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Table 4. Estimated water use in the Upper Colorado River Basin study unit, 1990 (D.W. Litke, U.S. Geological Survey, written commun., 1995) [Mgal/d, million gallons per day; ---, negligible] Ground-water use Surface-water use Consumptive Water use use Mgalld Percent Mgal/d Percent (Mgal/d) Offs4ream water uses Commercial 5.1 l9 0.4 --- 1.5 Domestic (self, public) 2.6 9 34.2 1 11.1 Industrial 1.3 5 0.5 --- 1.2 Irrigation 12.4 47 3,394 97 75R Power 0 0 8.6 --- 0.08 Livestock 1.0 4 61.4 2 3.7 Mining 4.3 16 0.02 --- 1.1 Total offstream 26.7 3,500 777 Hydroelectric power Reservoir evaporation Instream water uses 3,132 ~ 110 110 predominantly is accounted for by irrigation in the basin. The remaining water-use categories account for less than 2 percent of the consumptive use. Besides offstream water uses, there are instream water uses that are dependent on the amount of water flowing into a stream or the amount of water stored in a reservoir (table 4). The main instream use is for hydroelectric power generation, which accounts for about 3,132 Mgal/d of water. Reservoir evaporation accounts for about 110 Mgalld of water. Hydrologic Characteristics The hydrologic characteristics of the basin can be represented by a generalized water budget (table 5). The budget listed in table 5 was estimated using 1993 data on water supply, distribution, and use in the study unit. This generalized budget can provide an under- standing of the hydrologic system and the volume of water in the basin. The estimated average annual water input to the basin is about 20,980,000 acre-ft/yr. The source for water input in the area is precipitation (average precipitation is 22.1 in. distributed across the basin's 17,800 mi2). Because the Upper Colorado River Basin is a headwaters system, no surface-water inflow occurs; the remaining water inputs by interbasin water transfers and ground-water inflow are negligible. Water outputs from the basin are more diverse, but the predominant output is from evapotranspiration from nonirrigated land, which accounts for about 70 percent of the total water output. The other major outputs for the basin are surface-water outflow at about 21 percent of the total basin output, consumptive water use (primarily evapotranspiration from irrigated lands) at about 4 percent, interbasin water transfers at about 3 percent, and reservoir evaporation at about 1 percent. Surtace Water Streamflow has been measured at about 400 gag- ing stations in the study unit, and in 1993, 113 of these stations were active. The first gaging station in the area was established in 1894 on the Gunnison River near Grand Junction; however, a station on the Gunnison River below Gunnison Tunnel has operated continu- ously since October 1903. This station has the longest record of operation in the study unit. Most active stations are located in the headwaters area of the moun- tains. Flow data are more scarce for the low-altitude areas. Upper Colorado River Basin streams are classi- fied in this report into three general types: high-altitude streams, low-altitude streams, and mixed-type streams. High-altitude streams are at altitudes above 7,500 ft and receive 20 in. or more of precipitation annually, mostly in the form of snow. The basins for high- altitude streams have steep slopes and thin soils, which 18 Environmental Setting and Implications on Water duality, Upper Colorado River Basin, Colorado and Utah