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CRDSS <br />TASK MEMORANDUM 2.09-01 <br />Consumptive Use Model <br />Irrigation Water Requirements in the Upper Colorado River Basin <br />1.0 ISSUE <br />This task memorandum describes the computation of irrigation water requirement (IWR) in the <br />Upper Colorado River Basin for years 1985 to 1990. The estimates are based on the computation of <br />irrigation water requirements using the Soil Conservation Service (SCS) Blaney-Criddle method as <br />implemented in the CRDSS-CU model using the CRDSS recommended weather stations. <br />2.0 DISCUSSION/ANALYSIS <br />The estimation of the consumptive uses and losses for the Upper Colorado River basin can be <br />performed either by county (Eagle, Garfield, Grand, Mesa, Pitkin, Routt, Gunnison and Summit) or <br />by hydrologic unit, HU (14010001, 14010002, 14010003, 14010004, 14010005, and 14010006). The <br />U.S. Bureau of Reclamation (USBR) used the intersection of counties and HUs as the subdivisions <br />for its Colorado River System Consumptive Uses and Losses Report, 1981-1985. To maintain <br />consistency with these published reports, the Upper Colorado River basin has been subdivided into <br />fourteen subareas that are the result of intersecting the counties and the HUs. These include subareas <br />(1) Grand county HU14010001, (2) Grand in HU14010002, (3) Summit in HU14010002, (4) Eagle in <br />HU14010001, (5) Eagle in HU14010003, (6) Eagle in HU14010004, (7) Pitkin in HU14010004, (8) <br />Garfield in HU14010001, (9) Garfield in HU14010004, (10) Garfield in HU14010005, (11) Garfield <br />in HU14010006, (12) Mesa in HU14010005, (13) Mesa in HU14010006, (14) Routt in HU14010001, <br />and (15) Gunnison in HU14010004. <br />The irrigated acreage, including the type of crop cover for each subarea, is obtained through a <br />Geographic Information System (GIS) using a 1993 irrigated acreage map developed by the State <br />Engineer's Office and the USBR. Because of the lack of a better methodology for estimating the <br />1985-1990 acreage data, the 1993 acreage data was used directly for the years 1985-1990. An <br />alternative method that uses the 1985-1990 county statistics along with the 1993 acreage map to <br />extrapolate the 1985-1990 acreage data for the Gunnison River basin was presented in a CRDSS <br />consumptive use sub-committee meeting. Because of the large discrepancies in the Colorado <br />Agricultural Statistics from year to year, direct use of the 1993 acreage data was suggested. (Task <br />Memorandum 1.14-19 details the issues and problems of using the Colorado Agricultural Statistics <br />data.) The total irrigated acreage by crop in each subarea is shown in Table 1 along with the total <br />irrigated area (272,700 acres). Approximately 37 percent of the total area is located in Mesa county <br />while Gunnison county has the smallest area. Approximately 75 percent of the irrigated area is <br />devoted to pasture and 14 percent to alfalfa. In decreasing order of percentages, the remainder (11 <br />%) is devoted to corn, small grains, orchards, beans and vegetables. <br />The weather stations recommended by CRDSS (shown in Table 2 and 3) are used. Thirteen weather <br />stations are used. The data sets were prepared to support the input requirements for estimating <br />evapotranspiration by the SCS Blaney-Criddle method. The data set for Penman-Monteith <br />calculations was not prepared, mainly because of a lack of representative daily weather data for the <br />entire Colorado River Basin. <br /> 1 <br />11.26.96 12.09-01- IDS/CSU <br />