如题所述
Rivers passing through a region underlain by a phreatic aquifer ( and in special cases even by a confined aquifer) may either contribute w ater to the aquifer or serve as its drain. M uch of the low w ater flow in streams ( base flow ) is derived from groundw ater w hose w ater table elevations in the vicinity of a stream are higher than the stream. Such streams are called effluent streams ( Figure 9. 1 a) . On the other hand，w hen the w ater level in a stream is higher than the w ater level in an adjacent ( or underlying ) aquifer，w ater w ill flow from the river to the aquifer. The river is then called an influent river ( Figures 9. 1 b，c) . W hen a stream cuts through an impervious layer，establishing a direct contact w ith an underlying confined aquifer，the stream may be either an influent one or an effluent one， depending on w hether the piezometric heads in the aquifer are above or below the w ater level in the stream ( Figure 9. 1 d) . The same stream can be an influent one along one stretch and an effluent one along another. Or，it can be both influent and effluent at thesame point，as show n in Figure 9. 1 e.
Obviously，the entire discussion presented above is based on the assumption that the river bed is not completely clogged and that w ater can flow freely through the river bed. Otherw ise， there is no hydraulic contact betw een the w ater in the river and in the aquifer and no relationship exists betw een the tw o. It is possible that the profile of a stream is such that its deeper part， accommodating for low flow s，is completely clogged，w hile above a certain level，the river bed is pervious ( Figure 9. 1 f) .
When the w ater table under an influent stream is sufficiently deep，a mound is formed in the former by the percolating w ater ( Figures 9. 1 c，f) .
The volume of w ater contributed to an aquifer by streamflow ( or drained into a stream from an aquifer) ，is part of the regional w ater balance. The rate of flow in either direction， also w hen the stream bed is partly clogged，can be calculated.
In view of the different possible situations show n in Figure 9. 1，the river may play several roles w hen solving a groundw ater forecasting problem.
1) The river may act as a boundary of specified head to the flow domain in the adjacent aquifer. This situation is show n in Figures 9. 1 a，b，d，e. We have in mind an aquifer type flow ，w hich is based on the assumption of essentially horizontal flow in the aquifer，so that the shape of this boundary is a curve in the xy plane. This is a good approximation，overlooking the details of the flow net under the stream. A somew hat different boundary condition should be employed w hen the river bed is semipervious.
Figure 9. 1 River-aquifer interrelationships a. Effluent stream: groundwater drains into stream; b. lnfluent stream ( shallow water table ) : river contributes to groundwater flow ; c. Influent stream ( deep water table ) : river contributes to groundwater flow ; d. lnfluent stream ( piezometric surface B) ，or effluent one ( piezometric surface A) intersecting a confined aquifer; e. A stream which is both influent and effluent; f. A partly clogged influent stream
2) The river serves as a source，contributing w ater to the aquifer. The rate of leakage depends on the depth of w ater in the river and on the permeability of the river bed. This can be a line ( actually curve) source，or a strip of some w idth，w hen the river is sufficiently w ide. The main point is that the rate of seepage is independent of the w ater levels in the aquifer ( Figures 9. 1 c，f) .
Thus，Depending on the elevation of the w ater table，both situations ( a) and ( b) above are possible at different periods of time for the same stretch of river ( and certainly for different stretches of the same river) .
Finally，w e note that w hen a river is sufficiently large in terms of the rate of flow ，the exchange of w ater betw een it and an aquifer practically does not affect the flow ，and hence the depth of flow ，in it. How ever，in small streams，the leakage itself may low er the w ater level in the stream and even completely dry up the stream. We may encounter a passage from condition ( a) to ( b) as defined above.
Often，the type of situation to be realized is not know n a priori w hen solving a forecasting problem，and some trial and error，or iteration technique is required. ( Source: Bear，1979)
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