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<br />GROUND-WATER DEVELOPMENT, <br />SUSTAINABILITY, AND WATER BUDGETS <br /> <br />A ground-water system consists of a mass of <br />water flowing through the pores or cracks below <br />the Earth's surface. This mass of water is in motion. <br />Water is constantly added to the system by recharge <br />from precipitation, and water is constantly leaving <br />the system as discharge to surface water and as <br />evapotranspiration. Each ground-water system <br />is unique in that the source and amount of water <br />flowing through the system is dependent upon <br />external factors such as rate of precipitation, loca- <br />tion of streams and other surface-water bodies, <br />and rate of evapotranspiration. The one common <br />factor for all ground-water systems, however, is <br />that the total amount of water entering, leaving, <br />and being stored in the system must be conserved. <br />An accounting of all the inflows, outflows, and <br />changes in storage is called a water budget. <br /> <br />Human activities, such as ground-water with- <br />drawals and irrigation, change the natural flow <br />patterns, and these changes must be accounted for <br />in the calculation of the water budget. Because any <br />water that is used must come from somewhere, <br />human activities affect the amount and rate of <br />movement of water in the system, entering the <br />system, and leaving the system. <br /> <br /> <br />Some hydrologists believe that a pre- <br />development water budget for a ground-water <br />system (that is, a water budget for the natural <br />conditions before humans used the water) can <br />be used to calculate the amount of water available <br />for consumption (or the safe yield). In this case, <br />the development of a ground-water system is <br />considered to be "safe" if the rate of ground-water <br />withdrawal does not exceed the rate of natural <br />recharge. This concept has been referred to as <br />the "Water-Budget Myth" (Bredehoeft and others, <br />1982). It is a myth because it is an oversimplifica- <br />tion of the information that is needed to under- <br />stand the effects of developing a ground-water <br />system. As human activities change the system, <br />the components of the water budget (inflows, <br />outflows, and changes in storage) also will change <br />and must be accounted for in any management <br />decision. Understanding water budgets and how <br />they change in response to human activities is <br />an important aspect of ground-water hydrology; <br />however, as we shall see, a predevelopment water <br />budget by itself is of limited value in determining <br />the amount of ground water that can be with- <br />drawn on a sustained basis. <br /> <br />15 <br />