Studies reports in hydrology 29

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20. Hydrological m a p s . Co-édition 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 o n the hydrological regime and on water quality. 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 o n water resources planning and management.

27. Socio-economic aspects of urban hydrology.

28. Casebook of methods of computation of quantitative changes in the hydrological régime of river basins due to human activities

29. Surface water and groundwater interaction.

* Quadrilingual publication : English — French — Spanish — Russian.

For details of the complete series please see the list printed at the end of this work.

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Surface water and

groundwater interaction

A contribution to

the International Hydrological Programme

Report prepared by

the International Commission on Groundwater

Edited by C . E . Wright

lunssco

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legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.

Published, in 1980 by the United Nations Educational, Scientific and Cultural Organization 7 place de Fontenoy, 75700 Paris

Printed by

Imprimerie de la Manutention, Mayenne

I S B N 92-3-101862-0

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Preface

The "Studies and Reports in Hydrology" series, like the related collection of "Technical Papers in Hydrology", was started in 1965 when the International Hydrological Decade ( I H D ) was launched by the General Conference of Unesco at its thirteenth session. The aim of this undertaking was to promote hydrologi- cal science through the development of international co-opera- tion and the training of specialists and technicians.

Population growth and industrial and agricultural develop- ment are leading to constantly increasing demands for water, hence all countries are endeavouring to improve the evaluation of their water resources and to m a k e more rational use of them.

The I H D was instrumental in promoting this general effort.

W h e n the Decade ended in 1974, I H D National Committees had been formed in 107 of Unesco's 135 M e m b e r States to carry out national activities and participate in regional and international activities within the I H D programme.

Unesco was conscious of the need to continue the efforts initiated during the International Hydrological Decade and, following the recommendations of M e m b e r States, the Orga- nization decided at its seventeenth session to launch a n e w long-term intergovernmental programme, the International Hydrological Programme (IHP), to follow the decade. T h e basic objectives of the I H P were defined as follows: (a) to provide a scientific framevork for the general development of hydrological activities ; (b) to improve the study of the hydro- logical cycle and the scientific methodology for the assessment

of water resources throughout the world, thus contributing to their rational use; (c) to evaluate the influence of man's activi- ties on the water cycle, considered in relation to environmental conditions as a whole; (d) to promote the exchange of infor- mation on hydrological research and on new developments in hydrology; (e) to promote education and training in hydrology;

(f) to assist M e m b e r States in the organization and development of their national hydrological activities.

The International Hydrological Programme became opera- tional on 1 January 1975 and is to be executed through succes- sive phases of six years' duration. I H P activities are co-ordinated at the international level by an intergovernmental council composed of thirty M e m b e r States. The members are periodi- cally elected by the General Conference and their representa- tives are chosen by national committees.

The purpose of the continuing series "Studies and Reports in Hydrology" is to present data collected and the main results of hydrological studies undertaken within the framework of the decade and the new International Hydrological Programme, as well as to provide information on the hydrological research techniques used. T h e proceedings of symposia will also be included. It is hoped that these volumes will furnish material of both practical and theoretical interest to hydrologists and governments and meet the needs of technicians and scientists concerned with water problems in all countries.

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i. Introduction

1.1 Purpose and Scope of the Report

There has been a tendency in past years for separate departments to develop specialising in either surface water or groundwater systems. For this reason the understanding of the interaction between surface water and groundwater and techniques for its analysis have tended to be less well advanced than those for either discipline. In recent years the traditional division between the disciplines has tended to be reduced with the result that some useful advances have been made in understanding the interaction between surface water and groundwater. This report does not attempt to review all the relevant research of recent years but rather to emphasise and illustrate the importance of the subject.

Improvements in understanding the interaction can provide information useful in the management of water resources. For example existing schemes may be operated more efficiently and new techniques may be considered in planning the future development of resources. In all areas where water is a relatively scarce commodity there is a positive requirement to define the interaction accurately. One of the main purposes of this report is to assist developing countries, especially those in arid areas, in the management of their water resources.

However, there are likely to be benefits arising as a result of accurately defining the interaction in other regions such as those where the demand for water represents a high proportion of the total resource and where changes in the interaction caused by man have a marked

beneficial or detrimental effect.

Detailed consideration has been confined to one part of the hydrological cycle, the interaction between surface water and groundwater. In temperate regions the main aspect of this process is the flow of groundwater to rivers, and in arid regions the flow is frequently in the reverse direction with surface runoff recharging groundwater. Subject areas covered by other Working Groups such as irrigation, groundwater models, water quality and low river flows are referred to but not covered in detail. For example the effect of irrigation is included in a case study, and sections of the report contain a discussion of groundwater models and water quality. Changes in water quality due to the effect of man in polluting either surface water or groundwater is a major topic that is referred to only briefly. Methods of assessing

the interaction between surface water and groundwater are described together with an

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assessment of their accuracy. Four case studies are included. Two describe investigations in temperate regions and two describe aspects of the interaction process in arid regions.

Publications referred to in the text are listed in the references and an additional list of selected relevant papers is given from symposia held during 1979 at Dortmund (FRG), Vilnius

(USSR) and Canberra (Australia).

1.2 Formation and Activity of the Working Group

The second session of the Intergovernmental Council of the International Hydrological Programme (IHP), was held in June 1977, when the decision was taken to:

invite the Secretariat in co-operation with the International Commission on Ground Water (ICGW) of the International Association for Hydrological Sciences (IAHS). to prepare a technical report 'Improvement of methods of assessment of the interaction between groundwater and river flow' and report on the progress of this project to the third session of the Council.

Two sessions of the Working Group have been organised by the ICGW Secretary with the assistance of the IHP Secretariat of Unesco. The first session was held at the Unesco

headquarters in Paris from 12 to 16 June 1978 and the second was held at Dortmund from 7 to 11 May 1979. The Working Group was composed as follows.

