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Hypothetical Examples of How Ground-Water Systems Change in Response to Pumping Consider a ground-water system in which the only natural source of inflow is areal recharge from precipitation. The amount of inflow is thus relatively fixed. Further consider that the primary sources of any water pumped from this ground- water system are removal from storage, decreased discharge to streams, and decreased transpiration by plants rooted near the water table. If the above-described ground-water system can come to a new equilibrium after a period of removing water from storage, the amount of water consumed is balanced by less water flowing to surface-water bodies, and perhaps, less water avail- able for transpiration by vegetation as the water table declines. If the consumptive use is so large that a new equilibrium cannot be achieved, water would continue to be removed from storage. In either case, less water will be available to surface- water users and the ecological resources dependent on streamflow. Depending upon the location of the water withdrawals, the headwaters of streams may begin to go dry. If the vegetation receives less water, the vegetative character of the area also might change. These various effects illustrate how the societal issue of what constitutes an undesired result enters into the determination of ground- water sustainability. The tradeoff between water for consumption and the effects of withdrawals on the environment often become the driving force in determining a good management scheme. In most situations, withdrawals from ground-water systems are derived primarily from decreased ground-water discharge and decreased ground-water storage. These sources of water were thus emphasized in the previous example. Two special situations in which increased recharge can occur in response to ground-water with- drawals are noted here. Pumping ground water can increase recharge by inducing flow from a stream into the ground-water system. When streams flowing across ground-water systems originate in areas outside these systems, the source of water being discharged by pumpage can be supplied in part by streamflow that originates upstream from the ground-water basin. In this case, the predevelop- ment water budget of the ground-water system does not account for a source of water outside the ground-water system that is potentially available as recharge from the stream. Another potential source of increased recharge is the capture of recharge that was originally rejected because water levels were at or near land surface. As the water table declines in response to pumping, a storage capacity for infiltration of water becomes available in the unsaturated zone. As a result, some water that previously was rejected as surface runoff can recharge the aquifer and cause a net increase in recharge. This source of water to pumping wells is usually negligible, however, compared to other sources. In summary, estimation of the amount of ground water that is available for use requires consideration of two key elements. First, the use of ground water and surface water must be evalu- ated together on a systemwide basis. This evalua- tion includes the amount of water available from changes in ground-water recharge, from changes in ground-water discharge, and from changes in storage for different levels of water consumption. Second, because any use of ground water changes the subsurface and surface environment (that is, the water must come from somewhere), the public should determine the tradeoff between ground- water use and changes to the environment and set a threshold at which the level of change becomes undesirable. TItis threshold can then be used in conjunction with a systemwide analysis of the ground-water and surface-water resources to determine appropriate limits for consumptive use. Systemwide hydrologic analyses typically use simulations (that is, computer models) to aid in estimating water availability and the effects of 22