perception of their size, with basins being much larger than watersheds and typically composed of many watersheds. In the United States, "basin" is often used to mean a large riverine drainage system. Within a watershed or basin, water moves both on and below the surface. Aquifers are generally bounded by subsurface divides similar to surface features that separate watersheds. Often the boundaries of basins are not as obvious as those of watersheds, and aquifers may underlie and be common to several surface watersheds. Geologic strata that are tilted counter to the topography can conduct water in the opposite direction from topographic surface slopes. Large, confined aquifers may underlie smaller, unconfined zones that conform more closely to the surface topography. Because aquifers may be connected, the availability and quality of the ground water within them may be regional issues, defined by both surface and subsurface topography. The three-dimensional nature of aquifers is not generally well understood and is rarely considered in modeling for management applications. The condition and characteristics of a given aquifer are determined by the hydrologic cycle and by anthropogenic modifications in the hydrologic cycle.
The hydrologic cycle can be usefully depicted on both global and basinwide scales. In the global hydrologic cycle, water can be transferred from one location to another and transformed among the solid, liquid, and gaseous phases, but the total amount of water remains the same. From a basinwide perspective, the fact that water can be transferred from one basin to another means that specific basins can experience gains and losses in the total amount of water. This is an important concept in that the quantity of water in a basin can be depleted, whereas the total amount of water remains the same as it cycles among the various basins. The hydrologic cycle is depicted from a basin perspective in Figure 2.1. Precipitation is the pathway by which water enters the basin. Evaporation and transpiration, along with stream flows, are the principal pathways by which it leaves. Runoff, which is overland flow, can be augmented by interflow, which operates below the surface but above the water table, and by base flow, which refers to the discharge to streams from the saturated portion of the system. Infiltration of water into the subsurface is the ultimate source of both interflow and recharge to the ground water. Ground water recharge, defined as the portion of infiltration water that reaches the ground water, represents the replenishment of ground water supply.
The quantity of water stored in an aquifer can be characterized over time by accounting for inflows and outflows according to the following expression:
Change in storage = recharge - depletion