Assessment of the ground-water component of streamflow

Subdivision of total streamflow into the surface and subsurface components is necessary in investigations concerned with the water balance of catchments and the relationship between surface water and ground water. Such a subdivision enables an estimate to be made not only of the proportion of ground water in total streamflow but also of the ground-water resources of the area draining to the stream.

Commonly the only possible method of assessing the ground-water resources on a regional basis may be to estimate ground-water discharge to streams and interpret the result in the light of known hydrological and hydrogeological conditions.

Streamflow is considered to consist of three components, surface run-off, interflow and ground-water discharge. The first two are collectively described as direct run-off. Surface run-off represents that part of precipitation which flows directly over the land surface into stream channels. Interflow is that part which moves laterally through the soil zone and is discharged to streams relatively quickly after infiltration and without reaching the zone of saturation. Ground-water discharge is that part of precipitation which reaches the zone of saturation and contributes temporarily to ground-water storage before being discharged from springs and seepages into a stream system. The ground-water com- ponent of streamflow may include bank storage; this is water which infiltrates the banks of a stream when the stream stage rises and which is gradually released as the stream level falls. The volume of water comprising bank storage depends upon the maximum height of the stream stage during periods of high run-off, the length of time the high stage is maintained, and the intrinsic permeability of the deposits forming the stream banks and contiguous areas.

The components of the streamflow have different recession characteristics and the recession of each can be approximated to a straight line when the logarithm of streamflow is plotted against time, the recession equations being of the general form

5.4(1)

kt Q t = Q o

where :

Qt is discharge after time t;

Qo is initial discharge;

k is a negative recession constant (Barnes, 1939).

There is usually a time lag between peak discharge of the various components, inter- flow occurring after surface flow and ground-water discharge after interflow (Fig. 5.4a).

Because each component tends to overlap the previous one in time, the recession curve of streamflow, as defined by a semi-logarithmic plot of discharge against time, may be curvilinear in form.

Where more than one aquifer occurs in a catchment, each makes a varying contribution to streamflow. This tends to mask the basic simplicity of the recession curve as discussed above. The hydrogeological properties of the individual aquifers determine the extent of their individual contributions, both in time and volume, and hence their effects on the over-all form of the stream hydrograph.

The percentage of the total flow of a stream derived from ground-water discharge depends upon a variety of factors but principally the distribution and intensity of precipitation and the nature of the surface deposits within the catchment.

Even in simple hydrogeological areas, the recession curve of a stream hydrograph representing that part due to ground-water discharge may approximate a straight line (when flow is plotted logarithmically) only in the relatively early stages of recession.

The over-all curve tends to be curvilinear; this has been explained as being due to the

5.4 page I

Ground-water studies

5.4a. Separation of components of streamflow.

composite form of the curve reflecting areal variations in intrinsic permeability of the aquifer contributing ground water to the stream, variations in head of water in the aquifer and losses due to evaporation from the riparian zone (Ineson and Downing, 1964).

Meinzer and Stearns (1929), Sherman (1932), Snyder (1933), and others considered the problem of determining the ground-water component of streamflow and methods of differentiating the subsurface components of streamflow have been discussed by Kalinin and Abalyan (1957), Riggs (1953), Snyder (1939) and others. Kunkle (1965) showed that differences between the specific electrical conductivity of surface and ground waters could be used to separate the two components. In a similar manner any property, for example, temperature, alkalinity, beta activity, etc., which distinguishes the two com- ponents can be used.

The determination of ground-water discharge has been applied locally to the assessment of ground-water resources in the United Kingdom, as for example by Ineson and Downing (1965) and Downing and Williams (1969). Wundt (1953) determined the sub-

5.4 page 2

Defining the wuter Idunce

{IJ,

I I

E O m ilc o+-

1 !;/

P h

-

+Ol +o1 40 Y

c c C

Y C

c

Y C

Y C ù e 3

-I d

Groitnd-water stirdies

surface discharge to tributaries of the Upper Danube from records of the lowest stream discharge for a number of years.

In stream basins where only one aquifer contributes to ground-water discharge, the analysis of the stream hydrograph to assess this component may be relatively simple.

