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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />interests were completed, and the irrigators began to look to larger <br /> <br /> <br />proposed U.S. Bureau of Reclamation projects. The Colorado-Big Thompson <br /> <br /> <br />Project represents one proposed project that was implemented. <br /> <br />Well development in the valley-fill or alluvial aquifer began in about <br /> <br /> <br />1900. This development probably occurred partly as a result of most of the <br /> <br /> <br />reliable direct flow water being appropriated and partly as a result of <br /> <br /> <br />technological progress on electrical pumps. Droughts that occurred in the <br /> <br />early 1930's and early 1950's stimulated additional well development. <br /> <br /> <br />Irrigators whose surface water priorities were such that less than a full <br /> <br /> <br />water supply could be obtained under these drought conditions constructed <br /> <br /> <br />wells for a supplemental water supply. In addition, many farmers drilled <br /> <br /> <br />wells to put previously dry-farmed land under irrigation. other factors, <br /> <br />such as increasingly economical electric power rates during the 1950.s and <br /> <br /> <br />1960's and the anticipation of new ground water laws during the 1960's, may <br /> <br /> <br />have made well development more attractive. By 1970, it was estimated that <br /> <br />more than 3000 large irrigation wells had been constructed in the South <br /> <br /> <br />Platte River valley (Hurr and others, 1975). <br /> <br />At the present time, the dominant irrigation water use in the South Platte <br /> <br /> <br />River basin occurs downstream from Denver. It has been estimated that <br /> <br /> <br />diversions for irrigation purposes in this part of the basin averaged about <br /> <br />1.4 million acre-feet annually during the 1947 through 1970 period (Hurr <br /> <br /> <br />and others, 1975). This total amount of water withdrawals during this <br /> <br /> <br />period can be broken down into about 980,000 acre-feet through surface <br /> <br /> <br />water diversions and about 420,000 acre-feet through ground water <br /> <br /> <br />pumping. In 1979, irrigation withdrawals from surface water and ground <br /> <br /> <br />water sources in the basin totaled an estimated 1.3 million acre-feet <br /> <br /> <br />(John Huyler, personal communication, Colorado Water Conservation Board, <br /> <br />June 1981). <br /> <br />Not only is the total volume of water diverted for irrigation a <br /> <br /> <br />consideration, but the net consumptive use (consumptive use minus effective <br /> <br />-41- <br />