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CRDSS <br />TASK MEMORANDUM 2.09-04 <br />Consumptive Use Model <br />Irrigation Water Requirements in the Dolores and San Juan River Basins <br />1.0 ISSUE <br />This task memorandum describes the computation of irrigation water requirement (IWR) in the <br />Dolores and San Juan River Basins for years 1985 to 1990. The estimates are based on the <br />computation of irrigation water requirements using the Soil Conservation Service (SCS) Blaney- <br />Criddle method as implemented in the CRDSS-CU model using the CRDSS recommended weather <br />stations. <br />2.0 DISCUSSION/ANALYSIS <br />The estimation of the consumptive uses and losses from the entire Dolores and San Juan River <br />Basins can be performed either by county (Archuleta, Hinsdale, La Plata, Mineral, and Montezuma) <br />or by hydrologic unit (HU) (14080101, 14080102, 14080104. 14080105, 14080107 and 14080202). <br />The U.S. Bureau of Reclamation (USBR) used 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 Dolores and San Juan River Basins have been <br />subdivided into 21 subareas that are the result of intersecting the counties and the HUs. These <br />include subareas (1) Archuleta county HU14080101, (2) Archuleta in HU14080102, (3) Hinsdale in <br />HU14080102, (4) La Plata in HU14080101, (5) La Plata in HU14080104, (6) La Plata in <br />HU14080105, (7) Mineral in HU14080101, (8) Mineral in HU14080102, (9) Montezuma in <br />HU14080202, (10) Montezuma in HU14080107, (11) Dolores in HU14030002, (12) Mesa in <br />HU14030001, (13) Mesa in HU14030004, (14) Montrose in HU14030002, (15) Montrose in <br />HU14030002, (16) Montrose in HU14030004, (17) Montezuma in HU14030002, (18) San Miguel in <br />HU14030002, (19) San Miguel in HU14030003, (20) Montezuma 14080203, and (21) Dolores <br />14080203. <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 (230,897 <br />acres). Approximately 43 percent of the total area is located in La Plata county while Mineral county <br />has the smallest area (approximately 0.5 percent). Approximately 85 percent of the irrigated area is <br />devoted to pasture and 12 percent to alfalfa. In decreasing order of percentages, the remainder (3 %) <br />is devoted to small grains, orchards, corn, beans and vegetables. <br />1 <br />11.26.96 2.09-04 IDS/CSU <br />