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Regardless of the amount of water with- drawn, the system will undergo some drawdown in water levels in pumping wells to induce the flow of water to these wells, which means that some water initially is removed from storage. Thus, the ground-water system serves as both a water reservoir and a water-distribution system. For most ground-water systems, the change in storage in response to pumping is a transient phenomenon that occurs as the system readjusts to the pumping stress. The relative contributions of changes in storage, changes in recharge, and changes in discharge evolve with time. The initial response to withdrawal of water is changes in storage. If the system can come to a new equilib- rium, the changes in storage will stop and inflows will again balance outflows: Pumpage = Increased recharge + Decreased discharge Thus, the long-term source of water to discharging wells is typically a change in the amount of water entering or leaving the system. How much ground water is available for use depends upon how these changes in inflow and outflow affect the surrounding environment and what the public defines as undesirable effects on the environment. In determining the effects of pumping and the amount of water available for use, it is critical to recognize that not all the water pumped is necessarily consumed. For example, not all the water pumped for irrigation is consumed by evapotranspiration. Some of the water returns to the ground-water system as infiltration (irriga- tion return flow). Most other uses of ground water are similar in that some of the water pumped is not consumed but is returned to the system. Thus, it is important to differentiate between the amount of water pumped and the amount of water consumed when estimating water availability and developing sustainable management strategies. The possibilities of severe, long-term droughts and climate change also should be considered (see Box B). Long-term droughts, which virtually always result in reduced ground-water recharge, may be viewed as a natural stress on a ground-water system that in many ways has effects similar to ground-water withdrawals-namely, reductions in ground- water storage and accompanying reductions in ground-water discharge to streams and other surface-water bodies. Because a climate stress on the hydrologic system is added to the existing or projected human-derived stress, droughts repre- sent extreme hydrologic conditions that should be evaluated in any long-term management plan. Consideration of climate can be a key, but underemphasized, factor in ensuring the sustainability and proper management of ground-water resources. 19