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<br /> <br />o <br />(j <br />(.A,) <br />... <br />o <br />N <br /> <br />, <br />,) <br />!, <br /> <br />" <br />i <br /> <br />DEPLETION OF SURFACE WATER SUPPLIES <br /> <br /> <br />11 <br /> <br />POTENTIAL DEPLETIONS BY IRRIGATION <br /> <br />There is obviously enough land susceptible of irrigation on the <br />western slope to consume all surplus water if there were no economic <br />barriers to such unlimited expansion of agriculture. On the other <br />hand, it is equally apparent that there can be no material enlarge- <br />ment of the presently irrigated area unless the costs of construction <br />of irrigtl,tion projects be subsidized. The extent to which the existing <br />surplus;of water in Oolorado River and its tributaries may be depleted <br />by new agricultural uses will thus be dependent upon the extent to <br />which the costs of such new projects may be borne by the citizens of <br />the United States collectively. . <br /> <br />. '-.:~.." ,.;., <br /> <br /> <br />UNIT CONSUMPTION OF WATER <br /> <br />In the report of the Engineering Advisory Oommittee to the Upper <br />Oolorado River Basin Oompact Oommission there are set forth the <br />detailed'methods followed in determining consumptive uses of water. <br />It was found that the depletion ca,used by the irrigation of 790,600 <br />acres of cropped land amounted to 821,400 acre-feet per year and <br />that there were consumptive uses of 178,700 acre-feet on 106,800 <br />acres additional asa result of irrigation of a,djacent areas. The total <br />consumptive use causing stream depletion was thus determined by <br />the Engineering Advisory Oommittee to have been almost exactly <br />1 million acre-feet per year. The Engineering Research Oommittee <br />which is ,advising the Oolorado Oonference Oommittee now finds that <br />1,035,000 acre-feet per year are being consumed on the western slope <br />as a rest1-lt of irrigation of 824,072 acres of land and incidental uses <br />on 106,812 acres additional. Stream depletion at the sites of use <br />thus averages 1.26 acre-feet per acre of cropped land, or only about <br />1.11 acre-feet per acre spread over both cropped areas and incidental <br />areas cOIJ.Suming water. <br />Recent computations by the United States Bureau of Reclamation <br />of strea~ depletions which probably would result from development <br />of a large number of irrigation projects on the western slope indicate <br />somewhat larger consumptive uses. The average depletion estimated <br />by the BUreau for these new projects is only 1.16 acre-feet per acre <br />of all lands expected to receive water, but 30 percent of the area in <br />these potential projects is now being irrigated and only supplemental <br />water wojl1d be furnished to such lands. If the consumptive use per <br />acre of land given supplemental service should be one-half' of the <br />consumptive use' on new lands, a depletion rate of 1.40 acre-feet per <br />acre of new land is indicated. It is not clear why there should be <br />this increase from 1.26 to 1.40 acre-feet per acre per year, because <br />the potential projects are geographically scattered throughout the <br />area in the same relative locations as existing irrigated lands, and the <br />nature of the crops grown should be similar to those which have been <br />customary. <br />It is ev~dent, in any event,that the resulting stream depletion for <br />each addi~ional 100,000 acres of land which may be brought under <br />irrigation will be not less than 125,000 ,acre-feet per year nor more <br />than 150,000 acre-feet per year after allowance for all consumptive <br />uses on nqncropped lands which may be seeped or otherwise receive <br />water as a result of irrigation. '. <br /> <br /> <br />; <br /> <br /> <br /> <br />(. <br />