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<br />EM 1110-2-1406
<br />5 Jan 60
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<br />CHAPTER 6
<br />
<br />EFFECT OF Sr\OWPACK CO::\DITION ON RUNOFF
<br />
<br />6-01. GENERAL. S,re,unllow annly,is lor wintl'r or early spring p(.rio,ls (either lor d"y-tO-llny
<br />forecasts of strt'ilmflow or for <.lPsign f1ood:-;) requin's particular cognizanc(' of tl\(\ storagf' ef1'('ct
<br />of the sl1owpack. 11:(' storflJ.~(' eIT('rt of 01(' sJlOwpn('k all runoff is dr!prmined primarily by the
<br />cOlHlitions of jempl.'rallll'C' llnd liquid watl'!" within the puC'k at a givrn timp, TI]('se conditions
<br />continually change d:lring tll(' Jwriod of SIlOW 11C'C'unmlati(lll, so that. it is llot possible to g('n('rali;~('
<br />the storage effect, fU:, a. simple COllstant. amount.. Ob~(l'nLiions of slHm-pack tempc'l'flturp :1Jld
<br />liquid watf'I' conditions [IN' not gr]'rl';111y o;nlila}'lp for pl'iljl>d, basins; accordingly. tl1(')' must he
<br />estinlut('d illdir('ctl:~ from kllowll'dg(' of el1angl's \\~ith!Jl Ihe :-;IlO\\-paek that. hnH' ])(,{,ll known ~,o
<br />OCCUI'. During thr normal spring S'lO'.nnt>!j IWl'iod. tltl' slLO\ypnck i:-:- cOllditiollrd to pl'oduC'o rundt'
<br />early in the IJl'rio<..l, n.nd thnl' isgl\Tll'rnl1>-little storng:(' dt'f'Cl ftft('r tit,. initial priming has taken plncl'.
<br />
<br />6-02. CHARACTER OF THE SNOWPACK. Thl' lorlll:1tioll of lhe Sllowp"ck ]w~ills wilh the
<br />deposit of !H'w-fallell SIlOW of rrbti'.(\Iy lo\\~ Lkll~it,\,. (i.t'", ~i)(\('ifi(' gr,lvitYJ. 'Yith time, the sno\\--
<br />pack chunges: the dt>!i(':Itf~ crystal::: of ~1l0\\' bcrollw ('0,11':"(; ;..(rnill'3~ nnd tht' dellsity of the pack
<br />incn',ls('s. The change from Ioos('. dry, and slIhfrel'zin;.:: SllO\yp;l('k of low drllsity to a ('onr~(',
<br />granular, llnd llloist :-,nowpa('k of high drflsity is ~()llldjJlI{,s :'.;!)()kl\1l of itS uriJ1(\ning" of the snow-
<br />pack. .A ul'ipe" S!w\'.-pa('k is said to he <iJ1rilllrd'~ tn produc(' nl1w/1' \\'h('n it-; liqllid-water-holdillg
<br />capacity ha~ })('(\Il rr<lchc'd. ~\t tllis point. thl\ ()TlI,'-~ :-;tlll'n,~'(' dred ol' tlll\ ~n(1\\-p,lck is that ,jf
<br />((tnnsitory" stol'a~f', rrsllltillg ill ternpor<lJ'.\' (](.11l,'-" of liquid \\',111'1' ill tr<lllsit. throu~h tile p,:c;,;:.
<br />Allbollgh iiripr" snO\~- is lbllallx ,1...:;~oej<ltL'd wit]1 :1 l"cLttiy('!y dellsC'., ('o,ll'se~gr:lj!l('d SlIowp,\{'k
<br />charnct(\ri~tic of tht' spring period, therp is no r('~triciio!l a:-i to tlli~ tilde of Yt\ar that the S11I)\\'-
<br />park may yidd liquid water to the lInd('rlyjng' groHlul :-'llrfan', .\lid\yilltt'J' rainfall or SnO\\"Ille!t
<br />may satisfy the "cold content" ant: liquid-wntpr-holding r:lpacity of till' sno\\-pnck. .Aftef tho~,e
<br />deficiencies linn' 1)('('11 mot. nllY furth\'j' input of liqllid \\-at('r at that lillle \\.ill p;l:-;~ thruugh tIte
<br />snowpuck as dl'ainagr ];y gr:l\'it:ltioll~tl for{'e.
