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<br />10 <br /> <br />o <br />i::") <br />W <br />~ <br />~ <br />~ <br /> <br />precipitation, and frost dates, a <br />consumptive use rate was computed for <br />each major crop in each ot the reporting <br />years. For the purpose ot this report, <br />the cons~ptive use rates were <br />computed ~ing the modified <br />Blaney-Crilidle evapotranspiration <br />formula in'the version described in the <br />Soil Conservation Service Technical <br />Release No. 21, "Irrigation Water <br />Requireme:nts," revised September 1970. <br />Irrigation Consumptive rates were <br />determine4 by subtracting the effective <br />precipitation from the consumptive use <br />rates. Effe,ctive precipitation for the <br />Upper Basin was computed using the <br />Soil Conservation Service method, This <br />method is reterenced in "SCS Technical <br />Release No. 21." (It should be noted <br />that this method estimates less effective <br />precipitation than the Reclamation <br />method. Previous reports used the <br />Reclamatio,n method of computing <br />effective pziecjpitation.) The values of <br />irrigation consumptive use rates were <br />applied to ~e estimates of irrigated, <br />acreage to [yield the final values of <br />irrigation Consumptive use, <br /> <br />An exception to this procedure occurred <br />in the Lower Basin in the "low desert" <br />regions of Arizona and Nevada where <br />the Blaney'Criddle formula was used to <br />estimate the crop consumptive use, This <br />departure was based on the research <br />results of IJ.eonard Erie, et al. Seasonal <br />crop consumptive use factors ("K") for <br />the lower elevation desert areas were <br />selected from Conservation Research <br />Report N~ber 29, "Consumptive Use ot <br />Water by Major Crops in the ' <br />Southwestern Uniteli States; issued <br />May 1982 by the United States <br />Department of Agriculture. Effective <br />precipitati()n was derived from criteria <br /> <br />developed for ,the area by Wayne D. <br />Criddle, former Utah State Engineer. <br /> <br />These theoretical consumptive use <br />calculations were based on the <br />assumption of full water supply during <br />the crop growing Season. However, it is <br />estimated that in an average year, about <br />37 percent of the irrigated lands in the <br />Upper Basin receive less than a full <br />supply of water, either due to lack of <br />distribution facilities or junior water <br />rights. The degree to which these lands <br />suffer shortages varies widely from year <br />to year, depending in large part on the <br />magnitude of runoff. For this study, an <br />estimate of the short supply service <br />lailds was made, for each subbasin, <br />primarily on the basis of reports and <br />investigations collected for the <br />comprehensive framework study. A <br />streamflow gauging station was selected <br />within each subbasin and the magnitude <br />of the recessional portion of the annual <br />hydrograph was used as an index to <br />select the date at which consumptive <br />use calculations ,should be terminated <br />for the short supply lands. <br /> <br />Comprehensive framework studies of the <br />incidental consumptive use of water <br />associated with irrigation indicated that <br />this use varied between 5 and <br />29 percent of the irrigation consumptive <br />use, depending upon the location of the <br />study area within the Colorado Basin. <br />These percentages were used in the <br />Upper Basin and an average value of <br />15 percent was used in the Lower Basin <br />to adjust the calculated consumptive <br />use. <br /> <br />The agricultural data is generally <br />adequate for use in this report. With <br />the exception of Utah, each state <br />prepared annual county irrigated <br />