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<br />The preceding two field examples illustrate <br />some of the complexities associated with the use <br />of water budgets to determine the development <br />potential of a ground-water system. Knowledge <br />of the sources and discharges of water to and <br />from the system and how they change with <br />continuing development is needed to understand <br />the response of ground-water systems to develop- <br />ment, as well as to aid in determining appropriate <br />management strategies and future use of the <br />resource. <br /> <br />The examples discussed here and those <br />in the following chapters illustrate several of <br />the principles summarized by Bredehoeft and <br />others (1982) in their article on the "water-budget <br />myth" and earlier by Theis (1940): <br /> <br />. Some ground water must be removed from <br />storage before the system can be brought <br />into equilibrium. <br /> <br /> <br />. The time that is required to bring a hydrologic <br />system into equilibrium depends on the rate <br />at which the discharge can be captured. <br /> <br />. The rate at which discharge can be captured <br />is a function of the characteristics of the <br />aquifer system and the placement of pumping <br />wells. <br /> <br />. Equilibrium is reached only when pumping is <br />balanced by capture. In many circumstances, <br />the dynamics of the ground-water system are <br />such that long periods of time are necessary <br />before even an approximate equilibrium <br />condition can be reached. <br /> <br />In the next three chapters we discuss in <br />more detail the effects of ground-water develop- <br />ment on ground-water discharge to and recharge <br />from surface-water bodies, the effects of ground- <br />water development on ground-water storage, and <br />water-quality factors affecting ground-water <br />sustainability. <br /> <br />29 <br />