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regulated releases from Lake Powell at Glen Canyon Dam. Annual reservoir evaporation consumed between 1.0 and 1.2 million acre-feet. Apparent channel losses ranged from 90,000 acre-feet in 1976 and 1977 to 430,000 acre-feet in 1979. Note that apparent channel losses were calculated over the entire Lee Ferry to International Boundary reach instead of only the Davis Dam to International Boundary reach as in the 1971-75 report . Total mainstream depletions de- creased from 1976 to 1980, from 6.1 to 5.8 million acre-feet, with minimum depletions of 5.7 million acre-feet in 1979. This reflects decreases of 140,000 acre-feet for irrigation depletions and 130,000 acre-feet for exports to California from 1976 to 1980. Depletions for irrigation in 1980 were 1.6 million acre-feet and exports to California were 4.2 million acre-feet the same year. However, municipal and industrial water use, including thermal power- plants in Nevada and Arizona, increased depletions 18 percent from 123,000 acre-feet to 150,000 acre-feet in the 5-year period. Much of this demand is within the rapidly expanding population area of southern Nevada. Lower Colorado River Tributaries Tables LC-4 through LC-8 show yearly water uses by tributary area, State, and type of use. Onsite consumptive use in 1976 was estimated to be about 4.1 million acre-feet. By 1980, consumptive use was about 4.8 million acre-feet as a result of a substantial increase in both irrigated acreage and population. Most of the increase in water use, irrigated land, and popula- tion has occurred in the Gila River basin. Significant ground water usage occurs in Arizona, Nevada, and New Mexico. For the purpose of this report, groundwater overdraft has not been taken into account in the computa- tion of tributary consumptive use. Also tributary channel loss and salvage were not evaluated. It should be noted, however, that present ground water overdraft in Arizona has been estimated to be' approximately 2.2 million acre-feet per year. Consumptive use for the irrigation of crops represents between 80 and 85 percent of the total water use in the Lower Colorado tributary areas. Estimated annual consumptive use per area for the Lower Basin during the 5-year period averaged about 3.5 acre-feet per acre, varying from approximately 1 acre-foot per acre in parts of New Mexico to over 4 acre-feet- in the western portion of the basin. Estimated crop consumptive use varied considerably from year to year on the basis of climatic conditions, from a low of 3.3 million acre-feet in 1976 to a high of 3.8 million acre-feet in 1980. Irrigated lands amounted to 1,067,000 acres in 1976, decreased to 1,037,000 acres in 1978, and in- creased to 1,132,000 acres in 1980. The consumptive use of water for municipal and industrial purposes is estimated to have increased from approximately 444,000 acre-feet in 1976 to approximately 500,000 acre-feet in 1980 . Water supply conditions were charac- terized by near normal runoff in 1976, exceptionally poor runoff in 1977, and extremely large runoff in 1978, 1979, and 1980. In addition to replenishing storage reservoirs in the basin, the 1978 through 1980 runoff produced an outflow below Painted Rock Dam of as high as 2,385,000 acre-feet during 1980. 17