In large and complex river basins it may be necessary to study the contributions made by different aquifers to the ground-water components of river flow from the basin as a whole.

Such studies involve consideration of the relationship between hydrogeology and hydrology, including analyses of the surface and ground-water characteristics of the basin, definition of the hydraulic connexion between ground and surface waters, differentiation of the components of river flow during the entire period for which records are available, determination of the volume of ground-water discharge to streams during different seasons, and determination of the basic parameters controlling ground-water discharge.

The theoretical basis for the method has been described by Kudelin (1960).

The dynamics of ground-water discharge to streams from difl’erent water-bearing horizons are determined by the occurrence of and recharge to the unconfined and confined aquifers in the river basin and by the position of the ground-water surface relative to the stream stage. The following typical relationships between ground and surface waters may be identified: (a) aquifers having no hydraulic connexion with a stream; (b) aquifers having a constant hydraulic relationship with a stream; (c) aquifers having a periodic or intermittent hydraulic relationship with a stream.

In considering the conditions of a stream fed by ground-water discharge the following cases m a y be established: (a) permanent or intermittent hydraulic relationship between ground and surface water prevails; (b) the stream drains several aquifers, each being characterized by a different form of hydraulic relationship with the stream; and (c) ground- water discharges into the stream from both unconfined and confined aquifers. The ap- proach adopted in analysing stream hydrographs to assess ground-water discharge depends upon the hydraulic relationships existing in the basin (Fig. 5.4b).

Aquifers not connected hydraulically with a stream display a flow régime similar to that of surface run-off except that the peak of ground-water discharge is less pronounced and lags behind the peak of the stream stage as discussed above. A n increment of ground- water discharge is added to the stream in the flood period (Fig. 5.4b, col.1).

A water-level rise in a stream which is in hydraulic continuity with an adjacent aquifer changes bank storage by decreasing the hydraulic gradient in the aquifer and conse- quently the discharge of ground water into the stream. Thus the ground-water flow régime is dependent upon the stream régime with periods of peak ground-water discharge corresponding to periods of minimum surface run-off and vice versa (Fig. 5.4b, col. 2).

During a flood when the stream rises above the ground-water level there is a reversal of hydraulic gradient with infiltration of surface water into bank storage. A s the stream stage declines, the hydraulic gradient again reverses and water is returned to stream from bank storage. The net contribution to stream flow from an aquifer connected hydraulically with a stream is relatively small during a flood period.

The ground-water flow régime from a water-bearing horizon having an intermittent connexion with a stream is classified as ‘mixed’ (Fig. 5.4b, col. 3). At low stream stages the régime is characterized by an aquifer that is not hydraulically connected to the stream, at high stages by an aquifer hydraulically connected to the stream.

The size of the drainage basin and the areal distribution of the ground-water discharge must be considered in the analysis of a stream hydrograph for the determination of the ground-water discharge from aquifers hydraulically connected to the stream. Tn com- paratively large basins the movement of a Hood wave down the river may affect the volume of ground-water discharge to differing degrees in different reaches of the river.

Unconfined ground-water discharge may not have appeared in the stream flow measured at a gauging station in the lower part of the basin, although ground-water discharge may have occurred already in the upper reaches of the basin and may be in transit down

Defining the water balance

the stream channel. In this event the analysis of the stream hydrograph requires data relating to the beginning and the end of the flood in the upper reaches of the basin and to the time of travel of the water.

To illustrate the preceding discussion consider a stream basin with a gauging station at its outlet and containing only one aquifer, hydraulically connected to the stream. The task is to separate the ground- and surface-water components of the hydrograph shown in Figure 5 . 4 ~ representing a basin-wide flood that started on 23 March. Ground-water discharge to the stream obviously ceases at the time of the sudden rise in stream stage and discharge. Thus, in this example, the ground-water component, which is dominant most of the time, is separated from the surface-water component, which dominates the flood period, by the vertical straight lines AB and DE. However, the ground-water discharge that entered the stream channel in the upper reaches of the basin before 23 March continues moving downstream with the flood wave. Having calculated the velocity of stream flow from the movement downstream of the flood peak and knowing the distance from the stream headwaters to the basin outlet, it is possible to determine the time interval during which some ground water wjll still be included in the flow passing the gauging station. Thus, if the velocity of water travel is 50 km/day and the distance from the headwaters to the outlet is 250 kni, the ground-water discharge willbe moving downstream for 5 days and that portion derived from the remotest parts of the basin will

1,300

i.zoa

1,l O0 1,000

goo 800 700 600 500 400 300 200

I 100

.

o

C

I

0

Surface run-off Ground-water discharge

Time (months)

FIG. 5.4~. Stream hydrograph for gauging station at basin outlet.

