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<br />GENERAL EFFECTS OF DROUGHT ON ~ATER RESOURCES
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<br />For the Southwest as a whole, the fluctuations in
<br />ground-water storage, inrerred rrom fluctuations or
<br />water levels in observation wells, run the gamut rrom
<br />close correlation to no apparent correlation with cli-
<br />matic fluctuations. In some areas where wells have
<br />withdrawn large quantities or water ror many decades,
<br />the changes in storage have resulted almost entirely
<br />rrom pumping. In other areas there is even now prac-
<br />tically no ground-water development, and changes in
<br />storage reflect solely the differences between the fluctuat-
<br />ing rates or recharge and or natural discharge, In still
<br />other areas, ground water has been developed and used
<br />only in recent years, and the fluctuations or water level
<br />have reflected natural conditions in early years and
<br />artificial conditions subsequently. From the available
<br />records, no generalization can be made about the effects
<br />or the current drought upon ground. water storage in
<br />the Southwest as a whole,
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<br />NATURAL STREAMFLOW
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<br />By J. S. GATEWOOD and AI,FONSO 'VILSON
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<br />Precipitation can be disposed or in so many ways that
<br />one may well conclude that the relation between pre-
<br />cipitation and streamflow (ir any) must be obscure.
<br />Actually only a small proportion or the precipitation
<br />in the Southwest, generally less than 5 percent and in
<br />large areas less than 1 percent, appears as streamflow.
<br />Many or the streams in the region have large tributary
<br />areas that are classed as "noncontributing" because of
<br />the inrrequency or runoff rrom them.
<br />There is no simple relation between the amount and
<br />distribution .or precipitation and the amount and dis-
<br />tributiou or streamflow; rather complete knowledge
<br />or precipitation would yield only a generalized knowl-
<br />edge or runoff, Most or the water that ralls as precipi-
<br />tation is lost through evaporation and transpiration
<br />rrom the soil, and other ractors may modiry the original
<br />pattern or time distribution or precipitation to such an
<br />extent that the resemblance to the pattern or resulting
<br />runoff is small,
<br />From the general rule that runoff on a yearly basis
<br />is more variable than precipitation, it rollows that
<br />yearly runoff is generally a more sensitive measure or
<br />drought than is precipitation. An important difference
<br />between records or precipitation and or runoff is that
<br />the precipitation record gives a measure or events at
<br />or near a single point, whereas the runoff record gives
<br />the integrated measure or events over an entire drain-
<br />age basin. Thus the runoff record or a single perennial
<br />stream may reveal as much hydrologically as would
<br />the records ror a large number or precipitation gages
<br />in the drainage basin, It is ror this reason that a study
<br />or the drought is possible on the basis or the relatively
<br />few available records of natural streamflow.
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<br />For a given area the severity and duration or drought
<br />as measured by precipitation and by runoff may be dif-
<br />rerent, as indicated by the ract that there is no fixed
<br />quantity or runoff rrom the same precipitatio/l in two
<br />equally long but different time periods. The modifying
<br />ractors that occur between precipitation and runoff
<br />commonly change with time. A long-term change in
<br />mean temperature, although or only 10 or 20, may make
<br />enough change in the rates or evaporation and tran-
<br />spiration to cause a relatively large change in the pre-
<br />cipitation-runoff relationship, A change in the time
<br />pattern or precipitation or in vegetative cover and
<br />land use may change the relationship. There is evidence
<br />in the Southwest that such changes in man's use or
<br />land or water have been and are changing the relation-
<br />ship and that, in general, there has been a greater de-
<br />crease in runoff during the recent drought than can be
<br />accounted ror by the decrease in precipitation alone.
<br />This subject is explored in more detail later in this
<br />report,
<br />The effect or climatic fluctuations upon the flow or in-
<br />dividual streams is shown by runoff rrom six streams
<br />widely distributed in the Southwest. To approximate
<br />the natural flow, the records or water-year runoff have
<br />been adjusted ror storage or diversion where appropri-
<br />ate and possible, and estimates have been made ror
<br />periods or missing record, usually by correlation with
<br />records rrom nearby streams. The adjusted records ror
<br />these six streams are included in table 1.
<br />1. The San Gabriel River near Azusa, Calir., drains
<br />the San Bernardino Mountains east or Los Angeles.
<br />The record was adj usted ror change in contents in,
<br />and evaporation rrom, Cogswell, San Gabriel, and
<br />Morris Reservoirs; ror diversion by Azusa Canal;
<br />and ror water imported rrom the Colorado River and
<br />then discharged in to the stream above the gaging
<br />station.
<br />2, The Virgin River at Virgin, Utah, drains parts of the
<br />Utah High Plateaus and Basin Ranges in the lower
<br />Colorado River basin; There is diversion for irriga-
<br />tion or about 3,500 acres upstream rrom the gaging
<br />station, but the ,recorded runoff was /lot adjusted for
<br />that diversion.
<br />3. The Verde River below Bartlett Dam, Ariz.,
<br />draini/lg central Arizona, has the longest record in
<br />the Southwest. The runoff was adjusted for storage
<br />in Horseshoe and Bartlett Reservoirs, but /lot ror un-
<br />measured diversions ror irrigatio/l or about 12,000
<br />acres nor ror the ract that the station has bee/l moved
<br />several times and the drainage area in recent years
<br />is less than it was originally.
<br />4. The Gila River near Red Rock, N. Mex" drains the
<br />western side or the southern part of the Continental
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