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<br />e <br /> <br />e <br /> <br />e <br /> <br />EM 1110-2--1406 <br />5 Jan 60 <br /> <br />For winter Sllo\\'Illf'lt during rain, 1lH'lt rutl'S using tt'JIlpl'rnture indrxps arc difl'cl'ent from <br />those for springtillll' snowlllPlt. For fOI'estE'd areas \",'hel'e wind is relatively unimportant, the <br />melt fnctor is approxillllltdy 0.08 inch pel' dpgn>(-'-day above' ;)20 F. (s('(' equation 21), as compared <br />with 0.Oi5 for lnT('r:lg'f' springtime SIlOW!lH'}t (Sel' fig. 7). At {'xposrc! locutions where wind may <br />beeamr nn import!wt lllelt COlllpOlwllt, deg-rce-day factors would be higl1el'. For ('xamplc, at an <br />open site, with a 20-IlliJc.-pN-llOlll' willd at tll(' 50-foot level, tlIp corresponding llwlt faetor based <br />on air telllpf'I'atun> alOlH' would be about 0.20 inch pl'I' degree-day. <br /> <br />5-03. BASIN SNOWMELT INDEXES. For essentially unfol'ested an'as, statistical studies from <br />Snow Laboratory bfisills shmv that thr correlation is pOOl' hrtw('('n daily SnO\VHwlt und air teIllpera- <br />ture. For for<>stcd basins, temperatlln~ indexC's of snowlllPlt are fairly reliable for estimating <br />springtime snowmelt, and eit.her maximum or mCilIl daily ternp('rlltllres may be used. Upper <br />air t<'mpcraturrs by tllf'Illselvf's are not as l'rliable us surface trrnp<'ratllres for indexing snowmelt. <br />Upprr air temperatures) however) may be used in conjunction with other indexes to improve <br />est.imates of daily snowmelt variation. <br />Daily snowmelt runoff for a basin can be rdated to one Dr more indexes by use of Illultiple <br />correlation techniques. To be realistic, such indexes should be rdated to runoff from the snow- <br />covered area (i.e.) runoff in inches per unit area of snow cover). This) of course, requires knowledge <br />of the areal extent of snow cover, which is a major variable during the active snowmelt period. <br />For the mountainous regions of Western linited States, it Illay be necessary to simplify further <br />the procedure for determining indexes of daily snowmelt runoff for individual basins, One method <br />is to eon sider a fairly long Jll'riod of active springtime snowmplt (from 10 to 30 da,'s), after the <br />time WhC'll deficiencies in soil and snow liquid water requireffi<,nts ure kllO\vn to have been satisfied <br />throughout the contributing area. The total gpnprated runoff for the period (induding changes in <br />ground-water storage estimated by recrssion unalysis), C'xpresscu as inches runoff from the mean <br />snow-covered area, can he rclat€'d to the sum of the valu('s of a particular snowmrlt index. A <br />mean melt rate can ~)e derived for temp€'ratures above an assumed base value of the index at which <br />snowmelt runoff becomes zero. The application of melt rates so derived can be refined for basins <br />with significant un forested arras by consid<~ring adJustments to exprpss the effect of varying snow <br />allwdoes, Such adjustment is particularly important to ,'valuate the effeet of a new ;nowfall <br />which may occur during the melt period and thereby change the mdt rate. <br /> <br />19 <br />