Groimd-writer stiidies

pass the gauging station on 28 March. The point Fis plotted on the hydrograph to reflect zero ground-water discharge on this date. The straight line BF is then regarded as rep- resenting the declining ground-water discharge passing the gauging station. Assume further that the flood ended at the headwaters on 16 April and at the basin outlet on 8 May. The latter date corresponds to point D shown on the hydrograph. From 16 April ground water can again discharge into the stream channel in the headwaters of the basin aiid be reflected in the flow past the gauging station in 5 days, that is on 21 April. This event is shown by point G on the hydrograph, and the straight line GD is regarded as representing the increasing ground-water discharge passing the gauging station. The damming effect of the higher sta.ges on the ground-water discharge during the flood event willhave ended over the entire basin by 8 M a y and the stream willhave reverted entirely to base flow that reflects only the ground-water discharge.

The techniques to be adopted in the analysis of hydrographs when ground-water dischaige into a stream is of the descending type may be developed by considering the dynamics of the discharge from an aquifer having no hydraulic connexion with the stream. The discharge is determined by the régime of spring flow, the form of which is seeps and springs. When the necessary information concerning the spring régime is lacking, a less accurate but sufficiently practical method of hydrograph separation niay be devised by considering the general form of ground-water discharge in the basin.

Makarenko (1948) has devised a method for computing the annual ground-water discharge, QLw, from aquifers not hydraulically connected to a stream using the relation:

Qsw qki

+

qkz

+

...

+

^qk12 5.4(2)

where q is the minimum monthly base flow of record for the stream, which is arbitrarily taken as the unit of ground-water discharge into the stream; ki, k2, ... kiz are monthly constants for adjusting the q factor throughout the year in terms of the total discharge pattern of key springs.

Thus, the coefficient, k3, is the ratio of the average March discharge oi the key springs,

Q3, to the minimum March discharge over the period of the record, Qmin, i.e. k3

=

Q3,/Qmin. For the year in which the minimum March value occurred ks

--

1; for all other years k3 is greater than 1.

The method depends upon the existence of a relationship between the discharge of a key spring and total ground-water discharge to the river system, a relationship which may be devised graphically (Bleasdale et al., 1963). Similarly, an estimate of ground-water discharge can be obtained by establishing a relationship between ground-water levels in an aquifer and ground-water discharge to a stream (Rasmussen and Andreason, 1959;

Schicht and Walton, 1961).

The mixed type of ground-water discharge to a stream (Fig. 5.4b, col. 3) from aquifers, some of which are hydraulically connected with and others not connected with that stream, is found comnionly in river basins. Separation of the stream hydrograph, in this case, is accomplished in two stages. First, the hydrograph of ground-water discharge from the water-bearing horizon not hydraulically connected with the stream is drawn, using as appropriate some of the principles described above. Then, the flow from the aquifer hydraulically connected with the stream is plotted separately beneath the fist hydro- graph. Graphical summation of these two hydrographs shows the total ground-water discharge from the aquifers.

The nature of the hydraulic connexion between aquifers in the basin under study and the streams m a y be ascertained by analysing in detail the graphical and hydrological relationships along selected reaches of the stream course. The selection of a particular technique for analysing a stream hydrograph should be based on consideration of all the known facts relating to ground-water discharge in the basin under examination;

choices should then be made as to which aquifer (or aquifer complex) is the basic source of ground-water discharge and which aquifer(s) can be neglected.

