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. o /Xl W o . . PART II 1. HISTORY OF WATER RESOURCE DEVELOPl'IENT A. Irrigation Development Irrigation development in the Upper Basin took place gradually from the heginning of settlement about 1860 but was hastened by the purchase of land from the Indians in 1873. About 800,000 acres (324,000 ha) were irrigated by 1905. Between 1905 and 1920 the development of irrigated land continued at a rapid pace, and by 1920 nearly 1,400,000 acres (567,000 ha) were irrigated, The development then leveled off and increase since that time has been slow. In 1965, 1,600,000 acres (648,000 ha) were under irrigation in the Upper Basin. Since 1965, there has been very little change. The slow growth in irrigated acreage in the Upper Basin in the last 50 'fears is ascribed to both ph'fsical and economic limitations on the availability of water. By 1920 most of the lower cost and more easily constructed developments were in operation, and, although some new developments have taken place since that time, they have been partially offset by other acreages going out of production. Irrigation development began in the Lower Basin about the same time as in the Upper Basin. Development was slow because of difficult diver- sions from the Colorado River with its widely fluctuating flows. Devel- opment of the Gila area began in 1875 and the Palo Verde area in 1879. Construction of the Boulder Can'fon Project in the 1930's and other down- stream projects since that time has provided the means for a continued expansion of the irrigated area. In 1970 an additional 21,800 acres (8,800 ha) were irrigated by private pumping either directly from the Colorado River or from wells in the flood plain. In 1974, there were nearl'f 849,000 acres (343,000 ha) in the United States irrigated from Colorado River diversions below Hoover Dam. About 25,500 acres (10,300 ha) of Lower Basin lands in Utah and 12,000 acres (4,900 ha) in Nevada are also now under irrigation. B. Streamflow Depletions Development and utilization of the basin's water resources result in depletions of streamflows, Consumptive use of water by irrigated crops and exports to other basins produce the greatest flow depletions. Reservoir evaporation and consumptive use of water for municipal and industrial purposes also produce significant depletions, The 1976 estimated consumptive use of water by irrigated crops and municipal and industrial usert in the Upper Basin was more than 2,200,000 acre-feet (2,700 1< 10 m ). Depletions related to irrigation such as evaporation from irrigation reservoirs (not Colorado River 12