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CRDSS <br />TASK MEMORANDUM 1.14-26 <br />Consumptive Use Model <br />Irrigation Consumptive Uses and Losses in the San Juan River Basin <br />1.0 ISSUE <br />This task memorandum formalizes the computation of irrigation consumptive use in the San Juan <br />River basin for water years 1985 to 1990. The estimates are based on the computation of irrigation <br />water requirements by the Soil Conservation Service (SCS) Blaney-Criddle method. <br />2.0 DISCUSSION/ANALYSIS <br />The estimation of the consumptive uses and losses from the entire San Juan River basin can be <br />performed piecewise either by county (Archuleta, Hinsdale, La Plata, Mineral, and Montezuma) or <br />by hydrologic unit (HU) (14080101, 14080102, 14080104. 14080105, 14080107 and 14080202). <br />The U.S. Bureau of Reclamation (USBR) uses the intersection of counties and HUs as the <br />subdivisions for its Colorado River System Consumptive Uses and Losses Report, 1981-1985. To <br />preserve consistency with these published reports, the San Juan River basin has been subdivided into <br />11 subareas that are the result of intersecting the counties and the HUs. These include the following <br />subareas: (1) Archuleta county HU14080101, (2) Archuleta in HU14080102, (3) Hinsdale in <br />HU14080102, (4) La Plata in HU14080101, (5) La Plata (North) in HU14080104, (6) La Plata <br />(South) in HU14080104, (7) La Plata in HU14080105, (8) Mineral in HU14080101, (9) Mineral in <br />HU14080102, (10) Montezuma in HU14080202, and (11) Montezuma in HU14080107. Note that La <br />Plata is divided into northern and southern areas. The subarea in Archuleta county and HU14080101 <br />will be referred to by the county and the last digit of the HU (for example, Archuleta2). This <br />convention is used in the rest of the document. <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, it was decided that the 1993 acreage data be used directly for 1985-1990. <br />An 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 (169,063 acres). Approximately 55 percent of the total area is located in La Plata <br />county. Mineral county has the smallest area (approximately 0.5 percent). Approximately 85 <br />percent of the irrigated area is devoted to pasture and 12 percent to alfalfa. The remaining 3 percent <br />is devoted to small grains, orchard, and corn, in decreasing order. Unlike the Gunnison River basin, <br />vegetables, beans, and tree farms are not cultivated in San Juan River basin. <br />Two different data sets are prepared. The first data set involves crops that are assumed able to <br />complete their growing season with an adequate supply of water. The second data set incorporates <br />Page 1 <br />A275 03.07.95 1.14-26 Garcia <br />