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rr~J3~~ GENERAL EFFECTS OF DROUGHT ON WATER RESOURCES B49 regulations, and compacts and treaties-all which have originated because of the fact that some water users were in a favored position to monopolize specific sources of water. The resulting systems of allocatiou may be relatively simple or highly complex, but generally they define the rights and prerogatives of and also the limi. tations on those who are thus favorably situated. Owners of land contignous to a spring, lake or stream, or overlying a ground-water reservoir are in a favorable geographic position to use the water from those sources, and to deny access to the water to others. Similarly, many water users are favored by topographic position in that they are upgradient from other users and there- fore can divert and use water that would otherwise flow naturally to those downstream. In many arid Stares where from the earliest days of settlement it was recog- uized that water rather than land was the limiting fac- tor in development, the first users of water have been given a favorable position by the doctrine that "first in time is first in right." And in many places 'some uses of water are recognized as of greater economic value to society and are therefore favored above other uses. Thus systems of allocation may define the rights and limitations to use of water on the basis of (1) land- ownership involving both geographic and topographic positions, (2) priority of beneficial use,(3) designated preferences as to type of USe on the basis of public benefits derived, or (4) a combination of these. In the allocation of water for use, it is important to recognize that, SOlnB of the water resources are replenish- able by precipitation and thus constitute a perennial supply; but others are stored in quantitics which once removed cannot be replaced under present climatic con- ditions and are thus available for one-time extraction only, as are our resources of petroleum, coal, and the various metallic ores. Surface waters, including those collected in nat.ural or artificial reservoirs, are generally replenishable, although because of climatic variations full replenishment may not occur every year. Many of our ground waters are similarly replenishable at rates tlUlt. vary with the climate. Many ground-water reservoirs also contain such large volumes of water in storage that it is possible for a time to pump water far in excess of the rate of replenishment. The precipitation deficiency of drought reduces the rate of replenishment of water in surface or subsurface reservoirs and encourages an increase in the with- drawals from those reservoirs for use. Thus the overall effect of drought is generally a depletion of reservoir storage. The question, how far is it safe to draw down the storage and yet be iusured of future replenish- ment-the question of "safe yield"-has been asked in many areas of intensive ground-water development, And statures in several Stares have the objective of pre- venting ground-water draft from exceeding the safe yield of the reservoir. A period of drought is obviously not the time upon which to base calculations of the safe yield from a hy- drologic unit, whether of surface water or ground water or both, unless one is seeking a very conservative esti- mate. It is, however, an excellent time to test the cal- culations that had been made on the basis of past ex- perience. In Utah, Nevada, and New Mexico-three States which specify appropriation as the exclusive method of obtaining a water right-there are several ground-water reservoirs which have been declared to be fully appropriated and which therefore- have been closed to furt.her development, In Las Vegas Valley, Nev" there is evidence of progressive depletion of ground-water storage, but this causes no concern be- cause water is available to the valley~physically, economically, and legally-from Lake Mead on the Colorado River (Thomas and others, 1963c). In the Roswell Basin of New Mexico (Thomas and others, 1962) and Cedar City Valley of Utah (Thomas and others, 1963a) there has been a progressive depletion in storage during the numerous drought years preceding 1957, but this is to be expected during drought, for the recharge is less than average and the pumping is greater than average to make up for the deficiency in preCipi- tation. Even with 30 years of record for. each basin we cannot be certain that the present development ex- ceeds the safe yield, because there is some- possibility that rainfall in the future may be sufficient to increase- recharge and reduce the demand upon wells to the point where the recent withdrawals from storage maybe- re- placed, at least in large part, In both Roswell Basin and Cedar City Valley, the- annual pumpage has increased significantly during the- period of State control. Thus, if the use of water had reached equivalence with the safe yield at the time- the basins were closed to further deve-lopment, the present pumpage necessarily exceeds that quantity, For the most part this contravention of the spirit of the law has been accomplished legally: in New Mexico by drilling wells outside the declared area but tapping the same ground-water reservoir; in Utah by increasing the yield of existing wells to the claimed maximum that had been used beneficially, thus upsetting the- equilibrium condi- tions extant when controls were impose-d. In most of the Southwest the areas first settled and now most densely populated are the fertile but arid lowlands, From the first days of settlement the econ- omy of these areas has been sustained by streams whose headwaters are in relatively uninhabired-if not unin. habitable-mountainous areas, Security in the econ.