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nOil702 WaIN yield, ItHmdofl', incrl!aSt!S rapidly with pre- cipitation. frollllwar ZNu inlhc deserts to as much as ..UJ inches i/1 illpiOt~ ilrt'as. Water ~'ield impron~menl. as it pertains to forest and rangelands in the Colorado Ri\'er Basin. is based on the JlWmiSI! Ihal slrt'<Imf!u\\" tlnd!nr ground waler are im:rea:.ed by .In amount equal In the net reduction in t~\'apntrallspiralion. Little op. potlunity exi~ls 10 reducf' transpiration \\"hen~ pre- cipitation is It!ss than about Ie inches {fig. 31 and is exceeded by pnlt!lllial I'Vilpntranspiration (W.trIfl. dry portions in fig. 3). bt'callse rains du flull.ume- tralp far into the soil, and onp cover type is about as efficient as another in lIsinR, i1vailahlt! Willer. At the ulht!t extn'rne (the cold. wet climate). the oppor- tunity In d,!cr'~asp lranspiration is limited Lel:aUSt! waler use by plants already is ]ow, and furtlwr Ct'ductions ilre difficult 10 obtain. The greatest op- portunity exists where precipitation exceeds 18 inches and potential cVilllOtranspiralion {(Ieter- mined Lv the Thornthwaile method) excel!ds 15 inches. This kind of dimatt! promotes \'igotClus growlh of n~gt!tiltion capable of IIsinH large amollnts of water. :-"1odifying cover under these conditions can suhstantially increase wah:r yield. Riparian vegetation represents a special situation bec<ltlse of water a\'ailability 10 pl':Hlts in addition to Ilc1tural precipitdtion in areas wht!ft! tIlt! polen- tial for evapotrallspiration is often quite high. The besl opportunity to increilst~ watl'r yield b}' snow mana~(-,IIlt'nt is in cold dimah!s where blowing SIlOW can be concentratt!d in forest oplm- ings or trapped in large drifts to reduce evapora- tion, Vegetation (:;10 bl' managt'd in sl!veral ways to rmluee l'\'apotranspiralion: 1. Reduce stand dl'lIsity by various practices 10 tI~(lllCll transpiratiun and intcrcpption. 2. Con\'ert from DlH! cover typt-' to another thaI uses less wilh~r (typc convcrsion). 3. Create ojwnings in forest cover to reduce transpiration and to redistrihutf' snow, COil- centrating it to wduce evaporation anti in. crease snllwllwlt contrihution to sttt~amnow. 4. ESlablish Iwl's or large shrubs in windswept. trec]ess areas to pile snow in large drifts. thereby reducing evaporation. Snuw management can Sil\'e considerable water (Tabler 19731.-As snow particles art' blown along by wind in open terrain, thl';.' undergo significant ~~\'apurati()n (sublimationl loss and may com- pletely sublimah~ atter being transported a few thousand fed. Hy trapping SIlOW Iwhind natural or artificial harriers. such as snO\\' fl'nees, e\'aporation is reduced and snowmelt water is cunr:.mtrated to 40 I I I . I I Plrl)"on- . I Juniper I. Ponderosa Pine Big . I . ~I~ed Conifer 5 b hi I-\spen age rus . Subalpine L_______ .Atpine . . ~ v o Choponal Mountain Ekush ..arm and .of NOrm and ,,' o 30 2 e . ~20 " ~ c > . 10 ~ - o . " a. cotdand ,,' cold ond _of o o 10 20 30 40 50 Precipitation (Iflches) Figure 3.-Gen.rll rllltionship 01 wlg.lltion to pol.nU,1 evapotrlnspiration (PE) (ThornlhWlite Ind "'Ither 1957) Ind preclpllltion (PI In the Colorado River Slsin, The rang.s in PE Ind P are brOider than indicated by tha average "alues plotted lor eaCh v~elallon type. Asp.n, lor ell-ample, overllps Ihe area between ponderosa pine and subalpine. The dashed lines represent approll-imale marginl' conditions lor improving waler yield by meniputatlon 01 V9ge1l1lon; Ihe potential Is greater in the direction 01 wlrm and wet climate. enhance its delivery to the stream. In the forest. specific managempnt pradices can be IIsed to promote redistrihution of sno\... by wind from uncut areas of the forest to specially designed openings. \rtlPn concpntralt'd in this way, more of the snow melts .lnd contrib\lt~s to streamflow than if SIlOW is uniformly distributed over the entire forestpd ilwa The method of lreatnlent will var)' with vegeta- tion type, climate, soils. topography, and with so- cial. !!collumit:, and t'Ilvirunmental considerations. C1earcutting and t~'ptl conversion usually produce maximum incw<lst~S in waler ~'it'ld compared to thinning and patch cutting. An exception may be in areas of heavy sllowfall. whNl~ palch cuts or strip cuts of appropriate si7.e and spacing appear nlore efficient then large c1tMrcuts (Leaf EI7S). Thinning, the uniform reduction of fort'st COVer, is probably tnt' least dfecttvt! method of increasing watN \"idd in tilt' Colorado RiVl~r Basin, where summt;r moisture uSllalJ}' fall... shor1 of plant water demands. As soil water deplt~t~s during the grow- ing season. plants compete for a\'ailable rnoisturtL In this situation. plants lhat remain after thinning are capable of usin~ additional watN if it becomes .H'ailable by remO\'al of nearby plants (fig. 4), As a general rule. llniforllllhinning 01 forest dnd brush stands in the Rocky :\lollntains <Ind Southwest must rmllovt! about 5fJ<\, of the crown cOVer before water yield appwciahly increases. For this mason. sildcllltural systems that use sheltcrwood or indi- 5