THE INS AND OUTS OF GROUND-WATER FLOW IN BASINS OF THE SOUTHWEST

Ground-water withdrawals in basins in the Southwestern United States have consequences related to removal of water from storage in the areas of withdrawals as well as consequences related to alteration of inflow to and outflow from basin ground-water systems. Consequences of removal of water from storage occur over intervals on the order of decades. In contrast, consequences related to alteration of inflow and outflow occur over intervals from decades to centuries. Principles that dictate ultimate responses of inflow and outflow to ground-water withdrawals, although simple, are not well understood by many water managers and ground-water scientists. Some of these principles are (1) a sustained ground-water withdrawal will eventually result in increased rates of inflow and (or) decreased rates of outflow equal to the rate of withdrawal, (2) a withdrawal that continues for a time and then ceases will increase inflow and (or) decrease outflow by a volume equal to the volume withdrawn, (3) in the basins of the Southwest, withdrawals are much more likely to decrease outflow than to increase inflow, and (4) a withdrawal in a bedrock aquifer near a basin margin can capture recharge (inflow) from an adjacent hydraulically connected basin. Factors such as water temperature and chemistry, ground-water age, aquifer geometry, and aquifer properties are not relevant in determining the total ultimate effect of a withdrawal on a basin hydrologic budget. These and other factors, however, may be relevant in determining the time period over which a withdrawal alters inflow and outflow and which particular inflow and outflow components are most affected. To help in the management of ground-water systems, scientists analyzing effects of withdrawals should consider ultimate effects of the withdrawals and provide society with sound information on when and where inflow and outflow components might be changed.