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CRDSS <br />TASK MEMORANDUM 1.14-24 <br />Consumptive Use Model <br />Evaluation of the U.S. Bureau of R eclamation?s Procedure for Determining <br />Irrigation Water Requirement That is Not Met <br />1.0 ISSUE <br />In Colorado, the amount of water an irrigation canal or ditch can divert to meet the irrigation water <br />requirement is often limited by legal and physical availa bility. In the U. S. Bureau of Reclamation's <br />(USBR?s) Colorado River System Consump tive Uses and Losses Report, 1981-1985, a procedure was <br />identified for estimating a calendar date each year when the available water suppl y would be unable to meet <br />the irrigation water requirement of alfalfa and pa sture grass. In the Phase I CRDSS Task Memorandum <br />1.14-1, it was recommended that the acreage and crop ev apotranspiration (ET) estimates that are limited by <br />the available water supply (alfalfa-short and pasture- short) be reviewed and verified. This memorandum <br />evaluates whether the USBR cutoff dates for the deliver y of water to water-short alfalfa and pasture grass <br />are appropriate or if new proce dures for estimating cutoff dates should be developed for the Gunnison River <br />basin. <br />2.0 DISCUSSION/ANALYSIS <br />The USBR?s Colorado River System Consump tive Uses and Losses Report, 1981-1985, identified 15 U.S <br />Geological Survey (USGS) hydrol ogic/county units (HCU). The USBR procedure for determining <br />consumptive use on water-short acreages in each HCU de termines a consumptive use cutoff date for water- <br />short acreages based upon the recession portion of th e annual stream hydrogra ph at a specific gaging <br />station. When the streamflow at a specific gaging station falls below th e cutoff flow rate, all consumptive <br />use of the alfalfa-short and/or past ure-short acreage is assumed to cease. Table 1 lists for each HCU, the <br />stream gage for determining the cuto ff date, the cutoff flow rate, the pe rcent of total irrigated area in 1981 <br />that was determined to be alfalfa- short, and the percent of total irri gated area in 1981 that was pasture-short. <br />Evaluation of the USBR procedur es are based upon the following: <br />A study period from 1985 through 1990. <br />? <br /> <br />Analysis of the daily USGS streamflow records at the three stream gages listed in column 3 of <br />? <br />Table 1 to determine cutoff dates. <br /> <br />Monthly estimates of actual irrigation consum ptive use provided by the CSU-IDS group. This <br />? <br />consumptive use was calculated using the Soil Conservation Service (S CS) modified Blaney- <br />Criddle method and using crop distributions and irrigated acr eage provided by State of Colorado <br />(State) personnel participating in the CRDSS. <br /> <br />Records of daily ditch diversions and monthly to tals for a number of ditch structures in the <br />? <br />Gunnison River basin identified by the State. The diversions were provided by RTi. <br /> <br />1 <br />A 275 01.09.95 1.14-24 Walter <br />