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CO" 1 3"[' Il d i 0 186 Essr:nlinls fOT Optimum Use 0/ (;rounc, nier Resources meant some terrific harrIes for other waters which are surplus at the moment but certain to be needed far future requirements somewhere. A prime objective of efficient management of ground-water reSQurces would of course be to provide assurance of continuing supply far optimum use. Thus, it would provide assured water supply in lieu of the anxiety that breeds controversy over water rights. A BRIEF REVIEW OF FUNDAMENTAL HYDROLOGY We are almost ready now to <.:onsider the physical essentials [or optimum use of ground water, but no.t quite. First we must realize that the hydrologist and the general public may not use the same language when speaking of the physic<ll hasis of w3tcr SUPplY. A siz:lble part of the total population still believes in water witching--the location of wells hy m:lgical divination- as shown by a very recent study.12 One cannot read the newspapers without soon getting the idea that water levels are dropping everywhere. :lnd the only relief to this monotony is an occasional claim that someone has drilled a well that taps an "inexh;]ustible" supply. like as not in one oE our most unprepossessing deserts. J\-fany people have heard enough o[ the hyclrologic cycle to regard ground water ;]s a renewable resource, and many are sold on the idea thtlt the way to assure this renewahility is to "hold the rain. drop where it falls," as recommended by conservationists. Some people believe that primary water (water appearing for the first time from the earth's interior) is available in large quantities to anyone who kno\\ls how to f!l1d it. These arr.: just. a rC\.... of the cliches, myths, and half-truths that are associated with ground water, even by the "best people." In what Bcrtrand RusseIl13 call5 "the divorce of science and culture," it is possible to' spend many years in education, and still not learn to discriminate Olll1ong science, pseudoscience, and hokum. How larg.e is our resource of fresh WOlter? As shown in a recent summary by Nace,14 the average annual precipitation on the United States is nearly .5 billion acre-feet, of which about a third runs off to the oceans. This 5 billion acre-feet is our replenishing. replenishable resource. In addition there are tremendous accumulations of water within our nation~d boundaries: <lbout 15 billion acre.feet of surface water in Jakes, the overwhelming bulk of it in the Great Lakes, and nearly 200 billion acre-feet of ground water within a few thousand feet of the land surface. If :lll ground-water reservoirs could be drained completely. only a fraction of the national rainfall wallI(! go to their replcIlisJlJn(~lIt cilch y<:ar. and the resen'nirs could not be renewed in our lifetime. In other words, most of our ground-water resource is 110t replenishable. The renewable part is not the Olccumlllaterl resource, but only the overOow from full reservoirs. As with mO'st generalizations concerning ground water, the national picture includes a very wide range of conditions, from some resen'oirs that are drained and refilled every year, to others where the calculated accumub- tion is hundreds and even thousands of times. larger than the estimated annual replenishment. J\fany of the re~.crvoirs where replenishment is very low in comparison to total accumulation are in ollr arid lands. l~E. z. Vagt, and Ray Hym:JIl, IVaferw:lcllillg U. S. A.} Chicago, Unil,'. Chicag-o Pre,;" (19!l8). J3BcrtrJntl Russell. "The Divorce of Science ~~nd .Cliltlln..:'''. lJNF.SCO Cnurier, vol. II, no. 2, p, 4 (19,,~), HR. L. Nnce, "Waler Manngelllcnt, ..-\;::-riclIhufIC, and Ground-\Vater Supplies," u. S. Ceo!. Sur....ey open-file rept., 23 pp. Prl'senlCtI ;H Am. Assoc. Adv. Sci. meeting-, Phil;ldelphi;1. DecemlJer 1~I!lR.