally head) and is related to it by a constant called it is necessary to stress that the importance of the permeability (hydraulic conductivity), which evapotranspiration is dependent on a number of is measured in centimetres per second. Sandy soils climatic factors and so, although clearly at a have, on average, larger pores than clay soils and maximum in summer, losses are very variable hence a higher conductivity value. When soils are (Table 1.1). unsaturated, water still flows, but in fewer pores. In this case the flow rate also depends on the ratio of pores still containing water to those containing Infiltration, throughflow and overland flow air (the soil moisture content) and it is very much When precipitation has completely wetted any vegetation, lower than the saturated rate. Moisture content is the remainder is available to wet the soil or to often measured as a percentage relative to the run off over the surface (overland flow). The process water-holding capacity at saturation. whereby water enters the soil is called infiltration Most soils are on sloping land and so soil- and it is normally measured in centimetres depth water movement takes place towards the slope of water per time period. base. The vertical component of this movement Soil acts as a sort of giant sponge containing is called percolation, the downslope component a labyrinth of passages and caverns of various sizes. throughflow (Fig. 1.5), both are normally measured The total space available for water and air is called in centimetres per second. Unless the soil is very porosity and is expressed as a percentage of the dry or there is a storm of great intensity, all soil volume. Some of the passages and caverns (soil the effective rainfall will infiltrate the soil, filling pores) are interlinked, although others are culde- up empty soil pores and establishing a zone of sacs and conduct no water. The passage of water higher water content near the surface. The leading from soil surface to stream bank or waterconducting edge of the newly penetrating water is called the rock (aquifer) thus takes place in a very tortuous wetting front and it separates a zone of high way through small-diameter passages, so it is not permeability above from one with drier soil and surprising that water movement is slow. The speed a lower permeability below. If the storm is sufficiently of conduction of water is determined by the proportion prolonged, the wetting front eventually reaches and size of the interlinked waterfilled pores. If the soil base and all storage is filled. Thereafter all pores are full, the soil is said to be saturated water moves downslope by throughflow, except and the plane defining the upper surface of the in the special case of the part of the soil adjacent saturated zone is called the water table. Figure to a stream. Here, because the soil is in contact Figure 1.4 The position of a water table. Figure 1.5 Water flow on a hillside.