First Session Second Session Mr V V Kuprianov (USSR) Mr 0 V Popov (USSR) Mr J Soveri (Finland) Mr J Soveri (Finland) Mr C E Wright (Chairman, UK) Mr C E Wright (Chairman, UK) Mr H Zebidi (Tunisia) Mr M Ennabli (Tunisia) In addition the following experts were invited to attend the sessions :

First Session Second Session Mrs N Kapotova (USSR)

Mr C Pollett (Australia) Mr J A Rodier

Mr G Castany Mr M G Bos

(IAHS) (IAH)

(ICID Committee on Irrigation Efficiencies)

The report has been prepared by the members of the Working Group together with the following invited authors :

Mr C van den Akker (Netherlands) Mr D A Kraijenhoff van de Leur (Netherlands)

Mr B R Payne (IAEA) Mr J A Rodier (France) Mr K R Rushton (UK)

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Mr H J Colenbrander, Mr C E Wright and Mr Y N Bogoyavlensky were responsible for the final editing.

1.3 Relationship with other IHP Working Groups

Parts of the subject area of this report could overlap or are closely linked to the work of other IHP Working Groups. The subjects associated with these Working Groups are listed to enable further information to be obtained if required.

Project 5.1 Assessment of quantitative changes in the hydrological regime of river basins due to human activities (1975-1980) - Preparation of a casebook on methods of computation (1975-1979).

Project 5.4 Investigation of water regime of river basins affected by irrigation (1975-1980) Preparation of a technical report (1978-1980).

Project 7.3 Investigations of processes of quality and quantity changes of groundwater resources due to urban and industrial development.

Project 8.1 Physical and mathematical models for investigation and predicting the changes in groundwater regimes due to human activities.

Project 8.2 Study of groundwater recharge, including water quality aspects.

Part of the terms of reference for project 7.3 include a review of the present knowledge of the interaction between surface water and groundwater in the urban environment. Therefore this report (project 3.6) does not include a section on the urban environment.

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2.

Definition of the interaction

2.1 Part of Hydrological Cycle Considered

The interaction between surface water and groundwater is a part of the hydrological cycle that has been examined in some detail in recent years. There are two main aspects of this process,

firstly the flow of groundwater to support river flow and secondly the flow from rivers to groundwater. The former is a common occurrence in temperate regions whereas the latter occurs widely in arid regions. Figure 1 is a simplified conceptual model that illustrates the subject area of this report. There is considerable scope for modifying the figure to allow for

local conditions. For example in highly permeable areas the surface storage component could be negligible and therefore be omitted.

poration

Capillar Rise

Precipitation

1

Surface Storage

<

Infiltratior

Storage in Unsaturated Zone

i

1 Groundw

Rechar

Groundwater Storage

ater ge

Overland flow

Interflow

Base flow

<

Direct Runoff

Total Runoff

Figure 1 A conceptual model

River flow is derived essentially from precipitation less evaporation and the routes by which precipitation becomes river flow are shown in Figure 1. In a natural river system with

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negligible abstractions and discharges there are two main components of river flow, namely direct runoff and base flow. Direct runoff may be subdivided into channel precipitation, overland flow and interflow, whereas base flow is that part of river flow that is derived from groundwater. Groundwater flow is defined as flow within the saturated zone. In catchments with more than one aquifer the base flow component may be subdivided according to the

contributing formation. The proportion of' direct runoff or base flow in total river flow may vary substantially from one basin to another and from month to month because of the effect of different soil types, geology, land use, topography, stream patterns and changes in

precipitation, evaporation and temperature.

In temperate regions groundwater recharge is derived mainly from precipitation less evaporation, where evaporation is defined as including transpiration and interception losses from vegetation. However, in arid regions, where annual potential evaporation exceeds precipitation, groundwater recharge is frequently derived from temporary rivers that are in flood. More generally both flood water and base flow from mountain rivers can recharge

aquifers in the foothills and adjacent relatively dry low lying areas. In addition groundwater recharge may occur from lakes, canals and excess irrigation. If the groundwater table is near to the surface of the ground, then the capillary rise may enable evaporation to deplete

directly the groundwater storage. The infiltration process and the movement of water in the unsaturated zone are not discussed in detail in this report.

The storage, flow and quality characteristics of surface water and groundwater are frequently dissimilar. For this reason the interaction is important in water-resource

development since advantage may be taken of the differing characteristics to increase yields or improve the quality of water supplies. Changes in one part of the hydrological cycle may induce beneficial or detrimental changes in another part of the cycle. A definition of the water balance and its elements or component parts has been given by Brown et aZ.t (1972) . 2.2 Recharge of Groundwater

2.2.1 Recharge by Precipitation

The main source of groundwater recharge is generally directly from precipitation particularly in those areas where annual average precipitation exceeds potential evaporation. Evaporation may deplete water held in surface storage, in the soil or in the aquifer as shown in Figure 1.

Groundwater recharge occurs when the residual precipitation (precipitation less actual

evaporation) has infiltrated to the groundwater table. This may occur from several hours to several months after the precipitation event. If the precipitation is in the form of snow then infiltration is delayed indefinitely until there is a thaw.

To fully understand the characteristics of aquifer storage it is necessary to investigate the characteristics of precipitation, evaporation, temperature and the unsaturated zone which collectively determine the temporal distribution and rate of recharge. In some parts of western Europe, consecutive monthly totals of precipitation may be regarded as independent

(random) events that are uncorrelated with past or future monthly totals, whereas evaporation has a strong seasonal (cyclic) pattern that is repeated year after year. In such areas

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significant random and cyclic components are observed in time-series recharge data. Where precipitation and evaporation data display different properties the characteristics of recharge data will vary accordingly and in colder regions will be significantly influenced by temperature as discussed in section 3.3.

In arid areas the direct recharge of groundwater from rainfall is likely to be insignificant because of several factors.

1. for most of the year, rainfall is relatively small compared with potential evaporation,

2. storm intensity frequently exceeds the infiltration capacity of the ground surface resulting in overland flow,

3. the unsaturated zone tends to dry out and may therefore absorb a significant volume of infiltrating water,

4. semi-permeable crusts may form in the unsaturated zone comprising fine sediments that impede infiltration.

During the relatively few days that rainfall exceeds evaporation in arid areas, the storm intensity is frequently sufficient to induce surface runoff thus effectively removing the potential recharge water to a location downstream. Any water that does infiltrate tends initially to reduce the soil moisture deficiency then evaporate rather than recharge groundwater. Where rainfall is infrequent and irregular, direct recharge from precipitation is likely to be even less frequent.

