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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Some of the consumptive use requirements of each crop is satisfied <br />by effective rainfall. The data base of the Colorado Cl imate Center <br />at Colorado State University in Fort Collins, Colorado (7) was <br />accessed to retrieve rainfall records at four stations in the San Luis <br />Valley: Del Norte; Monte Vista; Saguache; and Alamosa. The rainfall <br />records for the period 1956 through 1986, a total of 31 years, were <br />retrieved. This interval was selected as the study period for the <br />reservoir operation study since it includes severe drought years <br />(1956, 1977), very wet years (1979, 1986), and was sufficiently long <br />to evaluate precipitation and irrigation trends. <br /> <br />Eighty percent of actual precipitation was assumed to be effective <br />rainfall. The net irrigation requirements for each type of crop in <br />the San Lui s Vall ey were then detenni ned by subtracti ng the effective <br />rainfall that occurred during the months of the study period from the <br />monthly consumptive use requirement of each crop. <br /> <br />2. Water Losses <br /> <br />Losses incurred duri ng transport mus t be i ncl uded in the wa ter <br />demands for each segment of the system (canal and ditch losses). The <br />San Luis Valley, however, is a unique irrigation situation. ~luch of <br />the area is irrigated with center-pivot sprinklers supplied by ground- <br />water pumping. The canal flows serve to maintain the aquifer levels <br />as much or more than they provide direct irrigation water, and in some <br />cases the canal bottoms have been disked or plowed to increase ground- <br /> <br />IV - 2 <br />