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CRDSS <br />TASK MEMORANDUM 2.09-03 <br />Consumptive Use Model <br />Irrigation Water Requirements in the Yampa River Basin <br />1.0 ISSUE <br />This task memorandum describes the computation of irrigation water requirement (IWR) in the <br />Yampa River Basin for years 1985 to 1990. The estimates are based on the computation of irrigation <br />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 from the e ntire Yampa River basin can be <br />performed either by county (Moffat, Rio Blanco, and Routt) or by hydrologic unit, HU (14050001, <br />14050002, and 14050003). The U.S. Burea u of Reclamation (USBR) used the intersection of <br />counties and HUs as the subdivisions for its Colorado River System Consumptive Uses and Losses <br />Report, 1981-1985. To maintain consistency with these published reports, the Yampa River basin <br />has been subdivided into seven subareas that are the result of intersecting the counties and the HUs. <br />These include subareas (1) Moffat county HU14050001, (2) Moffat in HU14050002, (3) Moffat in <br />HU14050003, (4) Rio Blanco in HU14050001, (5) Rio Blanco in HU14050002, (6) Routt in <br />HU14050001, and (7) Routt in HU14050003. <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 1985-1990. An alternative <br />method that uses the 1985-1990 county statistics along with the 1993 acreage map to extrapolate the <br />1985-1990 acreage data for the Gunnison River basin was presented in a CRDSS consumptive use <br />sub-committee meeting. Because of the large discrepancies in the Colorado Agricultural Statistics <br />from year to year, direct use of the 1993 acreage data was suggested. (Task Memorandum 1.14-19 <br />details the issues and problems of using the Colorado Agricultural Statistics data.) The total irrigated <br />acreage by crop in each subarea is shown in Table 1 along with the total irrigated area (86,963 acres). <br />Approximately 59 percent of the total area is located in Routt county. Approximately 95 percent of <br />the irrigated area is devoted to pasture and 4 percent to alfalfa. The remainder one percent is devoted <br />to small grains. Corn, orchards, beans and vegetables are not grown in the Yampa river basin. <br />The CRDSS recommended weather stations (shown in Table 2 and 3) are used. The data sets are <br />prepared to support the input requirements for estimating evapotranspiration by the SCS Blaney- <br />Criddle method. The data set for Penman-Monteith calculations was not prepared, mainly because of <br />a lack of representative daily weather data for the entire Yampa River Basin. <br />Table 4 shows the annual irrigation water requirement (IWR) for 1985-1990 for the ent ire Yampa <br />River basin as estimated by using the SCS Blaney-Criddle method. The 6-year average annual IWR <br />is 99,299 acre-ft. The peak IWR is in 1988 (117,391 acre-ft) while the lowest IWR occurred in 1986 <br />(90,195 acre-ft). A more detailed tabular result is in Table 5, showing the 6-year average annual IWR <br />1 <br />11.26.96 12.09-03- IDS/CSU <br />