Aquifers may be divided into two types, fissured and arenaceous, depending upon whether the storage of water is essentially within fissures or intergranular. However, some aquifers may be a mixture of both types with, for example, storage contained substantially within the granular interstices, but flow mainly through fissures. The delay between a precipitation event and the consequential rise in the water table is dependent upon the aquifer properties discussed in section 3.3. Where permeable soils overlie highly fissured deposits such as the Karst Limestones, high intensity rainfall may infiltrate rapidly to depths from which

evaporation is negligible. An example of this phenomenon has been described by Downing and Williams (1969) for the Lincolnshire Limestone of eastern England and Rushton (1976) estimated that rapid recharge through 'swallow' holes and fissures contributes up to 40 per cent of the total recharge to this aquifer. In these conditions groundwater recharge may be derived in approximately equal proportions from precipitation (directly) and surface runoff.

Long period records of weather conditions, river flows and groundwater levels are valuable aids in the analysis of water resources. Provided that the records are accurate the longer the record the more accurately defined are the annual and monthly means and the

variation about the mean. In addition long terms trends and cycles may be detected. For example in a study of the 1972 to 1973 drought in northern Nigeria some trends and cycles were detected in the hydrological data (Sonuga, 1977) . Unfortunately long term records are not

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available in many areas and the assessment of rainfall characteristics may be complicated by a high variability of daily, monthly and annual rainfall within relatively small areas (Balek,

1978) .

Where long period weather records exist and a suitable model is available, it may be possible to synthesise long sequences of aquifer recharge data. An abstract from such a synthesised record is shown in Table 2.2.1 (Morel and Wright, 1978) which illustrates the random and seasonal components of aquifer recharge for the Chalk of eastern England (West Suffolk). This area has an annual average rainfall of 600 mm of which 450 mm is evaporated.

Recharge occurs mainly during the four months December to March, but may be negligible if winter rainfall is insufficient to restore the soil to field capacity. The exceptionally dry weather of 1972-73 and 1975-76 resulted in negligible groundwater recharge for periods of

18 months. From the long synthesised record it is apparent that such events may occur in this area on average three or four times every 100 years.

Table 2.2.1 Typical values of monthly aquifer recharge in eastern England (1965-76) (millimetres)

Year 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976

Jan 0 20 23 39 57 49 70 51 0 0 54 0

Feb 0 54 36 22 49 50 10 30 0 46 13 0

Mar 12 0 0 1 37 23 23 24 0 2 64 0

Apr 10 9 12 0 0 29 0 0 0 0 22 0

May 0 0 21 0 20 0 0 0 0 0 0 0

Jun 0 0 0 0 0 0 0 0 0 0 0 0

Jul 0 0 0 0 0 0 0 0 0 0 0 0

Aug 0 0 0 13 0 0 0 0 0 0 0 0

Sep 0 0 0 56 0 0 0 0 0 0 0 0

Oct 0 0 0 24 0 0 0 0 0 0 0 0

Nov 24 24 0 28 0 0 2 0 0 88 0 0

Dec 91 67 40 42 0 0 15 0 0 21 0 17

Total 137 174 132 225 163 151 120 105 0 157 153 17 2.2.2 Recharge by Rivers and Canals

Recharge may occur whenever the stage in a river or canal is above that of the adjacent groundwater table, provided that the bed comprises permeable or semi-permeable material. This type of groundwater recharge may be temporary, seasonal or continuous. Also it may be a natural phenomenon or induced by man. For example intermittent recharge may occur in arid regions when temporary rivers are flowing in valleys that are usually dry (Besbes et al., 1978), seasonal flow can occur to and from bank storage (Popov, 1969), and there may be a

continuous flow to groundwater from rivers and canals (Smiles and Knight, 1979). In New Zealand several groundwater bodies near the coast are recharged mainly by seepage from river beds, and it is probable that similar processes occur in other places around the world

(Woudt et al., 1979). When there is seepage from a canal or ditch overlying a shallow water

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table, water-logging of the soil at points some distance from the canal is a distinct possibility (Bruch, 1979).

Man can induce groundwater recharge from rivers by lowering the water table adjacent to rivers or by raising the river stage. The former is a relatively common occurrence which may be caused by groundwater abstractions for supply or by mine drainage, and the latter may be caused by reservoir releases (Kemp and Wright, 1977), weirs or other engineering works. A serious deterioration in groundwater quality may result if the recharge water is saline or significantly polluted. This is discussed in section 2.4.

Since the replenishment of groundwater by temporary rivers is frequently the main source of aquifer recharge in arid regions, much of this section describes the process in such areas and additional information is contained in two case studies. However, groundwater recharge from rivers also occurs in other regions where the geological conditions are favourable, especially where there are Karstic rocks.

In arid areas groundwater recharge from precipitation is generally limited because of high rates of potential evaporation and other factors described in section 2.2.1. On the other hand the replenishment of groundwater by rivers in flood is frequently the major source of recharge. Temporary rivers are formed in the valleys, or wadis, following intense storms in the hills which are sufficiently severe to generate surface runoff. These temporary rivers may terminate either in spreading zones where the flood water infiltrates to the aquifer below, or in chotts or sebkhats which are low lying areas where temporary lakes are formed. Water that accumulates in these depressions evaporates leaving behind its salt content. In both cases the aquifers are recharged mainly in the foothills, or piedmont zones, where the surface runoff is concentrated and where topographical conditions and soil permeability tend to be more favourable for infiltration to the saturated zone.

Several factors combine to enable recharge to take place in the piedmont zones:

1. in such areas there is a thickness of permeable detritus comprising sand, gravel and talus (detritus fallen from a cliff face),

2. the beds of the wadis are higher than the groundwater table, 3. water may flow horizontally through the banks,

4. the surface water spreads out over the ground thus accelerating the process of infiltration and subsoil saturation,

5. the finer sediments that could impede infiltration are carried to the downstream periphery of the recharge zone.

Groundwater recharge from temporary rivers is very irregular in both time and space, just like the storms that produce it. In contrast to direct recharge from precipitation it is relatively localised and concentrated with a rapid and divergent groundwater flow at the point where the valleys open out into the plain. After each flood event there is a period

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during which the aquifer is recharged causing a rise in the water table in that area. The observed changes in the piezometric surface are the result of the superimposition of the recent and previous flood events, so that the effect of recharge from a specific flood is superimposed upon the preceding recession of the water table.

