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CRDSS <br />TASK MEMORANDUM 1.14-25 <br />Consumptive Use Model <br />Total Irrigation Water Requirement of Areas Associated with Selected <br />Diversion Structures in the Gunnison River Basin <br />1.0 ISSUE <br />It was recommended in Phase I that 4 to 7 diversion structures be identified, and that the diversion <br />records be analyzed against the actual total water requirement of the crops in the areas served by these <br />structures. The purpose of this analysis is to determine whether the U.S. Bureau of Reclamation (USBR) <br />water supply cut-off dates are appropriate, or if recommendations for new cut-off dates are necessary. <br />Thus, the overall task involves two necessary prerequisites: the acquisition of the diversion records and <br />corresponding irrigated areas, and the computation of the total water requirements of the crops in the <br />irrigated areas. This task memorandum serves to meet the latter prerequisite (the computation of the <br />total water requirements). The results will be used to evaluate the appropriateness of the USBR cut-off <br />dates. <br />2.0 DISCUSSION/ANALYSIS <br />The irrigated acreages, including their crop covers, are identified through a geographic information <br />system (GIS) using a 1993 irrigated acreage map. The total area is 99,582 acres and is broken down into <br />45 subareas, the largest of which is the Uncompahgre Project, which has a total area of 65,530 acres <br />(Table 1). The 1993 areas are used for the years 1985-1990 to estimate the total irrigation water <br />requirement (IWR) over the 6-year period. A total of 123 crop-soil combinations are obtained with each <br />combination exclusively associated with a particular subarea. The soil type is not identified but the <br />water holding capacity (AWC) has been calculated for each crop-soil combination. The crops that are <br />reported in these areas include alfalfa, pasture, dry beans, corn grain, orchard, tree farms, small grains, <br />and vegetables. <br />Seven weather stations are identified and shown in Table 2, including the adopted set of weather station <br />weights. In areas where more than one station is recommended, the weight distribution recommended <br />for use as part of the CRDSS project is adopted. <br />The CRDSS consumptive use (CU) model is used to calculate the total IWR for the years 1985-1990. <br />The Soil Conservation Service (SCS) Blaney-Criddle, with and without the CRDSS enhancements, is <br />applied. The annual carry-over soil moisture coefficient is assumed equal to 0.5. The management- <br />allowed depletion varies for each crop. Variation in the root length of the crop during the growing <br />season is assumed to be insignificant, and therefore the root length is kept constant. <br />The total IWR is computed with the assumption that irrigation water is adequate. Alfalfa-short or <br />pasture-short areas are not considered. Depending on whether the specified planting/ harvesting dates <br />are within the frost-free period, the growing season of annual crops is assumed to remain the same <br />regardless of the location. <br />1 <br />A275 01.09.95 1.14-25 Manguerra <br />