Subsurface Flow

the movement of subterranean water under the influence of piezometric head and the force of gravity. An integral part of the water cycle, subsurface flow characterizes the natural resources of subterranean water, which is under the draining influence of rivers, lakes, seas, or waterless negative landforms. Subsurface flow is expressed as a modulus (liters per sec per sq km) or as a layer of water (mm per year), as well as in cu m per day and cu km per year. In hydrogeological studies, the moduli and coefficients of subsurface flow are determined, indicating (often in percent) what part of the precipitation goes to feed the subterranean water.

In the USSR, the modulus of subsurface flow varies from 0.1–0.5l per sec per sq km on the plains of Middle Asia to 10 l per sec per sq km in the Western Pamirs and 20 l per sec per sq km in the Greater Caucasus.