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OVER VIEW 25 <br />uniformly distributed over time throughout the year. On a sustained <br />basis, perhaps only 25 percent of the water is available on average, so <br />that the consumption rate is thus quadrupled to 18.7 percent relative <br />to the sustained yield-still a small percentage of the total available <br />resource. A much higher percentage is extracted and recycled. The <br />U.S. Geological Survey (Solly et al., 1988) estimated that in 1985 a <br />total of 338 billion gallons of water per day was used for off-stream <br />purposes (Table 1.2). This represented approximately 15 percent of <br />the total resource, or 61 percent of the sustained yield. In-stream <br />uses were an order of magnitude larger. The production of hydropower <br />utilizes more than 3,000 billion gallons per day, an amount that ex- <br />ceeds the available supply but includes the repetitive use of water as <br />Miles <br />Alaska <br />(!9) <br />P ~ Kawai <br />~ Oahu Q o olokai <br />o~ Maui <br />Hawaii <br />(20J Hawaii <br />0 150 Miles <br />Caribbean (21) <br />Puerto Rico p° ~ <br />0 80 Miles ~ <br />FIGURE 1.2 Hydrologic units of the United States.. Source: U.S. Geological <br />Survey, 1987. <br />