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<br />DROUGHT IN THE SOUTHWEST, 1942-56
<br />
<br />GENERAL EFFECTS OF DROUGHT ON WATER RESOURCES OF THE SOUTHWEST
<br />
<br />By J. S, GATEWOOD, ALFONSO WILSON, H. E, THOMAS, and L, R. KISTER
<br />
<br />ABSTRACT
<br />
<br />The effects of drought afe most pronounced on soil moisture,
<br />because soil is the prime recipient of the water from precjpita~
<br />UOll, and upon streamflow, because it is the residual water that
<br />is not accepted by or that flows out from the soil and ground~
<br />water reservoirs. Studies by statistical correlation of records
<br />of natural streamflow and of dendrochronology indicate pat.
<br />terns of regional runoff that reflect precipitation trends in the
<br />principal meteorologje regions in the Southwest. By contrast,
<br />the effeets of drought upon ground ,vater vary with the natural
<br />characteristics and degree of utilization of individual aquifers.
<br />
<br />INTRODUCTION
<br />
<br />This report is a general discussion of the effects of
<br />the Southwest drought as shown by hydrologic data.
<br />Thomas (1962) defined drought as a meteorological
<br />phenomenon and presented some of the published and
<br />recorded conclusions. and ideas concerning the basic
<br />meteorological factors that influence the patterns of
<br />precipitation in the Southwest. It also considered the
<br />characteristics of that drought as indicated by meteoro-
<br />logical records, Subsequent parts of this professional
<br />paper provide more detailed evaluations of the effects
<br />of drought in individual river basins and specific
<br />localities.
<br />The fresh water upon which man depends for his
<br />existence and well-being is obtained from a great num-
<br />ber and variety of sources: soil moisture that sustains
<br />vegetation for food, fabric, forage, and forest prod-
<br />ncts; ground water from springs, wells, infiltration gal..
<br />leries, caverns, mines, tunnels, and other excavations;
<br />surface water from streams, lakes, ponds, reservoirs;
<br />water collected in cisterns from rain or melting snow.
<br />Practically all the water from these sources, which con-
<br />stitutes so large a proportion that the remainder is
<br />negligible, is meteoric in origin; that is, it has been
<br />derived from precipitation, Thus, it may be said that
<br />preci pitation is the ultimate source of all our fresh-
<br />water supplies.
<br />It has been noted that precipitation depends upon
<br />several meteorologic factors, of which an essential one
<br />is atmospheric water vapor, which in turn is obtained
<br />
<br />by evaporation at the earth's surface of ocean water, soil
<br />water, surface water, and ground water. Thus in seek.
<br />ing the ultimate source of fresh water we find ourselves
<br />working backward through a continuing cycle of
<br />events. If we reverse our direction and proceed foro.
<br />ward with this hydrologic eycle, it becomes evident that
<br />the relation of precipitation to soil water, ground water,
<br />and surface water is exceedingly variable frOm place to
<br />place and from time to time at a specific locality.
<br />The complexities of the hydrologic cycle have been
<br />summarized as follows (Thomas, 1951, p. 16-18) :
<br />
<br />Of the water that reaches the land surface by precipitation,
<br />some may evaporate where it falls; some may infiltrate into
<br />the ground, some may be evaporated; some may be absorbed
<br />by plant roots and then transpired; some may percolate down~
<br />ward to ground-water reservoirs or into voidS and crevices
<br />in relatively impermeable material. Of the water that enters
<br />gl'ound-"\vater reservoirs, some may move laterally until it is
<br />close enough to the surface to be subject to evaporation or
<br />transpiration; some may reach the land surface' and form
<br />springs, seeps, or lakes; some may flow directly into streams
<br />or into the oceans. Of the water in streams, some may accumu-
<br />late in. lakes and surface reservoirs; some may be lost by
<br />evaporation or transpiration of riparian vegetation; some may
<br />seep downward into ground-water reservoirs, and some lUay
<br />continue on to the oceans. The hydrOlogic cycle is completed
<br />by evaporation from the oceans and circulation of water vapor
<br />ill the atmosphere.
<br />Lest it appear that because of these apparent multiple choices
<br />the path followed by a particle of water is entirely fortuitous, it
<br />should be stressed that there are detinite priorities for that
<br />movement. Except for the water that evaporates at the surface,
<br />the soil or mantle-rock has top priority upon the water that falls
<br />as precipitation. Overland runoff does not occur unless or until
<br />precipitation exceeds the capacity of that surface layer to absorb
<br />the water. The soil holds water against the force of gravity
<br />until its field capacity is reached, that is, its capacity for hold-
<br />. ing water by molecular attraction, and only then does. water
<br />start to percolate downward under the force of gravity. In the
<br />intervals between storm periods soil moisture may be depleted
<br />by evaporation and transpiration, and this depletion must be
<br />made up during subsequent storms before there can be addi-
<br />tional downward percolation.
<br />Ground-water reservoirs, including those perched upon im-
<br />permeable rock layers, receive the water that percolates down-
<br />ward from the soil zone. These reservoirs, or aquifers, are com-
<br />
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