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. PART III. HISTORY OF W\TER RESOCRCE OEVElOPME:-.;r .... o ...... .... A. Irrigiltion Development Irrigation development in the Upper Basin took place gradually from the beRinning of settlement about 1860 but \oOS hastened by the purchase of land from the Indians in 1873. About 800,000 acres were firtRated by 1905. Between 1905 and 1920 the developm~nt of irriRated land contin- UN at a rapid pace, and by 1920 nearly 1,400,000 acres were irrigated. The development then leveled off and increase since tha t time has been slow. In 1965, 1,600,000 acres \or'en' urrler 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 SO years is ascribed to both physical and economic limitations on the dvailability of \.IJ.ter. By 1920 most of we lower cost and more easily constructed developments were in operation. and, although some new devel- opments have taken place since wat time, they have been partially offset by other acreages goinR out of production. . Irrigation development be~an in the Lower Basin about the Slme time as in the Upper B.:Isin. Development was slo\ol' 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 Canyon Project in the 1930's and other down- stream projects since that time has providtd the means for a continu(.-d ~xpansion of the irrigated area. In 1972, there were nearly 810,000 acres irrtRated from Colorado River diversions below floover Dam. About 25.500 acres of Lower Basin lands in Utah and 12,000 in ~~evada are also now under irrigation. An additional unknown acreage is irrigated by private pumpin~ from wells in the river aquifers in the Lower Colorado River Basin. B. Streamflow Depletions Development and utilization of the basin's ...ater resources result in depletions of streamflo....s. Consumptive USt' of water by irrigated crops .1.nd l'xports to other bilsins produce the greatest flow depletions. Reservoir evaporation and consumptive use of '"8ter for municipal and industrial purposes also produce significant depletions. . The 1973 estimated consumptive use of Witer by irri~ated crops and municipal and industrial in the Upper Basin WIS more than 2,200,000 acre- feet. Depletions related to irrigation such as evaporation from irri- gation reservoirs (not Colorado River Storage Project Reservoirs) was estimated to be about 150,000 acre-f",et per year. 15