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<br />'l! "J'I ~.,.,' . <br />'j}iJ\./!,.,() <br /> <br />GENERAL EFFECTS OF DROUGHT ON ~ATER RESOURCES <br /> <br />B7 <br /> <br />'I <br /> <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withdrawn large quantities or water ror many decades, 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, <br /> <br />NATURAL STREAMFLOW <br /> <br />By J. S. GATEWOOD and AI,FONSO 'VILSON <br /> <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. <br /> <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 <br />