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12 o a.l --J 00 PART III. HISTORY OF WATER RESOORCE DEVELOFMENI A. Irrigation Devel~ment 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. The slow growth in irrigated acreage in the Upper Basin in the last 45 years is ascribed to both physical 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 devel- opments 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. The development rate increased in the period 1900-10 with construction of the Yuma Project, the Palo Verde Canal and intake, and other irriga- tion projects along the river. Construction of Boulder Canyon Project in the 1930's and other downstream projects since that time has continued to expand the irrigated areas until about 25,500 acres in utah, 12,000 acres in Nev~da, and 789,500 acres below Hoover Dam are irrigated under organized irrigation systems. An additional unknown acreage is irrigated by private pumping from wells in the river aquifers in the Lower Colorado River Basin. B. Streamfl~W Depletion~ Development and utilization of the basin's water resourceS results 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 8lld in- dustrial purposes also produce significant depletions. For the 1941-68 period of record consumptive use of water by irri- gated crops in the Upper Basin was estimated to average 1,727,000 acre- feet annually. This is low in comparison to the irrigated acreage, b:1t some lands do not receive a full supply.