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A GAINING STREAM Flow direction ... \ Water table ... .... Saturated zone -- B LOSING STREAM Flow direction Water table \ ;.-""" ~- '-, - C LOSING STREAM THAT IS DISCONNECTED FROM THE WATER TABLE Flow direction Unsaturated zone I Water table ... ~ - Figllre 12. Interaction of streams and grollnd water. (Modified from Winter and others, 1998.) Gaining streams (A) receive water from tile grollnd-water system, whereas losing streams (B) lose water to tile ground-water system. For ground water to discharge to a stream channel, tile altitude of the water table in tile vicinity of the stream must be higher than the altitude of the stream-water surface. Conversely, for surface water to seep to ground water, the altitude of tile water table in the vicinity of tile stream must be lower tllan the altitllde of the stream surface. Some losing streams (C) are separated from the saturated ground-water system by an unsatur- ated zone. A pumping well can change the quantity and direction of flow between an aquifer and stream in response to different rates of pumping. Figure 13 illustrates a simple case in which equilibrium is attained for a hypothetical stream-aquifer system and a single pumping well. The adjustments to pumping of an actual hydrologic system may take place over many years, depending upon the physical characteristics of the aquifer, degree of hydraulic connection between the stream and aquifer, and locations and pumping history of wells. Reductions of streamflow as a result of ground-water pumping are likely to be of greatest concern during periods of low flow, particularly when the reliability of surface-water supplies is threatened during droughts. At the start of pumping, 100 percent of the water supplied to a well comes from ground-water storage. Over time, the dominant source of water to a well, particularly wells that are completed in an unconfined aquifer, comrnonl y changes from ground-water storage to surface water. The surface-water source for purposes of discussion here is a stream, but it may be another surface- water body such as a lake or wetland. The source of water to a well from a stream can be either decreased discharge to the stream or increased recharge from the stream to the ground-water system. The streamflow reduction in either case is referred to as streamflow capture. In the long term, the cumulative stream- flow capture for many ground-water systems can approach the quantity of water pumped from the ground-water system. This is illustrated in Figure 14, which shows the time-varying percentage of ground-water pumpage derived from ground-water storage and the percentage derived from streamflow capture for the hypothet- ical stream-aquifer system shown in Figure 13. The time for the change from the dominance of with- drawal from ground-water storage to the domi- nance of streamflow capture can range from weeks to years to decades or longer. 31