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LEE FERRY AVERAGE ANNUAL VIRGIN FLOW FOR SELECTE D PERIODS 17 16,8 W N 16 0 15.6 '""-l 15 14B [4 13,9 13.3 [3 12 11.8 11,8 II [0 9 8 .... 7 OJ OJ ... I 6 OJ '" ~ 5 z 0 :; 4 ..J i 3 2 0 N '" '" "' '" r:' ~ '" <t !!! ~ ~ <t l;; '" '" !!! en !!! en !!! I I , T T I T I '" '" '" I '" ~ <t III en en 0 <t '" "' "' !!! !!! !!! ij; Ql Ql Ql Ql WATER YEARS Figure 5, Lee Ferry average annual virgin flow for selected periods. areas. Naturally, data available on stream flows and water quality are based on river basins. The Upper Colorado River Basin divides naturally into three major drainages, the Colorado River Main Stem (UM), the Green River (UG), and the San Juan River (US), and these are comprised of some 39 hydrologic subbasins. Water use and economic data, however, are more easily obtainable for political subdivisions. Most of these data are summarized by county or aggregated by larger regions along county boundaries (such as the Water Resources Subareas (WRSAs) and Aggregated Subareas (ASAs) used by the Water Resources Council 1972). Consequently, for purposes of organizing these data into common geographical subdivisions, WRSA 1401-1408 (1-8) are chosen as subareas in this study. In order to estimate water availabil- ities for the analysis, the contribution to the natural river flow from each county was estimated. For this purpose the long term (1906-1973) average annual disc"harge of 14,994,200 acre-feet/year, used as the available supply in the Water Resources Council (1976) '75 Water Assessment Study, was taken as a base figure. Using hydrologic data on specific subregions (Table 1), irrigation consumptive use and exports were added to the gaged flow to estimate net basin contribution. This was proportioned upward to a total basin flow of 14.994 million acre-feet. Contributions of hydro- logic subbas ins were then aggregated to estimate the flow contributed by each WRSA. Similar calculations were made for 14.1 MAF and 13.8 MAF assumptions as shown in Table 2. Superimposed on the natural hydrologic system are the water uses in the basin. A number of studies have attempted to estimate the withdrawals and consumptive uses of water for various purposes in the basin. A com- prehensive set of estimates were made for the Upper Colorado River Framework Study based on 1965 data (U. S. Water Resources Council 1971). These data have been updated and modified by several recent studies (Colorado River Basin Salinity Control Forum 1975, Hyatt et a1. 1970, U. S. Department of the Interior 1975, U. S. Water Resources Council 1976, 1977, Western States Water Council 1974), Irrigation USe Irrigation is by far the largest con- sumptive use of water in the Upper Colorado River Basin, accounting for approximately 60 percent of the total deplet ion. Because of the arid nature of the entire region, the use of irrigation water on cropland is almost essential for significant production. Table 3 summarizes estimates of irrigation land and water use from the most recent studies. As shown in the table, the amount of irrigated land and water use in the Upper Basin has not increased appreciably over the last decade. In comparing the estimates, it should be noted that the data from the Agricultural Census are for water delivered to farms rather than consumptive use. These data are used to compute the irrigation consumptive use in Table 1 by disaggregating to county levels using Agricultural Census data. The water budgeting in Table 1 is used to derive the net availabilities in Table 2. Other water uses Water uses in the basin for purposes other than irrigation include municipal and industrial water supply, mining, livestock, recreation, fish and wildlife, and present energy development, mainly for coal-fired steam electric generation. The two largest sources of deplet ion other than i rr igat ion 9