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<br />possible to issue reliable year-in-advance forecasts of water reserves <br />for these regions. The economic health of large regions of the nation <br />rises and falls with available water reserves that depend on yet un- <br />known caprices of nature. <br /> <br />Mpreover, in rare periods of abundance we tend to view the ab- <br />normally plentiful water reserves as a "return to normal". We let <br />ourselves be lulled into complacency that results in declarations of <br />disaster when conditions of dry weather develop that are equally close <br />to normal. <br /> <br />, <br /> <br />WATER SUPPLY CONSIDERATIONS <br /> <br />(a) The atmosphere is our real resource of fresh water. <br /> <br />The atmosphere is man's real fresh water manufacturing plant, <br />distilling fresh water from the oceans. Direct use of ocean water may <br />ultimately become an important basic supply, but the problems of arti~ <br />ficial distillation are formidable, and the economics of the processes <br />now avail able are promising only where the premium value of fresh <br />water justifies extremely high costs. Nature, on the other hand, does <br />a magnificent job of distilling fresh water from the seas and at no cost. <br />Most of this returns to earth as rain and snow. <br /> <br />Where Nature falls down is in keeping the supply lines clear to <br />the places we want water. Moreover, she ignores our demand cycles. <br />She gives us heaviest precipitation in the seasons when we least need <br />it. And she turns on or off the supplies without advance warning, some- <br />times for disastrously long periods. <br /> <br />Dams and reservoirs can be used to retain river water for the <br />future. The larger the reservoir volume compared to the stream flow, <br />the larger its capacity to tide us over dry periods. But as the size <br />of the reservoir increases, its ability to store water does not increase <br />at a steady ratio. Beyond a certain size the storage ability increases <br />only slightly, because increased evaporation and seepage gradually <br />reduce the amount of water available for release downstream. These <br />factors become more severe as the reservoir size increases, as <br />Langbein>:' has shown. <br /> <br />Langbein estimates, for example, that doubling the reservoir <br />capacity of the Colorado River with the completion of the Glen Canyon <br />Dam will increase the water retention by 5%. Any future increase <br />will result in only a fraction of a percent increase in useful retention. <br />If the investment return from such a small increase in usable water <br /> <br />* W. Langbein, U.S.G.S. Circular No. 409, 1959. <br /> <br />4- <br />