The rise in groundwater levels is related to the size of the flood. However, there is a delay in the response of the water table due to two factors. Firstly there is a delay due to the thickness, permeability and porosity of the unsaturated zone, and secondly the horizontal propagation of the flood wave in the saturated zone is related to the diffusivity of the aquifer.

A recession of groundwater levels follows the rise caused by infiltrating flood water.

When flow through the unsaturated zone ceases, there is a recession in groundwater levels until such time as the next major recharge episode. In areas close to the wadis the variability of inflows may be sufficient to prevent groundwater flow from reaching the steady state condition, and the effect of major floods may be apparent even after several months. Further away from the recharge zone the amplitude of groundwater level fluctuations decreases and the flow approximates to or reaches a steady state condition.

In areas where aquifers are recharged by rivers or canals, the safe groundwater yield (Q) may be expressed as (Bochever, 1979) :

Q = Qe + Q± 2.2(1)

where Q is that part of the yield derived from natural groundwater sources and Q. is the total inflow from other sources such as rivers. The determination of Q. is dependent primarily upon analyses of the interaction between groundwater and river water.

2.2.3 Recharge by Lakes

In the United States a large number of small reservoirs are being built and small lakes are increasingly being used as a focal point in urban planning. This has given rise to pollution and amenity problems that for their solution require some understanding of lake hydrology of which the interaction between lakes and groundwater is an integral part (Cherkauer, 1977) .

In many studies of lake hydrology the precipitation, evaporation and inflow/outflow data are available. However, evaporation assumptions in particular may lead to errors in the water balance. If the residual is allocated to groundwater effects then serious misunderstandings could arise concerning the interaction of lakes and groundwater. To investigate this interaction, numerical model simulations were carried out (Winter, 1976).

Most natural lakes in the United States are caused by glaciation, and the studies by Winter (1976) apply especially to those conditions. The lines separating the various types of flow system, or divide, were obtained for various situations. In each case the groundwater levels surrounding the lake were assumed to be at a higher level than the lake surface, and a point was located on the divide where the head is a minimum. This minimum head may occur

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beneath the shoreline on the downstream side of the lake and is called the stagnation point.

The relationship of the head at the stagnation point to the lake level is fundamental to understanding the interaction of lakes and groundwater. If the head at the stagnation point is greater than the lake level it is impossible for water to move from the lake to groundwater.

If a stagnation point is located then the divide is continuous, the lake cannot leak, and it is the discharge point for the groundwater flow system. Alternatively if there is no stagnation point then the lake can leak through part or all of its bed.

2.2.4 Artificial Recharge

To increase the natural replenishment of aquifers, man has used artificial recharge in

addition to those methods described in section 2.2.2 (rivers and canals). Natural infiltration may be augmented in two ways. The first is through surface works, including recharge lagoons, ditches, the building of low dams to cause flooding of riverside tracts, and excess irrigation.

These methods are, as with natural infiltration subject to evaporation losses and may occupy large areas of land. The second means of augmentation is to inject the recharge water directly into the aquifer through shafts and boreholes. While this method avoids evaporation losses and reduces land use, there is the disadvantage that recharge water often requires extensive

treatment before injection to avoid serious clogging of the recharge wells. The normal source of recharge water is surface runoff, but treated effluents and cooling water have been used.

Artificial recharge dates from early in the nineteenth century in Europe and near the end of that century in the United States. More recently the experience in the United States has been summarised by Todd (.1960) , in Israel by Harpaz (1970) and in the United Kingdom by Rodda et dl., (1976). In arid and semi-arid regions, such as parts of the western United States, salinity increases have been observed in both groundwater and surface water due to the effects of irrigation practices. Much of the irrigation water is lost by evaporation, but some recharges the aquifers and provides an increment to river flow. In these areas the

concentration of dissolved solids tends to increase and may reach a level intolerable to many crops. In several places this effect is so pronounced that the quality of water rather than the amount available restricts water use. This has led to the development of computer models to predict changes in dissolved solid concentrations in response to varying hydrologie stresses

(Konikow and Bredehoeft, 1974).

The effect of excess irrigation upon aquifer recharge is such an important issue in arid and semi-arid climates that a case study (section 5.3) concerning this subject is included in this report. However, the subject area of irrigation and groundwater recharge is covered by other IHP Working Groups (5.4 and 8.2) and it is therefore not covered in further detail in this report.

2.3 Groundwater Component of River Flow

The groundwater component of river flow is derived from continuous and intermittent flows from aquifers that drain to the river under varying degrees of hydraulic connection. It is the term used to describe that part of river flow that has been formed by the complicated processes that

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result in groundwater inflow. The main features of the interaction between surface water and groundwater may be identified as a specific part of the hydrological cycle. On a regional scale the characteristics of these main features, including groundwater inflow to a river, may display a marked regularity in both space and time.

It is customary to subdivide the component of river flow derived from groundwater into continuous base flow from the main aquifers and intermediate flow, or sub-surface runoff, from temporary storage. However, it is frequently difficult to estimate quantitatively the varying properties of base flow, short-term groundwater flow and surface flow, or direct runoff, that are present in the measured river flow. These proportions tend to change due to the different rates of recession that characterise each flow component. The recession of short-term groundwater flow is more rapid than that of base flow, but slower than that of surface runoff.

However, the accurate estimation of each component of flow can be completed for specific rivers only on the basis of complex water balance investigations in representative and experimental basins (Toebes and Ouryvaev, 1971; Brown et al., 1972). Because of this it is more usual to group together short-term groundwater flow and surface flow as direct runoff.

The groundwater component of river flow may be subdivided according to its origin (i.e.

its genetic parts) with the detail depending upon the availability of hydrological and

hydrogeological information. Improvements in the methods of assessing the interaction between surface water and groundwater should be based upon more subjective and detailed separation of the groundwater component of river flow. Firstly, the base flow component should be identified by considering the river flow and basin characteristics. For this purpose conceptual models of groundwater flow to rivers are proposed based upon the classification shown in Table 2.3.1.

In constructing conceptual models it is important to consider the extent to which aquifers contribute to river flow. Also care should be taken to differentiate between the single-aquifer and multi-aquifer system. Difficulties may be encountered when estimating the base flow components of multi-aquifer systems, since it is then necessary to identify the contribution from each aquifer on the basis of available hydrogeological information. In the absence of sufficient information it is good practice to take account of the contribution from the main aquifer, or at least to estimate the total groundwater inflow without attempting further division.

