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<br />EM 1II0-2-HOd
<br />5 Jan 60
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<br />CHAPTEI{ 7
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<br />EFl<TCT OF GlWUXD COt\DITIO'\i' 0:\ RUNOFF AND LOSSES
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<br />7-01. GENERAL. Th~ ,::(rollnd ('oTlditioIlS I,both the soil mantlc and underlying ground-\Vatrf
<br />aquifers) I.lI'C importa1lt in CVUhl<ltillg' "lllluff, eithc'!' ,1S 'lpi'rnwncnt" losses or temporary delay cf
<br />fIO\\- Ly welter in tr:lJl5~t, lhrnu~h groUllll ~tOJ;lg('. The gLneral pl'inciph-s of water loss and delay
<br />caused by soil moist-'lIe deficit:-;, ('y,L}:ot~'llll~pil'nti()n, and grollnd-water storage that are llsrd in rai:l
<br />hydrology Ul'(' also nppJicl\blc to ~1l0W l:ydrolcgy. Thrrr (11'(' ~omc spcC'inl considerations that ml1~t
<br />be j aken int.o account, l1o\veYt'l'. ",IlI'I't' snow is a source of rUlloff.
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<br />7-(U. SOIL \IOISTURE. 'I'll(' SGil In-mtlc. Lmdio!ls ns n n.'servoir, storing water when availabl{',
<br />t.o bl~ uspd during IH~)'j()ds wIlrn potrnti<ll l'\'llpotmIlSpil'il1 ion excppds current supply. In snm\<T
<br />hydl'oloK\"", it. is aSSll)lll~d that (.,..,-;enti:dly no direct runoff lIlay occur until the soil storage is filIeu to
<br />its fielu eapilcit,y, whicfl i:; consitkr{'d to lJC thc' total amount of water 'which can be held figain~t
<br />gn1yjly. Th,; theordi(',d fllaxiruum c11pacity of tll(~ soil to withhold watc'I' permanently is detf'r-
<br />mined by the' difi'L'rl'n':~t: betweell the "prrmanrllt wilting point" and the "field capacity" of the soil.
<br />For typical IlloUIltain :':llils, the IllflXiltllllll soil storilge capacity rangC's approximately from 4 to S
<br />inC'lJc::; (If water, for tlh' ;,-,ont~ from which ~tnr('d "T<ltc'r may 1)(' f('llloVNI by trnnspiration or evapora-
<br />tioIl. Afte'r the field cnpacity of the ",nil bas hl'('n n':l('h('d, ('x cess wah'r may P,ISS through the soil
<br />uIlder gravitational fcrcc and appear latc!' a~; subsurface Of basl~ nO\V components of streamflow,
<br />The tilIlc-dday of transitory stOf<lge ill tll(' ~oil is integrat('d in the total basin storage rtfI'd,
<br />Dire('t IlI(':lSIlI'eIlll'!l(:-i of soil Tlloistlll'l' in pfujrrt b<lsin~ are g'rlll'rully lucking. \rhile attempts
<br />arC' lwillg made ill s(Jmt' areas tu oLtllltl t,jedrical resistatlCe type JlH't1surements of soil lllOistllre
<br />beIlt'atll till' snow pack, I'ertnill lilllitr tions prL's('Ittl.\~ restrict th(~ir llse to qualitative intcrpret.atior..
<br />The principal dim('l.~lti,_'s are prolJlcms .with calibration, b1Ck of ('buffer C'flect," inconsistency of
<br />results, dislllt rgration ()f the SCll:".illg Hni t, anclunrt'presen tn i in_'IlC'ss of illui \Tidual samples. Accord-
<br />ingl~T, estimatC's of bm;in SOLI moist 111'l~ ('oIHlitioll:-i are gl'[ll'rall,v hasrd OJl indirect relationships
<br />inv(>lYing nntecrdrnt pl'cejpittltiort, duration of l':lilllt>~s days, gr011I1d-\\"utcr ICTels) stremll di~,-
<br />cha.rge's, tinw of Y('ill', or other f[J('lOr:-i as'-)()cinted \yith soil moisture varia.tion. For arE'a.s of d('cD
<br />~,now <It'curndntioll, as in the mOll!ltniJl~ of 'Yesll'rIl l~Ilitcd Statc:3, the soil moisture drfiC'it. i~
<br />satisfied carly in the s,h1wmelt period, :lfld in maIl~~ nreilS iL may oftr!l be satisfied in thr fall frOLl
<br />ra.infall or stll)wmdt ]!J the latter (,,1"(', the soil brrwnth the snowpnrk f('nlilins at 01' above the
<br />field capaeity throughollt. tilt' \Yintt~r, nnd nny loc-;~ by ('\"apotrl1nspirn,tioa will usually he supplied
<br />by \\-inter :,llOWfIlelt or rnin[u.lI. For YP,l1':-i in which the ~oil moisture capacity is not filled b.y full
<br />or \\-int('r ruinfall or S!lOWI1wlt! it i~ Jl('ce~~ary tu estimate the condition of the soil frOlll preceding
<br />hyd mnH'tcorolog:iral evc!! ts.
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<br />7-0:1. BVAPOTRAr\SPIRATIOl'l LOSS. Loss 01 "a(p]' hy n~apo(rallsrira(ion can be os(ima(ed
<br />b,v any OliC of scnr'll f'mpiriea] formuhis (for pxamp]p, (hose dl'n']oped by Thol'llthwaitp, 50 ppn-
<br />I1l,Ul, 42 or IfalstcucP":I. Hl'sulb of COlllput<l tions using such formulas are useful in establishing watpr
<br />bn1an('{'::, for ,Heus iIlvoh.illg snow, and tbey IllUY serve as all nid in estimating SoillllOisture condi-
<br />tions at it pildicular ti "1"1('. Transpiration may occur in fon'stN] Uf(',lS during times WhNl snow is oa
<br />the gronnd, Since snowmelt and trampirntion arc both fll!l('tions of ('nergy exchange from the
<br />atmospht're, lnlllspiu.l.jon loss can bl~ expressC'd liS a funct:nn of snowmelt. The gcnprnlized lmsin
<br />snowT1wlt runoff equations presented in paragraph 4-()S ilccount for transpirntiouloss frum forested
<br />firea~~. III wnter bal:ulce COlIIplltlltiollS, hO\n_'\'er, the loss of \yuter from the snowpuek in forested
<br />areaS oy trun;piration during the spring m<'lt period can bl' tuken as "bollt 12 percent of the water
<br />equivalent of the sno\\~pack.
<br />Loss of .wllter by evaporat.ion fnolll the sIlowpack iLwlf is usuall:--- small, As shown in equation
<br />12 (p[lr. 2.-0n), YllpOr \'x('hange bcbn,pll the (ltmosph(.rc aIld t1J(' slliJwpack is a function of the
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