Obviously, the trough must also have an overflow that is flowing at the same rate. If a small pump is introduced into the trough and begins pumping continuously, then the overflow will soon be depleted by an amount equal to the rate of pumping. In many ground water systems, surface water supplies are ultimately depleted by an amount of water equal to volumes pumped and consumptively used. The effects of pumping on surface water sources are normally greatly attenuated relative to the horse trough analogy. The effects of pumping on surface water supplies may be distributed over years, or even decades, depending on the size and properties of the aquifer.
Difficulties arise in determining the timing, location, and magnitude of the impacts.
The degree to which ground water pumping depletes surface water supplies is dependent on several features of the particular basin. These factors are discussed in the following paragraphs. The degree of river and aquifer interconnection is of great importance in controlling the amount of surface water depletion resulting from ground water pumping. If a river is perched above an aquifer, ground water pumping has no effect on river flow. If the river is not perched, but sediments have accumulated in the riverbed, or the river only slightly penetrates into the aquifer, then the hydraulic communication between the river and aquifer may be limited.