<br />ChaIlgc\~ tlwt tnke plnc(' \\-itliin th!.... sIlO\q);lck art' c<lll:-,('d hy 8~\\"('r;d physical pro('('sc'(':::;,
<br />inclndiJlg (1.) heat eXclHtnge ,It t hI' SIlO\\- :'.;111"[;1('(', (2) p('!'rol:ttioll of IIlt,l t OJ' ra i II \VaU't' th),()llgh tl1(' SilO\\"-
<br />pack, (;-j) internal pr(~5Url' dlH' to tlte \\Tigllt of the :-;1l0\\".. (-t) \\'jnd, (,-I) telllpe/,(ltlll'l' and y,q)Or
<br />pn'SSllI'P yariation within tlll\ ::-iIlO\\pack, and (1)1 heat rXI'b:lrl.~C' ,It the ground ~\I]'f,j('t'. ,\.s l':l<!t
<br />new layer of snow is d(\positer], its upper surfnc(' is s1I1Jj",l'tl.d tn \H\,tlhel'illg PiTrch of rudi,ltioJ1,
<br />rain, alld \rind, the UTI<!Cl'SlJrLI('1' to gl'olllld lwat. ~.\s a l'(\Slllt, till' Sn()\\"lhlck is stratifi('d, show-
<br />illg distinct In.Yt\I'S and ic(: pbllllS or 1('lls(':-:; \\-hic!t :O:C!Ml'atl\ illdiyidlJ:d :-,J]t)\\'StOI'lIl de!HJsit:3. TI e
<br />interior of the pack is ~mbj('ctt'd to tlu' at'lion of pere(llatillg \r;lIl'r and djjrll~iJ]g wafer \'111'01',
<br />
<br />6-03. NOCTURNAL SNO\V CRUST. DurilJg till' mdt :"(';[0;011, OIl el('HI' night.--. ,1 rcJntin.ly
<br />shnllow surface layer of the ~ll()\qlack ;!(\Iwt'al!y cools ('on:,dLkr;lbly ll('!ow OC ('. O\\'iIlg to th~
<br />loss of lwat to the sky by IOJ]~""-;[\'t.' j"adiation; tll(' 1irlllld w;ltl'r will f]'(,c'/.(' ill this layer to as llluch
<br />llS 10 illclu's 1tl dl'pth, hut. hdo\\" thi~ :-:'llJ'LH'C bye]' tl)(, liqlJid ,y,\te]' ],f'lllnillS unfrozen.
<br />
<br />6-0t. Rl'iOWPACK TE:\IPERATVRE A:\'D DEXSITY VAlUATION. ObSelT<lliollS of SllO""-
<br />pn{'k ('onditil)flS at flu' snow bbor:ttorit's pl'ovid{' fadwtl jnfOrlll:ltioIl on tIll' t(,Tllpe]'aturc and
<br />densit.y variation within thr s!lowpnek. Pla\('] prrst'llts ,I time-profile of this 1,Tnriut.ion, together
<br />with the rt\lativL' settlement with tilllc 'Jf each horizun of !I('\\" SIlO\\T, for the U);j2-.5:.) snow season
<br />at Ccntral Sierra SnO\\ Laboratory, '. his illlJ:'>tratvs a typical pattrrn [or cl('cp,. mountain STlU\\'-
<br />paeks. A f(,flt.ure of this pnrti('ular sellson was tllP OCl'tJlTeIle{' 011 ,JalJlwry 8 illld 9, 19.3:1, cf
<br />about 4 inches of rnin Oil an initi,dly ('old :lnd dry snowpa.'k~ which rt'sultpd in substnlltinl runoff.
<br />Following this occurr,']]Cl', lhe \I"<'I1thel' conditi"m (':Lused refrcczin~ 01 the snowpllck during the
<br />latter part of January, and. the SIlOWIMCk contillllcd to 105(' heat, through tIle month of Fcbru-
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