Determination of sustainable ground water use

Scientists and engineers worldwide are challenged by the need to assess the availability of freshwater supplies, and to ensure that sources are sustainable in the long-term. The availability of water has become a serious concern for many communities, large and small, in the United States, where rapid population growth has forced the need to secure additional water from new sources. The United States is planning to conduct a national water census from 2009 to 2019 to assess the availability of water and to better predict the sustainability of ground-water supplies. An important definition of sustainable ground-water use involves the societal willingness to accept the consequences of a given level of development (Alley and Leake, 2004). In other words, ground-water pumping will have consequences at virtually all levels of use. Some consequences are negligible, and others are more serious, such as the reduction of flow to a stream or spring. To complicate matters, the effects may take decades to centuries to be observed. A determination of sustainability for ground water is informed by hydrologic budgets and ground-water flow models that incorporate estimates of aquifer properties. In its simplest form, a hydrologic budget is constructed by measuring or estimating the terms in the following equation:
Inflows = Outflows ± Storage Change.
Determination of sustainability with water budgets is most robust when independent estimates of budget components can be made. The most important inflow term is recharge, but it is also difficult to measure. Outflow terms include discharge to streams, evapotranspiration, and pumpage. Storage change can be estimated by systematic measurement of water levels in wells or by more direct measurement of mass change using gravimetric measurements. In the latter approach, a network of benchmark gravity stations and systematically repeated measurements are used to estimate storage change for a time interval. For example, in the Tucson Basin, Arizona, storage change has been measured since 1997, which has permitted an annual estimate of recharge (Pool and Anderson, 2007). Recharge, estimated by measuring storage change and ground-water pumping, averaged 70,000 acre-feet per year but was as much as 380,000 acre-feet per year during wet El Nino conditions. Understanding time-scale effects from pumping stresses is commonly critical. For example, the U.S. Geological Survey’s ground-water flow model (MODFLOW) was used to gain insights about sustainable ground-water use in northern Arizona. The model was used to simulate the effect of proposed pumping from the regionally extensive C-aquifer on the base flow in Clear Creek (about 25 miles away). The maximum reduction in flow simulated by the model occurred 80 years after pumping began but about 30 years after pumping ceased (Leake and others, 2005). The model was invaluable in projecting future change about consequences that may take decades to centuries to be detected.