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Show XII-64 UPDATING THE HOOVER DAM DOCUMENTS livery to Mexico. The interim deficit of water to the system will be replaced by water savings resulting from the construction of a concrete-lined canal generally parallel to the first 49-mile reach of the existing unlined Coachella Canal. The water saved, estimated at about 132,000 acre-feet annually, will represent a part of California's entitlement. However, until the water saved is required by these users, it can supplement or replace water from storage that has been released to Mexico and not counted as part of the scheduled treaty deliveries. Plans also call for the permanent replacement of reject brine water from the desalting plant. Tributaries Tables LC-4 through LC-8 show water uses by selected tributary areas, by States, and by type of use. Onsite consumptive use in 1971 was estimated to be about 3.8 million acre-feet. By 1975, consumptive use was about 4.5 million acre-feet as a result of a substantial increase in both irrigated acreage and population. Over half of the consumptive use is satisfied from ground water overdraft. Irrigated land was estimated to be about 954,000 acres in 1971, and 1,090,000 acres in 1975. Gain in population has been on the magnitude of about 100,000 new residents for each year during the period. Most of the increase in water use, irrigated land, and population has occurred in the Gila River Basin. Gila River Consumptive use for the irrigation of crops represents about 85 percent of the total water use in the Gila River Basin. Estimated annual consumptive use per area for the entire basin during the 5-year period averaged about 3.5 acre-feet, varying from less than 1 acre-foot per acre in parts of New Mexico to over 4 acre-feet in the western portion of the basin. Crop consumptive use varied considerably from year to year on the basis of climatic conditions. Favorable economic conditions for farming led to an increase in irrigated land of about 127,000 acres. The consumptive use of water for municipal and industrial purposes is estimated to have increased about 42,000 acre-feet during the 5-year period. Water supply conditions were characterized by exceptionally poor runoff in 1971 and 1974, near normal runoff in 1972 and 1975, and the occurrence of a major flood in 1973. In addition to replenishing storage reservoirs in the basin, the 1973 runoff produced an outflow below Painted Rock Dam of 412,700 acre-feet during the water year. About 100,000 acre-feet of the outflow reached the Colorado River mainstream. Estimated diversions during the 5-year period averaged about 5.6 million acre-feet, of which 4.1 million acre-feet were from ground-water pumpage. The recent "Inventory of Resources and Uses, Arizona State Water Plan, Phase I-July 1975," prepared by the Arizona Water Commission and based on 1970 development conditions, estimated annual ground-water overdraft to exceed 1.8 million acre-feet. In general, increased water uses within the basin since 1970 have added to the overdraft. The Central Arizona Project, scheduled for completion in 1985, would divert the remaining portion of the Arizona entitlement of Colorado River water to central Arizona, reducing ground-water pumpage and consequently the overdraft. Other Tributary Areas Outside the Gila River Basin, and within the remaining tributary areas to the Colorado River mainstream, water resources are generally limited and their development is less intensive. As shown in tables LC-4 through LC-8, total estimated consumptive use within the area increased from about 437,000 acre-feet in 1971 to 475,000 acre-feet in 1975. A lack of adequate surface-water storage facilities tends to make irrigated acreage subject to fluctuation from year to year based on the variable and somewhat undependable runoff. Localized ground water overdrafts occur in parts of the area. With the exception of Las Vegas Valley, population is predominantly rural. In Las Vegas Valley, municipal and industrial demands are increasing rapidly; however, these demands are being met by increased diversion from Lake Mead, as shown in table LC-3, and reliance on ground-water pumpage is being reduced. 18 |