Conceptual models that are used to estimate the components of groundwater runoff should be based upon the available and essential observational data. If such data are not available then an objective schematization of the complex natural conditions (i.e. that occur in the interaction between surface water and groundwater) should be adopted together with the

application of simplified schemes (Toebes and Ouryvaev, 1971; Popov, 1969b; Dobroumov et al., 1976) .

In some river valleys the position of the water table may vary in relation to the river stage at different points along the valley. This may give rise to local groundwater inflow, river water outflow and underflow through permeable deposits beneath the bed of the river. By

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carefully siting river gauging stations it may be possible to minimise some of the complications arising from these local conditions.

Table 2.3.1 Classification of groundwater discharge to rivers

Class Type Source of recharge

Hydraulic Connection Present (P),

Absent (A) 1. From

unconfined aquifers

A. Intermittent Groundwater (inter-flow) Temporary perched water

('Verkhovodka') in mountain rock

Water of raised bog Water from intermittent springs and geysers Intermittent flow from aquifer overlying permafrost Melt water from groundwater frozen at surface ("aufies") Return water or bank storage

(flowing period) Phreatic groundwater

Continuous flow from aquifer overlying permafrost

Groundwater flow between aquifers

Water flow below permafrost zone

B. Continuous

P, A P, A

P A

P, A

P, A P, A

2. From confined aquifers

(artesian)

A. Open flow

B. Close flow

Water of fen soil

Water of constant springs (spring flow)

Confined water, upper spring water discharging directly into the channel

Confined water moving into the overlying aquifer

P A

P, A

2.4 Influence of the Interaction on Water Quality

Abstractions from surface water and groundwater for supply purposes are limited by both quantity and quality considerations. Whenever there is a flow of water between the surface and aquifers, in either direction, there is a relationship between the quality of water in the two systems. Where pollution is tending to increase due to man ' s activities an understanding of

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the interaction is essential to reduce the effects of such pollution. Groundwater can pollute surface water and surface water can pollute groundwater. Alternatively there may be

improvements in quality.

2.4.1 Surface Water to Groundwater

The various methods of natural and artificial recharge of groundwater are described in section 2.2. Whenever groundwater is recharged by the infiltration of surface water, the quality of the former depends to some extent upon the quality of the latter. In natural

conditions the recharge of groundwater from surface water tends to cause some reduction in the quality of the groundwater with a consequential decreases in its usefulness. However, this reduction in quality may be minimal because the infiltrating water receives some purification caused by physical, biological and chemical processes, as it passes through the unsaturated zone.

Infiltrating water is mechanically filtered and some substances are adsorbed. Biological purification takes place either by oxic or anoxic dissimilation. The microbes on the soil particles tend to exert a greater purifying effect the longer the water remains in the soil stratum, and the slower the water flows. The most important chemical reactions are those involving carbon, nitrogen, calcium, iron, manganese and sulphur. These reactions depend upon the redox properties of the substances, but biological interference may change the approach to equilibrium conditions as determined from thermodynamically known potentials. Thus the

reactions may have an importance which differs from that in the purely physico-chemical system.

The purifying activity in surface waters always depends upon the oxygen content. The activity of microbes reduces the oxygen concentration with a resultant rise in the carbon dioxide concentration. Firstly, the microbes use up the dissolved oxygen, then use organically bound oxygen and the oxygen in nitrates and sulphates. Nitrate will be reduced only when the oxygen content is less than 0.5 mg/1.

Organic substances in the infiltrating water disintegrate rather quickly as shown by decreasing permanganate numbers. However the humic fraction does not disintegrate but forms humâtes with metal compounds which become bound to the soil. In the Nordic countries a problem exists because much of the surface water is derived from swamps and contains much soluble humic material which is not retained by the soil but filters through to the groundwater. Table 2.4.1 contains the mean concentration of various substances in surface water and groundwater in Finland.

Agricultural fertilizers may have some influence on the quality of groundwater. Organic nitrogen is readily oxidized to nitrate after passing the ammonia stage, and groundwater may

contain all the oxidizing stages of nitrogen. Phpsphorus readily becomes closely bound to the soil and thus groundwater tends to contain very little phosphorus.

Iron and manganese frequently detract from the usefulness of groundwater. These elements have a solubility which depends on the redox potential and the pH value. Because biological processes determine the redox state, certain organisms will influence the solubility of iron

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and manganese.

Table 2 . 4 . 1 Changes in composition of water from a sandy soil in Finland based upon analyses for snow-melt, lysimeter water and groundwater

Chemical Determinands

pH

Electrical Conductivity NO - N N H . - N NO - N P O , - P CI 4 Total Hardness

S 04 Na K Ca Mg Mn Cu Pb

Unit

m S / m

M / l ug/1

M / l

ug/1 mg/l mmol/1

mg/l mg/l mg/l mg/l mg/l

M / l

ug/1 ug/i

Snow-melt S w

4.4 2.3

410 230 5 8 0.6 0.02 2.0 0.3 0.2 0.4 0.1 25

3 8

Lysimeter Water

L w

7.4 22.6 73

3 1 7 1.0 0.53 28.0

1.6 1.5 9.3 0.9 72

7 4

Ground- Water

G w

7.4 23.0

73 3 1 7 1.0 0.8 2.6 1.5 0.8 5 . 4 0.8 110 190 30

Change in Value L - S

w w 3.0 20.3

-337 -227 - 4 -1

0.4 0.51

26.0 1.3 1.3 8.9 0.8 47

4 - 4

G - L w w

0 0 . 4

0 0 0 0 0 0.27

-25.4 -0.1 - 0 . 7 - 3 . 9 -0.1 38 183 26

G - S w w

3.0 2 0 . 7

-3.37 -227 - 4 -1

0.4 0 . 7 8

0.6 1.2 0.6 5.0 0.7 85 187 22

The influence of the-quality o f surface water on groundwater is primarily determined by the time lag and distance o f flow through the unsaturated zone. In general the quality of groundwater is quite good if the delay is two or three months or m o r e , depending upon the composition and permeability of the soil and underlying aquifer. When groundwater is recharged from watercourses, the quality o f the groundwater tends to improve with increasing distance from the recharge area. However, in arid areas salinity may increase where evaporation occurs from groundwater, such as in ' s e b k h a t s ' as described in section 2 . 2 . 2 . Geological factors including the structure of the aquifer and the mineral composition of the soil and bedrock also influence water quality. More substances will dissolve from minerals formed at high t e m p - eratures than from minerals which are less easily attacked and have crystallised at low temperatures. Minerals that are easily attacked include micas, dark thermal minerals and limestones which readily dissolve in water containing carbon dioxide. If the water is very hard, calcareous deposits may form. Iron and manganese dissolve in reducing environments, b u t may b e precipitated when the oxygen content of the water rises. Most light halic minerals, such as quartz and feldspar, which are the main minerals in granite, gneiss and quartzite will withstand chemical attack best, and little will dissolve from these minerals.

