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calculations. <br />Agriculture <br />The percentages of irrigation consumptive use compared to total consumptive use <br />averages around 60 percent for Colorado. This percent excludes Colorado River Storage <br />Project (CRSP) reservoir evaporation. Agricultural information, including acreage and <br />crop types, was obtained from the CRDSS effort by diversion structure or a combination <br />of diversion structures. The total irrigated acreage values for Colorado are shown in <br />Table 7. <br />Representative climate station combinations were generally selected based on the <br />location of acreage within a County and Hydrologic Unit Code area (HUC). Using <br />historical records of temperature, precipitation, and frost dates, an irrigation water <br />requirement was computed for each diversion structure, or combination of diversion <br />structures. For the purposed of this report, potential consumptive use was computed <br />using the modified Blaney-Griddle evapotranspiration formula in the version described in <br />the Soil Conservation Service Technical Release No. 21, "Irrigation Water <br />Requirements," (TR-21) revised September 1970. Irrigation water requirements were <br />determined by subtracting the effective precipitation from the potential consumptive use. <br />Effective precipitation for the Upper Basin was computed using the Soil Conservation <br />Service Method (TR-21). <br />The degree to which lands suffer shortages varies from year to year, depending in large <br />part on the magnitude of runoff. For this study, available diversion records, and an <br />assumed maximum use efficiency of 50 percent, were used to estimate water supply- <br />limited consumptive use. The water supply-limited consumptive use for structures <br />without adequate diversion records is estimated based on the calculated shortages of <br />structures with diversion records located in the same water district. These structures <br />without diversion records represent approximately 23 percent of the total irrigation water <br />requirement. <br />Irrigated acreage is based on GIS data, originally digitized by the USBR from 1985 aerial <br />photography. Acreage and crop types were field checked by the State Engineer's Office <br />local water commissioners and revised to reflect conditions in 1993. This information is <br />considered acceptable for use in developing irrigation consumptive use estimates for this <br />report, but more current data is suggested for use in the development of future reports. <br />Incidental consumptive use of water associated with irrigation is estimated to be 10 <br />percent of irrigation consumptive use. The actual amount of incidental seepage and <br />phreatophytic losses associated with irrigation should be updated and verified for future <br />reports. <br />Reservoir Evaporation <br />Reservoir evaporation is based on the results of the historic basin simulations using the <br />culossapp Page 8 of 24 09/30/1999 <br />