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EM 1110-2--1406 5 Jan 60 CHAPTER 4 BM;I:-~ SI\OW""IELT DUIUI\G RAI:\-FRBE PERIODS ,1--01. GENERAL. :Jl1J,innal drte]'mi:wtion of snnWIllcll 0\'<']' basins for clear-weather (rain-free) periods J\'C}llires somewhat ItlOl'(, CUlllple:'\: f(1rmllln~ than for periods during rain. Solar and t.c.r- rrstrinl rndia.t iOIl lwt h become iIllflortnnt Yarjabl\~s 1H tilt' balnI1ce of heat, exchange to the snowpack. They ma~T require direct c,-aluatiot1 for specific md(\orol()~;(l('al conditions, depending upon basin forest ('ovel'. COlly(ction and l'OndClIsatioll are gCIH'rull:: less important heat sources than radia- tion. The prin('ipl('~; ~l:t forth in the prcceJing ~('dion for ("raInation of basin sllO\\'lnelt duri:Jg rain nlaY Dc l~xtrlHj.ed 10 derive f'qllutions for rain-frrc periods. Studies of sno\\'rrl{'lt indexes for the Snow Laboratory basins of the eooperativc snO\v investiga- tions provide rxprrimeTllnl cyidencc of t11e l'rlativc importance of the different metcorologieal pnrarnctC'I's in eslima.tillg daily dcur-wr(lthcr snO\\-nlelt quantities for varying degrees of forest ('over. These studil's also inTe u:3cd in wt'ighting air t<'mperature and dew point temperature fuctor.-; for estimatir g; hrat transfc'r by ('011\'('('( ion llnd cOTH!f'TIsation. The rational analysis of sI!;)wmdt at a point pr{'srn!t:,d in rhlpter 211:1s lwrn comllinecl wit.h stulistically d(lrivcd ,vcightirgs from the stlldics of snowmelt inclexC's, 10 nrrive nt ~cIH'ralizcd and simplified basin snowmelt equa- tions for clc'tlr-weuther pc'riods. Basic forms of tbe SnOWIlH'lt equatioIl~, in relation to the amount of forest cov('r) involve the following consiclcra tions: a. Heavily foreE,lrd arra (generally >80% ('over): (1) \rind i~:; unimporttlnt ill eyallIllting cOIlvection and cOIHkns.ation melt. (2) Longwave radiation a'1lel convection IlH'lt may be combinrd into n singh-linear funct.ion of air tempernturr. (:3) COlldf~rsation lllelt may be evaluated as a linrar functioIl of dewpoint temperature. (,1; Shortwave radiat.ion mdt is unirnportftnt. ~lI1d may be assuIlIeu to compensate for the loss of water by eyapolraIlspiratioll, b. Fon'strd area (GO--SO% cover',,: Same as for f], above, except that- (n A ,vincI variable is spl'ci,ied for evaluation of conyection-condcllsation melt, together with II onsill convection-condensation lIwlt coefficient. (2) Longwnvc radiation r,lrlt is l'yaluult'd separately from the convection melt term. c. Partly forest'.:d area OO-GOS7o con'r;': Same as b above, with the addition that- (1) Shortwave radiation DICit should bl~ ('yaluated for the unforei-'ted portions of Lhc basin, considering incident solar radiation, snow surface albedo, and a basin short.wave radiation Dlelt eodlicil'llt, /.;', (:2) Longw,Lre radiation llwh for tlJe forc::stcd port.ion of the basin may be eyaluated lincarly as all air 1empt'rature functioIl. For the 1LnfOrrstl'u. portion, heat loss by longwuvc radia- tion Iua::; be aceolln1l'll for indirec.tly by reducing t he short w,t"~e radiation mel t coefficicn t, ther/'by allowing longwan' los-;; 10 br a funl'lion of net sllOrtwilYP radiation. d. C\flNl area (< 10% cover): (I) Short"",,c radiation is almost always the most important melt factor, Evaluation of this factor requjI'('t, est.imates of inl'idcnt solar radiation, snow surface albedo, and a basin short- wave radiation Bwlt ('OPfTiCiPIlt, k', (2) Heat exchange hy longwuy(' radiation may be e\'uluated on the basis of surface air temperatures durin~: cloud-free periods, During perLods with cloud co\'er, estimates of net long- wave radiation ex<:Lange Inu}' be ';xlsed on cloud t empcratul'e and cloud COY('I' amounts. (3) COIl" el~1 ion and condensation snowmelt du:::ing clear weather is usually of minor importance. It mE,)" be evaluated with an air temperature, dcwpoint temperature, and wind functioIl, as previously cliscuss('d. 12 _I . _ e