Introduction.to.phytoremediation.of.contaminated.groundwater.2011

the inflow and outflow amounts of water in a basin provided the foundation for early conceptual models of the hydrologic cycle and the framework to quantify this cycle in terms of a water budget. Because the total quantity of water in the earth is finite, it can be handled mathematically using continuity equations. Thus, for any particular valley or basin, such as the Seine River valley studied by Perrault (1674), we can state the following equality shown in Eq. 2.1: WInflow ¼ WOutflow ; (2.1) where WInflow refers to water that enters a representative basin, such as precipitation per unit time, and WOutflow refers to water that leaves a representative basin, such as by river discharge per unit time. Equation 2.1 represents steady-state or equilibrium conditions, such that the amount of water entering a basin during a certain period of time is completely removed from the basin during the 2.2 The Water Budget and Hydrologic Cycle 31 same time, with no storage, S, of water in either lakes or groundwater. Mathematically, this can be expressed as the change in storage, DS, over time, Dt, equals zero, or DS/Dt ¼ 0. This water-budget approximation can be considered valid when looking at the inflows and outflows of water for a particular basin over a period of many years, in which any changes in storage that do occur can be considered small relative to the larger quantities of inflow and outflow. When shorter timeframes are studied, however, DS/Dt will not be 0, and a…