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,) ,'. ,j GROUND WATER DROUGHT IN THE SOUTHWEST, 1942-56 B4 By H., E. THOMAS Rain or snow does not contribute directly to ground water, except in open wells. Thus precipitation affects ground water only indirectly, and the effect is modified by the passage pf the water through some other phase of the hydrologic cycle, Some of the water from pre- cipitation becomes ground water after passage through the zone of aeration (including the soil) that separates the land surface from the zone where all pores are filled with water under hydrostatic pressure, Other water from precipitation may filter into the ground along stream channels. The..contributions to a ground- water reservoir at any locality, therefore, are not neces- sarily proportional to the precipitation at that locality because of modifications promulgated in other phases of the hydrologic cycle, Surface water is the chief source of recharge for some ground-water reservoirs of the Southwest. Most such reservoirs are in arid basins that derive an im- portant part of their water supplies from adjacent mountains or plateaus. The water flowing from these highlands represents the residue after various and perhaps complex hydrologic processes in the highlands, and the correlation between precipitation on the high- lands and runoff to the arid basins may range from excellent to poor and from simple to devious, In many areas in the Southwest ground water is re- charged from precipitation upon the land surface by means of percolation through the rock materials above the water table; and in many more areas such recharge is presumed, because it appears to be the only possible source of ground-water replenishment, As' already noted, the proportion of precipitation that becomes ground water by this method depends upon several meteorological factors, including the rate and duration of preci pitation and the temperature and humidity (be- cause of their influence upon the rate of evapotranspira- tion) ; it depends also upon the physical characteristics of the soil and other materials through which the water must pass, On the basis of the rate at which water may move through them, Tock materials may be classified as aquifers, which are sufficiently permeable so that water moves freely by gravitational drive; aquitaTd8, in which pores are small enough that molecular attraction be- comes an.important force and gravity movement is re- tarded; and aquiol,vie8, in which the pores are so small that molecular attraction is the predominant force and gravity movement is small or nil. The aquifers, usually called ground-water reservoirs (or perhaps grouped to constitute a single large reservoir), are especially im- portant to mankind, because their water can be ex- tracted through wells, These three terms are relative, and they are generally applied only to rock materials within the zone of saturation, But rock materials with the same wide range in capability for transmitting water occur also in the zone of aeration, including the soil. Thus, from the time precipitation reaches the land. surface, the permeability of soils and rock mate- rials limits the proportion of water that can filter into the soil and also the proportion that can continue down- ward to become ground water. Although precipitation deficiency is the general rule in the Southwest, the rainfall during cloudbursts may be sufficiently intense that infiltration capacities are temporarily exceeded, These are the conditions under which flood runoff occurs, Although floods in the Southwest are newsworthy and sometimes spectacular, they occur rarely, whether oue considers the flooding stream in relation to the large number that are not con- currently in flood or the duration of a flood in compari- son with the length of time between floods on any specific stream. Of the total volume of precipitation that falls throughout the Southwest in most years, the great pre- ponderance falls where it does not exceed the infiltra- tion capacity of the soil. Under these conditions, the soil's capability for retention of water becomes the maj or factor in determining how much of the water from precipitation will become ground water, In any soil, the capability for water retention varies from time to time, depending upon the soil-moisture depletion be- tween storms; and, of course, the wide variety of soils in the Southwest provides a correspondingly wide range of capability in soil-moisture retention, Because of the variable proportion of water from precipitation that is retained iu the soil, the relation between total precipi- tatiOn and the ground-water recharge from the pre- cipitation may be vague and irregular, Water from precipitation upon uniform. sltnd, gravel, cavernous limestone, porous lava, or talus slopes may all become ground water, except for very small losses by evapo- transpiration. At the other extreme, many of the storms in desert basins contribute only to soil moisture, and ground-water recharge may occur only at intervals of several years or even decades, The rate of ground-water recharge from precipita- tion-either from individual storms or on a monthly, seasonal, or annual basis-is not measured directly, be- cause adequate techniques for such measurement have not yet been developed, For numerous ground-water reservoirs, however, the rate of recharge has been com- puted from the hydrologid equation: total inflow is equal to the sum of all outflow plus any increase or minus any decrease in storage during the period under consideration. Thus our conclusions concerning the