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. . .- N ,., C-.) . PART III. HISTORY OF WATER RESOURCE DEVELOPME~T A. Irrigation Development Irrigation development in the Upper Basin took place gradually from the beginning of settlement about 1860 but was hastened by the purchase of land from the Indians in 1873. About 800,000 acres 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 were irrigated. The development then leveled off and increase since that time has been slow. In 1965, 1,600,000 acres 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 years is ascribed to both physical and economic limitations on the availability of vater. By 1920 most of the lover cost and more easily constructed developments vere in operation, and, although some ne~ 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 ~idely fluctuating flaws. Devel- opment of the Gila area began in 1875 and the Palo Verde area in 1879. Construction of the Boulder Canyon Project in the 1930's and ather 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 were irrigated by private pumping either directly from the Colorado River or from wells in the flood plain. In 1974, there were nearly 849,000 acres In the United States irrigated from Colorado River diversions below Hoover Dam. About 25,500 acres of Lower Basin lands in Utah and 12,000 in ~evada 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 1973 estimated consumptive use of water by irrigated crops and municipal and industrial users in the Upper Basin was more than 2,200,000 acre-feet. Depletions related to irrigation such as evaporation from irrigation reservoirs (not Colorado River Storage Project Reservoirs) was estimated to be about 150,000 acre-feet per year. 10