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Show APPENDIX XII XII-57 UPPER COLORADO RIVER Irrigation Consumptive Use: The determination of annual irrigated acreage and crop distribution during the reporting period was made using the 1969 National Census of Agriculture, annual State Agricultural Statistics Reports, Bureau of Reclamation Crop Inventory Reports, and various inventory and planning reports issued by the Upper Basin States. Since most of these data were presented on a county basis, it was necessary to separate them into reporting areas and smaller sub-basins for computational purposes. This was accomplished by using land inventory maps and relationships developed for the comprehensive framework study. For purposes of computing irrigation consumptive use, the Upper Colorado River Basin was divided into 58 sub-basins to account for local consumptive use requirements. These sub-basins generally follow tributary stream basin and State boundaries. A representative climatic station was selected for each sub-basin. Using historical records of temperature, precipitation, and frost dates, a consumptive use rate was computed for each major crop in each of the reporting years. For the purpose of this report, the consumptive use rates were computed using the modified Blaney-Criddle evapotranspiration formula in the version described in the Soil Conservation Service Technical Release No. 21, "Irrigation Water Requirements," revised September 1970. Irrigation consumptive use rates were determined by subtracting the effective precipitation from the consumptive use rates. Effective precipitation was computed using criteria described in the U.S. Department of Agriculture, Agricultural Research Service, Technical Bulletin No. 1275. The values of irrigation consumptive use rates were applied against the estimates of irrigated acreage to yield the final values of irrigation consumptive use. The theoretical consumptive use determinations are based on the assumption of full water supply during the crop growing season. However, it is estimated that in an average year about 37 percent of the irrigated lands in the Upper Basin receive less than a full supply of water, either due to lack of distribution facilities or inferior water rights. The degree to which these lands suffer shortages varies widely from year to year, depending in large part on the magnitude of runoff. For this study, an estimate of the short supply service lands was made for each sub-basin, primarily on the basis of reports and investigations collected for the framework study. A streamflow gaging station was selected within each sub-basin and the magnitude of the recessional portion of the hydrograph was used as an index to select the date at which consumptive use calculations should be terminated for the short supply lands. Comprehensive framework studies of the incidental consumptive use of water associated with irrigation indicated that this use amounted to a magnitude ranging from 5 to 28 percent of the irrigation consumptive use depending upon location of the study area within the Upper Basin. Lacking an up-to-date inventory of incidental use lands, these percentage adjustments were retained for use in this study and applied against the annual estimates of irrigation consumptive uses. The total irrigation consumptive use and incidental consumptive use associated with irrigation are reported in tables UC-3 to UC-7. Reservoir Evaporation: A comprehensive listing of all reservoirs and stockponds in the Upper Basin was developed. This listing included information about major reservoir use, location, elevation, total capacity, and surface area at total capacity. The listing was brought up to date and is now kept current. Monthly content records were obtained for those reservoirs for which records are available. The average annual water-surface area was determined for each year of the reporting period. For those reservoirs lacking records, a "fullness factor" was estimated on the basis of reservoir use and historical hydrologic conditions. These "fullness factors" were then used to obtain estimates of average annual water-surface area for the unreported reservoirs. Regression equations relating gross annual reservoir evaporation to elevation, latitude, 11 |