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Two relatively common forms of groundwater pollution due to the activity of man arise from waste disposal and saline intrusion along coastlines and estuaries. Pollution from waste disposal occurs from a wide range of man's activities such as domestic sewage, industrial effluent and waste disposal tips. In addition serious pollution can arise due to accidents during transportation of chemicals if these occur adjacent to an aquifer. The storage of radio-active waste poses special problems due to the long life of the pollutant, its potency and the uncertain rate of groundwater flow at appreciable depths below the surface of the ground.

Saline intrusion is likely to occur if the water table is lowered by groundwater

abstraction at sites adjacent to the ocean of other salt water environments. For example there are long stretches of coastline in England along which aquifers are in contact with the sea, and the pumping of groundwater has resulted in saline water moving inland at a number of sites including the Humber, Mersey and Thames estuaries and along parts of the south coast. In Israel seawater may penetrate at depth to the Jordan-Dead Sea Rift Valley (Kafri and Arad, 19 79). Aquifers can also be contaminated by the upward flow of fossil brines where these occur at depth below freshwater.

2.4.2 Groundwater to Surface Water

During prolonged periods of dry weather a high proportion of river flow tends to be derived from groundwater seepage. Thus the quality of groundwater frequently tends to dominate the quality of dry weather river flows. Groundwater is generally of good quality but if it is polluted then there is the risk of surface waters becoming polluted, especially during low flow conditions when there is a minimum of dilution of base flow. A relatively common example of river pollution by groundwater is that caused by the discharge of mine drainage to watercourses. This type of pollution may occur when minewater is pumped or when there is a natural overflow from a disused mine.

Mine drainage can effect both the flow regime and the quality, tending to be relatively constant throughout the year and during dry periods may contribute significant flows to rivers.

However, in England the major effect results from the quality of mine drainage (Rae, 1978) . The River Pollution Survey of England and Wales (1970) shows that a large percentage of polluted and poor quality watercourses are in the coalfields. This is in part due to mine drainage. In a typical mine-drainage water the concentration of chlorides, sulphate, calcium, total dissolved solids and occasionally iron will be several hundreds of milligrams per litre.

This tends to decrease rapidly downstream of the discharge point leaving a ferruginous deposit on the bed. Although this deposit may not be totally destructive to the local biological system it is unsightly and may inhibit the use of rivers for water supply.

The hydrograph of total river flow can be divided into its main components of base flow and direct runoff as described in section 2 . 3 . The characteristics of flood waters are

frequently different to those of low flows (Subramanian, 1979). If each component tends to retain its own characteristics of quality and temperature then it is possible to construct mathematical models of river flow based upon hydrograph separation techniques and water

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quality considerations. Conservative or nearly conservative determinands such as alkalinity (as CaCo.) and ortho-phosphate (as PO.) have been modelled with some measure of success.

In arid regions the available water, because of its scarcity, may be used several times for various purposes. The re-use of water can cause quality problems which may be associated with the cycling of water from the surface to groundwater and then back to the surface.

Excess water applied for irrigation purposes may infiltrate to the water table, reach the surface water channels as base flow then be abstracted and used again for irrigation. This has caused severe water quality problems and reduced crop yields because of the build up of salt in the soil.

Quality and quantity changes may occur in surface water as a result of changes in land use, such as changes to or from arable, forest or urban environments. In arid and semi-arid areas a significant increase in salinity may occur in surface runoff after natural vegetation has been removed for agricultural or other purposes. This process has been observed for example in south-western Australia (Peck and Hurle, 1973). The removal of forest cover could reduce evaporation, increase aquifer recharge and increase stream flow; but the associated rise in the water table could cause some pollution of the aquifer by bringing the water table above a zone containing saline deposits.

Another form of groundwater and consequential surface water pollution may occur from inorganic fertilizers, sewage effluent and atmospheric sources. In England and Wales atmospheric sources provide the greatest amount of nitrogen annually followed by animal and human wastes and inorganic fertilizers (UK, CWPU, 1977). Although inorganic fertilizers

contribute the least to the total it is this source that has caused concern because of its steady increase from two per cent of the total in 1933 to rather more than 25 per cent in 1972.

The slow build up of nitrogen, or other substances, in groundwater can create surface water quality problems especially at times of low flow.

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3. M e t h o d s of assessing the interaction

3.1 Channel Water Balance

The interaction between surface water and groundwater may be determined by analysing their regime features throughout a drainage basin. International guides have been published that enable such studies to be carried out based upon water balance investigations (Brown et al.,

1972:, Sokolov and Chapman, 1974; Toebes and Ouryvaev, 1971). Generalized features of the interaction are reflected sufficiently in water balance calculations for channel networks, to enable objective studies of the interaction to include the compilation and analysis of the Channel Water Balance (CWB) for specific river reaches and river systems (Anon, 19 77a).

To estimate the CWB elements, observational data are required and the most rational method of calculation must be used consistent with the characteristics of each channel reach being studied-. The independent determination of the CWB elements provides the most

comprehensive information concerning the relationship between surface water and groundwater and the characteristics of their interaction.

The elements of the CWB equations are determined from a consideration of the characteristics of the regime for each reach and river system. Accordingly, various observational data for estimating the water balance and solving the CWB equation are obtained from valleys, flood plains and channels. In the absence of observational data the corresponding CWB elements can be determined by less rigorous methods. The values of elements that are within the limits of

the error of their definition are not included in the CWB computation. Channel Water Balance computations may be based upon a month, a year and for typical periods of the hydrological year. All the elements necessary for the CWB computation are defined in terms of the mean discharge for a given period with an indication of the quadratic error (see section 4.3.1).