5.4 page 6

Defining the wafer balance

Over a long period of time the average annual value of ground-water discharge is equal to the value of recharge to the ground-water reservoir. Therefore, separation of the ground-water discharge component from stream run-off by analysing the stream hydrographs for a period of many years makes it possible to determine the mean annual replenishment of the ground-water resource in the zone of intensive water circulation for entire or selected parts of river basins.

The preceding discussion has been concerned primarily with ground water in the zone of active circulation; that is, ground water stored temporarily in the relatively shallow portions of the earth’s crust, and,which is replenished annually by precipitation. Com- monly the ground-water discharge for a one-year period is expressed quantitatively as a layer of water so many millimetres in thickness.

The annual mean modulus of ground-water discharge (Mn, in l/sec/km2), also termed coefficient of base flow or ground-water discharge, may be computed from the relation :

3.16

x

10-5en

5.4(3) Mn = F

where Qn is the rate of ground-water discharge (m3/yr) determined by planimetering the separated stream hydrograph, and Fis the area (km2) of the river basin above the river gauging station.

The minimum modulus of ground-water discharge can be obtained without specific separation of the stream hydrographs, if it is assumed that the minimum mean-monthly flow of the stream is entirely ground-water discharge. Annual values of ground-water discharge, expressed as the thickness of a layer of water, represent infiltration into an aquifer (or aquifer complex), less losses due to evaporation and transpiration. The amount of infiltration

(Y,,,

in mm) equivalent to the annual ground-water discharge is calculated from the relation:

Q n

Y,, = 0.001

-

F ^5.4(4)

where Q n and F are as in Equation 5.4(3). The coefficient of ground-water discharge (Kn) is the ratio of that discharge to the amount of precipitation falling on the river basin under study expressed as a percentage. This coefficient is important because it shows the portion of precipitation that recharges the ground-water system in the zone of active circulation. The coefficient is computed from the relation:

0.0001 Q n

where N is the precipitation in mm per year and the other factors are as defined pre- viously. A related coefficient may be derived from the ratio of the ground-water discharge to total run-off in the stream. This coefficient characterizes îhe ground-water component in the stream run-off.

W h e n values for average ground-water discharge have been determined from available records for all existing gauging stations, the results are generally displayed in m a p form.

This allows the areal variations in ground-water discharge throughout the region under investigation to be shown by contouring or shading.

T o estimate the natural discharge from and replenishment to each aquifer contributing to the stream network, an analysis must be made of the ground-water conditions throughout the region under study. The analysis must consider the distribution, thickness, lithological composition and permeability of the individual water-bearing layers, and must indicate the degree to which each aquifer contributes ground-water discharge to the stream.

A n approximation of the contribution from a particular aquifer can be computed from the relation :

5.4page 7

Graurirl-water stirdies

5.4(6)

where :

MnL is the modulus of ground-water discharge from the single aquifer under study, in l/sec/km2;

MTL is the modulus of total ground-water discharge from all aquifers contri- buting to the stream network, in l/sec/km2;

Kjmjlj is the sum of the products of the transmissivity (product of hydraulic

'

ⁱ conductivity, K, aquifer thickness, m), and hydraulic gradients (I) for the

ïz individual aquifers discharging into the streams;

Ki, mi, li, are similar factors, respectively, for the single aquifer under study.

Investigations of the ground-water discharge and replenishment may be divided into three stages as follows :

1. Collection and analysis of all available data on the physiogeographical, hydrological and geological conditions throughout the basin; determination of the degree to which the principal aquifers contribute to the ground-water discharge of the streams, and the nature of their hydraulic connexion with the streams; preparation of contour maps showing configuration of the water table or piezometric surface of the principal aquifers; preparation of maps showing areal distribution throughout the basin of the different types of ground-water discharge; and an indication of the particular technique or techniques to be adopted for determining the components of stream hydrographs.

2. Location and establishment of a network of stream-gauging stations with arrange- ments for regular discharge measurements throughout the basin. The number of discharge-measuring sites or sections is determined by the complexity and variability

2. Location and establishment of a network of stream-gauging stations with arrange- ments for regular discharge measurements throughout the basin. The number of discharge-measuring sites or sections is determined by the complexity and variability

Dans le document Ground-water studiesl (Page 99-109)