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B2 DROUGHT IN THE SOUTHWEST, 1942-56 posed of materials sufficIently permeable that' water can move through them by gravity. Water accumulates untn the res- ervoir is filled sufficiently to cause underground flow, which may ultimately be discharged into lakes or stream channels or oceans, or at the land surface by springs or seeps. Where ground water is at shallow depth, it may be discharged by evaporation or transpiration. Streams are the spillways of the hydrologic cycle and carry off the surplus water that is not stored in lakes or underground or returned to the atmosphere by evapotranspiration. They have the lowest priority on water that falls as precipitation, for water enters a stream only if it falls directly in the channel or if it cannot get into the ground by infiltration or if it is dis.- charged into the stream, it may be lost by evapotranspiration or disappear by seepage into underlying ground-water reservoirs. The great differences in ground-water resources and in streamflow characteristics in various parts of the country are traced not only to differences in rates of rainfall and other clhnntic factors but to differences in the materials in and below the soil zone, through which the water may pass. In some places the soil is like a blanket over the earth, absorbing the rainfall even ot intense storms until it can hold no more, so that some starts movtng-duwuwuJ.d lulu nmlertytng rock mate. rials; in other places bare rock or other impermeable material or frozen or compacted ground cannot absorb the water even of moderate storms or of gradual snow melting, and the surplus may cause a stream to flood. The underlying rock materials may be very permeable and form PRrt of a ground-water reservoir capable of transmitting large quantities of water for considerable distances, finally dis. Charging the water at a fairly constant rate into streams. In other places, down.ward percolation may be stopped within a few feet or even a few inches of the surface by an impermeable layer, and the water collected above that layer may quickly reo appear in streams only a short distance away, perhaps soon enough to contribute to floods in those streams. Because of the many factors that cause variation in the effect of precipitation upon soil water, ground water, and surface water, it is to be expected that drought, which may 'affect all water resources, may not affect them all similarly or simultaneously, Particularly with respect to ground-water and surface-water resources, the effects of precipitation may be recorded months or years after the precipitation has occurred. As exam- ples, Piper (1948) has reported a lag of several years between fluctuations in precipitation and correlative fluctuations in the flow of the Metolius River in Oregon, and Jacob (1945) found that water-level fluctuations on Long Island, N, y" correlated more closely with the progressive 25-year average precipitation than with' averages for longer and shorter periods, Analyses for tritium have led to the conclusion that water pumped from sampled wells in central Nebraska has been under- ground for more than 50 years after it was precipitated (Begeman and Libby, 1957). It is likely that some of the water pumped from wells in several parts of the country may have been stored underground for tens of thousands of years since the last glacial stage of the Pleistocene. As a general rule, the most obvious and immediate effects of drought are observed in soil water and in streamflow-soil water because it is replenished directly by precipitation upon the land surface and streamflow to the extent that it is normally made up of storm run- off, Although runoff is influenced by many factors, studies of the variations in natural streamflow indicate similarities in pattern over broad regions that corre- spond approximately to the principal meteorologic zones of the Southwest (p, B27), A similar generaliza- tion cannot be made as to the effects of drought upon ground-water storage or discharge, because of the com- plications introduced by geologic factors and by the development and use of ground water. Other broad generalizations can be drawn concerning the effects of drought upon soil water, ground water, and especially surface water in the Southwest and are based upon the regional climatic patterns that have been described in preceding sections; these' general fea- tures are summarized later in this chapter, In each specific area or drainage basin, variations are super- posed upon this general pattern by the geologic en- vironment, These varations, described in the chapters concerned with individual drainage basins, pertain es- pecially to ground water but also surface water. SOIL WATER By H. E. THOMAS Most of the people of the Southwest have made them- selves independent of the water that enters the soil directly from precipitation, They have achieved this independence by irrigated crops, which are the predomi- nant source of farm income, and by urban development, which likewise depends upon surface water or ground water for its municipal and industrial supplies, But the total irrigated area in the Southwest is far less than the area that is not irrigated, and the nonirrigated vege- tation depends upon soil moisture derived directly from precipitation, The nonirrigated areas include range- land with grass, browse and brush cover, forested high- lands, barren lowlands with little or no vegetation, and some land used for dry farming, Most precipitation in the Southwest falls upon land that is not irrigated, because this land constitutes a large proportion of the total area and it includes the areas where precipitation is mostabundant. This report is concerned primarily with the effects of drought upon ground-water and surface-water re- sources and not with soil moisture, The soil zone is an important consideration, however, because of the inter- relations of the hydrologic cycle, and more specifically because the proportion of the precipitation that does contribute to the ground-water or surface-water re-