Examples of the CWB compilation have been described for examining various hydrological problems including studies of the interaction between surface water and groundwater (Anon,

1977a; WMO, 1975) . In the majority of cases the most appropriate method for estimating groundwater flow and river flow is that based upon the CW3 using hydrometric data (WMO, 1975).

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3.1.1 Compilation for River Reaches

The elements of the channel water balance are calculated by using the appropriate equation for each type of river reach. Thus four reach types may be defined taking into account natural and artificial factors.

1. without flood plain, reservoir and water intake,

2 . without flood plain or reservoirs but with water intake for irrigation or other purpose,

3. with flood plain or reservoir and without water intake, 4 . with a considerable flood plain.

When using the CWB method to study the interaction between groundwater and river water, the reaches should be chosen with homogeneous conditions of water exchange between rivers and aquifers. This enables a simple interpretation to be given to the CWB estimate in a design period. Therefore a further four types of reach should be identified:

a. with a continuous groundwater inflow to the river, b . with a continuous outflow of river water to groundwater,

c. where groundwater inflow may alternate with river water outflow (eg with bank storage phenomenon and groundwater table depressions adjacent to the river and below the river stage during the low flow season),

d. with sub-channel stream flow.

3 . 1 . 2 Equations for River Reaches

The CWB for reaches of type 1, i.e. without flood plains, reservoirs or intakes, is computed from equation 3.1(1).

2l +Qlr - 22í Qg lí Quí 20i Qw= 0 3.1(1)

where Q . and Q? are the discharges at the upstream and downstream cross sections respectively, Q^ is the intermediate inflow.

Er

0 is the channel regulation discharge, minus when water accumulates in the reach and plus during the abstraction periods,

Q , is the allowance for ice formation (minus) or ice melting (plus),

Q is the exchange between the river and aquifers, plus for inflow of groundwater to the river and minus for the reverse flow.

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0 is the residual or remainder term that characterises the discrepancy in the o

water balance equation due to computation errors and incomplete account taken of the CWB elements.

The sign of the residual term is defined on the basis of the relationship between the CWB elements thus :

*o *2 *1 *Er - *w - *gl - *u

3.1(3) The CWB for reaches of type 2 is computed using equation 3.1(2).

0, + Q„ - Q0 +Q - Q„ + Q + Q + Q = 0 3.1(2)

*1 Er "2 *<r *ß _ «gi _ *u - *o

where 0 is the total abstraction at a water intake in the reach,

oc

Q is the total water returned to the river,

p

The CWB for reaches of type 3 is computed using equation 3.1(3).

Q , + Qv + Q - Q n - Q - Q„ + Q + Q + Q + Q + Q + Q = 0

*1 *£r *p * 2 *EL Tit - * w - *gl - *u - *AM - AG - o where Q is the river flow due to channel precipitation,

Q„T is the total evaporation from the water surface and transpiration from EL

vegetation along the reach that draws directly on channel storage, Q is the evaporation from the flood plain and reservoir banks,

Q is the discharge corresponding to changes in the soil moisture storage of the zone of aeration,

Q is the change in the groundwater storage in the flood plain and reservoir banks.

The values of Q. and Q, are negative when the storage increases and positive when AM AG

storage decreases.

The CWB for reaches of type 4 is computed using equation 3.1(4).

*1 Er p 2 « Qß * QEL - QE t Í Qw i Qgl Í Qu

- *AM - AG - o 3.1(4)

3.1.3 Compilation for River Systems

To study the interaction between surface water and groundwater for river systems, the CWB can be compiled for the main watercourse to the downstream outflow point of the basin.

3.1.4 Equations for River Systems

The CWB for the main part of the river system is computed using equation 3.1(5)

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E QEr + E QP - Q2 - Z^ + ZQ& - £ QE L - 2 QE t + E ^ + 2 Qg l + EQU

± QA M ± 0A G Î0o = ° 3-1 ( 5 )

where EQ is the sum of the inflows to the main part of the river system above the downstream Er

cross section or outlet,

EQ is the total water added to the main river system from precipitation on the surface P

of the river channel, reservoirs and flood plain along the reach being studied, Q„ is the discharge at the downstream cross section or outlet,

EQ is the total abstraction at water intakes along the main river from its mouth up to the outlet,

EQ is the total returned surface water to the main river up to the outlet,

P

EQ is the total evaporation from surface water, including transpiration from EL

vegetation in reservoirs and along the flood plain of the main river from the mouth to the outlet, that draws directly on surface storage,

EQ„ is the total evaporation from exposed or dry flood plains or from the banks of Et

reservoirs.

EQ is the total discharge due to channel regulation and runoff controlled by reservoirs and the inundation of flood plains, located along the main river above the outlet, EQ is the total water discharge due to ice formation and ice melting,

ZQ is the total water discharge involved in the water exchange between the main river and aquifers along the reach up to the outlet,

£ QA M an(3 £QA r a r e the total water discharges corresponding to changes in the moisture content of the soil and sub-surface zone of aeration, and groundwater in dry flood plain reaches and reservoir banks along the main river above the outlet,

Q is the remainder term of the equation.

If there is no flood plain or reservoir in the main river up to the outlet, the CWB for the main part of the river system is calculated by equation 3.1(6).

Z QEr - Q2 - Z Q« + Z S ± Z Qw i Z Qgl ± ZQu ± Qo = ° 3"1 ( 6 )

3.1.5 Computation of the Elements

General information concerning the computation of the CWB elements is given in this section.

A more detailed description is given by Anon (1977a), Sokolov and Chapman (1974) and WMO (1975).

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3.1.5.1 Exchange between Rivers and Aquifers

The Channel Water Balance method may be used for the assessment of that part of the interaction between surface water and groundwater that relates to the exchange between rivers and aquifers.

Moreover the method enables the principal elements in the equation to be determined which are:

1. groundwater inflow to the river, 2. outflow from the river to groundwater, 3. water discharged to or from bank storage, 4. sub-channel stream flow.

