Long-term water usage can be no more than that which is naturally available through the hydrologic cycle. To help in the determination of the hydrologically desirable water usage from the surface and groundwater sources, a simple, idealized long-term analysis of water availability based solely on the natural hydrologic cycle is suggested. The concept of a hydrologic replacement time is introduced to determine the water availability from ground and surface water sources. Hydrologic replacement time is defined as the average time for water to complete a trip through a particular phase of the hydrologic cycle. The hydrologically desirable amount of water utilization from groundwater and surface water is proportional to the amount available from these sources. The available water is calculated on the basis of hydrologic replacement time, volume and any allowable water source depletion. The water availability problem is addressed at two spatial scales - global and continental. For each spatial two cases are considered: one in which no groundwater depletion is allowed and one in which a small amount of groundwater depletion is permissible. On the basis of the hydrologic characteristics of groundwater and surface water sources, it is desirable to utilize more surface water than groundwater for both spatial scales.
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