| OCR Text |
Show APPENDIX XII XII-59 amenable to direct measurement. Theoretical and indirect methods of estimating consumptive use must be relied upon in the tributary areas. In the New Mexico portion of the Gila River Basin, the annual consumptive use of water is reported by the New Mexico Interstate Stream Commission, pursuant to article VII of the March 9, 1964, decree of the United States Supreme Court in Arizona v. California, et al. Agriculture: About 85 percent of the consumptive use in the tributary area to the Colorado River mainstream is for irrigated agriculture. The annual irrigated acreage and crops grown within each reporting area were estimated principally from information in the yearly State Agriculture Statistics. Irrigated pasture and some minor crops not reported by the statistics were estimated from information in the 1969 Census of Agriculture, supporting information from framework studies, and various other local reports including county farm-agent interviews. In essence, the county data from the statistics were disaggregated into the reporting areas and sub-areas for computational purposes. The Blaney-Criddle empirical formula was utilized to compute the annual rate of crop consumption use. The formula is based on the assumption of a full water supply, among other things, and results in a theoretical water requirement rather than actual use. Seasonal crop consumptive use factors' "K" for the lower elevation desert areas were selected from Technical Bulletin 169 "Consumptive Use of Water by Crops in Arizona," issued September 1965 by the University of Arizona and the U.S. Department of Agriculture. In the higher areas, seasonal factors from the Soil Conservation Service Technical Release No. 21 were utilized. Effective precipitation, that amount of rainfall which satisfies a portion of consumptive use, is accounted for by criteria developed for this area by Wayne D. Criddle, former Utah State Engineer. Among the many variables affecting the actual use of water, the most important is individual farm water supply and its management. There is no adequate method to adjust computed annual requirements to actual water use over broad areas. Past studies of the incidental consumptive use of water associated with irrigation (water surfaces and vegetative areas on rights-of-way for canals, laterals, drains, roads, etc.) suggest that this use may be accounted for by adding 10 to 20 percent of the computed crop consumptive use. A factor of 15 percent is used herein to represent this use. In the heavily irrigated central Arizona area of the Gila River Basin, in-transit water may sometimes be considered a depletion. In-transit water is potential ground water recharge which, due to declining water tables, interception by impervious beds (perched water), etc., is presently irrecoverable. Although this water \s not truly consumed, it is not available for, use. This temporary loss of water has not been included in this report because of the lack of pertinent information to estimate its present magnitude. Evaporation from Reservoirs, Lakes, and Stockponds: Adequate data are available at most of the major reservoirs in the tributaries to estimate annual lake evaporation. Monthly net evaporation rates were derived from nearby climatic stations recording pan evaporation and precipitation. Stockpond evaporation was taken directly from framework study supporting data which were prepared by the Soil Conservation Service. In addition to major reservoirs and stockponds, there are many other reservoirs about which little information exists. For the most part, these reservoirs are small and are used for a number of joint purposes. Using available listings of these impoundments and other data, a total average surface area and a representative evaporative loss were estimated. No attempt was made to vary these losses or those from stockponds on a year-by-year basis. Municipal and Industrial: The base for estimating municipal and industrial uses is the urban and rural population within the reporting areas. Preparation of annual population estimates was guided by the 1970 Census, and various State and county statistical reviews and reports which include population esti- 13 |