The CWB elements concerning the various types of groundwater exchange included in the equation are defined on the basis of the analysis of hydrological and hydrogeological

information including in particular observational data of river and groundwater stages. The direction of the flow or exchange between rivers and aquifers depends upon the differences in stage and slope of the groundwater table adjacent to the channel. Groundwater exchange is estimated by hydrodynamic computations, water balance and other methods depending upon the natural conditions and the availability of hydrological and hydrogeological data (Anon, 19 77b;

Bochever et al., 1969; Brown et al., 1972; Kudelin, 1969; Popov, 1969).

To provide detailed quantitative estimates of the interaction between river water and groundwater is generally a complicated problem that requires for its solution special field observations of the hydrogeology and of the regime (Toebes and Ouryvaev, 1971; Brown et al.,

19 72). Therefore it is expedient to estimate the exchange between rivers and aquifers using hydrometric methods of differences by solving the CWB equation for the relevant elements. Thus suitable channel reaches are chosen bounded by two cross sections within which the inflow to or outflow from the river may be estimated (Kudelin, 1979; WMO, 1975). The remaining elements of the CWB equation can be calculated when the values significantly exceed the respective

computational errors (see section 4.3) (Anon, 1974).

In the computation of the CWB a proportion of the runoff may be derived from bank storage, Qh_f and some allowance may be necessary when the duration of a flood exceeds the limits of a design period. If a change in river stage takes place during a design period

(such as one month) causing some increase in bank storage of a backwater type (Popov, 1969) , then quantitative estimates of Q, may be obtained from equation 3.1(7). In such cases bank storage increases because of groundwater flow rather than outflow from the river to groundwater.

Q bC = Qc - AQ 3.1(7)

where Q is the initial outflow from bank storage in a quasi-stationary regime (before flood or spring flood),

AQ is the mean additional discharge of a design period as determined by water level

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changes in the channel (Anon, 1977a; Shestakov, 1973).

Along the flood plain, or inundating reaches, the augmentation of groundwater storage is defined at the expense of river water infiltration provided that the inundation occurs during a design period and after the water levels have risen, i.e. a backwater and infiltration type of bank storage is available (Popov, 1969). This value is introduced into the CWB equation for reaches of types 3 and 4 and is computed by methods that have been described in some detail (Anon, 1977a; Brown et al., 1972).

When computing the CWB for reaches with considerable sub-channel stream flow its values at the upstream and downstream cross sections must be estimated. In addition its magnitude relative to the error, or remainder, term in the CWB equation should be examined together with probable errors in the discharge measurements. The systematic study and computation of sub-

channel flow and its related problems has been described (Anon, 1977a).

3.1.5.2 River Flow, Intermediate Inflow, Abstractions and Returned Water

Techniques for the measurement of discharge and the procedure for computing runoff are given by Toebes and Ouryvaev (1971), Sokolov and Chapman (1974) and WMO (1975). The natural intermediate inflow between two cross sections is computed by summing the water discharge adjacent to the flood plain in the reach, from rivers, streams or valleys using available observational or theoretical data. The CWB computation allows for abstractions at water intakes and returned water from various sources (Anon, 1977c; WMO, 1975).

3.1.5.3 Channel Regulation

Channel regulation water is that which accumulates in the channel, flood plain or reservoir because of an increase in the stage. It is a particularly important component of the balance during a flood if the design period is relatively short. However, with longer design periods of up to a year the channel'regulation value may be approximately zero especially in areas where the hydrological regime has a pronounced annual cycle. Various methods of computing the channel storage suitable for the CWB calculation have been described (Anon, 1977a).

3.1.5.4 Precipitation

Discharge derived from precipitation on the surface of channels, reservoirs and flood plains must be considered in the calculation of the CWB. This includes snow melt within these areas.

If there is a considerable flood plain or reservoir and a relatively insignificant river flow then this is likely to be a major component. Alternatively in reaches with a negligible flood plain, precipitation and melt water have only a small influence on the CWB and are thus not included in the calculations.

3.1.5.5 Evaporation from Surface Water

This section includes all evaporation from surface water regions, such as from vegetation that draws on surface water (riparian areas) dried reaches of the flood plain and reservoir banks. A considerable proportion of natural flows may be lost by evaporation from the water

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surface and transpiration by vegetation in reaches with extensive inundations of the flood plain. However, in reaches without flood plains evaporation may account for less than one per cent of the river flow and is then not included in the CWB calculation.

During a period of flood plain inundation and reservoir filling a proportion of river water in the design reach is accumulated in the soil and zone of aeration thus increasing its moisture content. After the water has receded from the flood plain and when reservoir levels are drawn down, some of the accumulated water is evaporated and this has to be included in the CWB calculation. These elements of the CWB have to be estimated in the most objective way on the basis of the experimental data after investigating the water balance of the appropriate water body. Methods of computing the discharge for the elements have been described (Anon,

1977a).

3 . 1 . 5 . 6 Change of Stored Moisture

During inundations of the flood plain and reservoir filling some of the water accumulates in the soil and sub-surface zone of aeration, and a further part increases groundwater storage below the flood plain. In a dry period some of the water is evaporated, flows away or

increases groundwater storage. This causes a change in the moisture content in the zone of aeration and in groundwater storage which has to be estimated from observational data for soil, sub-surface moisture and groundwater levels (Anon, 1977a; Brown et at., 1972).

3 . 1 . 5 . 7 Ice Formation and Melting

In the autumn and winter periods a proportion of channel water may form into ice, which on melting in the spring increases the flow in the design reach. The effect on river discharge due to ice formation and melting is determined from changes in the volume of ice as indicated by observational data (Anon, 1977a).

3 . 1 . 5 . 8 Areal Definition of Flood Plain Reaches

To determine particular CWB components in reaches with flood plains (Q„,» Q x , Ç) , Q„„, Q,„) gj. bu at ¿AM Ala it is necessary to include the following elements in terms of water discharge: precipitation, ice, evaporation from inundated and exposed or dry reaches of flood plains, change in moisture content of the soil and sub-surface zone of aeration and groundwater storage. The areal

definition of flood plain inundations and their exposed reaches enables these components to be calculated.

3.2 Hydrograph Analysis 3.2.1 Flow Separation

Improvements in the methods of assessing the interaction between surface water and groundwater are closely connected with the development of computation techniques to determine the various genetic components of river flow. Flow separation should be carried out as objectively as possible and only in association with independent estimates of the genetic components from detailed water balance studies for representative and experimental basins (Brown et al., 1972;