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<br /> <br />~ <br /> <br /> <br />the two watersheds, one of which will not be changed. . Any, er9sion that <br />result.s,fr9m the timber cutting will be noted and the sedim.ent will be <br />caught and measured. <br /> <br />From previous studies in other forests:, the yield of water is ex- <br />pected to increase as much a.s 10 or 15. percflnt in, the first few years <br />after. the timber is cut. In other forests, hoWever, this increase, has <br />diminished as brush covered the logged area);. It probably will be <br />necessary to keep brush and trees out if the increase in water yield is <br />maintained. <br /> <br />.. <br /> <br />.. <br /> <br />Net Water Yields and Sediment Production <br />Of Semi.-Arid,R.ange lioands <br /> <br />The Soil Conservation. Service,'incooperation with soil conserva~ <br />tion districts and Agricultural Experiment 5,tations of Al'izona and New <br />Mexico, h;l.s begun study of the relationship between range conservation <br />and water yields in zones of relatively low precipitation. The Forest <br />Service has obtained valuable data about this relationship under forest <br />conditions and on grassland in higher elevat,ions and higher precipitation <br />zOnes. However, there are only fragm!lntary data for the semi-add <br />range grasslands which make up the bulk of! southwestern watersheds <br />and n9rmally receive less than 16 or 18 inches of precipitation annually., <br /> <br />The soil and water conservation progi:-am,of the Department on <br />range lands in the Southwest has been viewed with suspicion by State <br />engineers, compact commission, irrigation districts, and others. They <br />are as sincere in supporting the principles !of conservation as any of .us, <br />but h;l.ve feared that if erosion is controlled on water shed lands the <br />streams would virtually dry up. No data acceptable to these interests <br />are available, and they have refused to acc.eptour good intenti9ns as suf- <br />ficientsafeguards of their water supplies. ,Becau.se the Soil Conservati.on <br />Service serves more districts, farmersa~d ranchers in the downstream <br />irrigated areas th;l.n on watershed lands, i, also needs to know what effect <br />conservation operations on range, lands might h;l.ve on the interests of <br />dOWnstream clients. For eight year s hydr;Ologists of the Service have <br />studied all available information on this su,bject.Scattered data were <br />put together, with what we consideredso~d engineering reasoning to <br />fill gaps. Theoretical. analyses were mad~ of the effect th;l.t complete <br />watersh!ld co.ns!lrvaticm might h;l.ve on watfrsuppliu of the pecos R,iver, <br />:Elio Grande and Upper Gila:Eliver. Although no one can yet prove whether <br />a grass cover adequate tb reduceer.osion ~o a practical minimum will re- <br />duce or increase evapo--transpiration IOss"s from watersheds, we are <br />satisfied that th.e additional cOnsumptive U;8e l)f. water, if any, will be <br />small. This theoretical analysis h;l.s not ~atisfied many water users' <br />organizations and state and federal agencies. The Department is also <br />amdous to know definitely what h;l.ppens when semi..ari4 range lands <br />improve from a depleted-cover condition to 9ne with sufficient grass to <br />control erosion. From consultation with ~rrigation districts, and compact <br />commissions and engineers, it appears necessary to: (1) obtain actual , <br />records of precipitation a.nd water and se\l.iment yield, . related to vegeta.- <br />~ tlve ch;l.nges, on watershedslarg.e enough; tb haveall.ch;l.racteristics, <br />including channe110SBE!8, of 1ar.ge southw"stern basins, and .(2) record <br />these relationships before and after wate;tshed h:nprove:rnents take place. <br /> <br />. <br /> <br />.. <br /> <br />. <br /> <br />-10- <br />