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EM 1110-2-1406 5 Jan 60 IYLn)' be appli('(l to daily snowrndt values, t.o I'f'prrsent sno\\'mrlt excesses at t:~,. .>11u\vground- interfu('(', as follO\n~: Time,hrJIIT3 l'arndnf1'1ilv nll)'l'lIIcl! OROO- HOO__ llO(}- 20l)(L 2000-0200__ 020(}-OSOOn 1,1) GO 25 10 9-05. RAINFALL. Estimates of basin mmfaU arc made on the basis of point \"ldUL'S at hydro- meteorological stations, adjusted by normal UIlllUlll precipitation variation to repres('nt basin urnotlnts, In ruggE,d, headwater areas where snowmelt gcnerally occurs, areal coverage by ruilLfall I'Pportillg stations is likely to be incomplete, and determination of basin rainfall amounts cnn be considered only as an approximflt:on. During the major SnOWllH'lt runoff period, rainfall on l,arc ground is commonly lost to soil moisture deficits, so tllUt only that portion falling on the SIlO\\'- ('overed area is efJ'edive in producinJ runoff. However, very intense rainfall wiII nh\:ays produre somt' runoff from bare ground; also, there may be cases in whi('h snow-free grounu has brcome lIprimed)) by rainfall preceding sno\rmelt, so that moderate rainfall on sno\v-free areas may becoIlw axaiJabJe for runoff. 9--06. INITIAL LOSSES. Snowmelt early in the SL',,,on is not l'ntirdy effective in produl'ing runoff, because of required condit':oning of the snowpnck and soil. 'ratf'r balanc(' computations, based on preceding rainfall and snowmelt, may be llsl'ful in determining- the soil and sno\vpaek liquid water deficiencies as outliIlf,d in chapters 6 and 7. Reconstitutions arc usually carried out for prriods aftf'r snowmelt is \\-'ell under way, and the basin liquid-,\'atc'r deficiencirs have be'en met. There may be high-rlrvation zonps, h<Hvever, for wl1ich soil moisture losses are not COlll- pletely satisfied untJ nllwh Jatl'r than the lower zones. 9-07. GROUND-WATER RECHARGE AND BASE FI,OW. In hydrogmph analysis, base flow is gf'nrrally separaLc.d from surface and subsurfllce runoff. Snowlllelt runoff indexes may be related to direct runoff (excluding base flow) and computed discharges then added to a base flow amount which is chilI'ttctcristic of the basin and time of ycur. A more rigorous approach is used for application of th,~ general snowmelt equations. H('rr, the totul sllowrrwlt quantity is consido'cd to be available for all components of streamflow. That. portion infiltrating to tlw ground \yatrr aquifers would vary: drpt'nding upon tl1(' geologic stl'llCtUl'i~ of individual basins. For typical mountainous areas in \ypstrrn L~lliU'd :)tates. the ground water fi'cbargi' accounts for about 30 percent of the snowmelt. This w::1t('r is the source of ha~(' flow and returns to the strf'nm ovpr a pc'riod of several months. ]n the rrcollstitution of stl'l'Hlidlnw in the Sltlmon HiveI' Basin, Tdnho, it WitS estimated tlut drep pe!'colation amountrd to O.:W illch Iwr ony, and it was assumed that dnily snowmelt fi\":dnblc for surf<lcP and suhsurface rUllolI was that which was in excess of tbat nIllOllnt. 9-08. SURFACE Ai\D SeBSURFACE RUNOFF. "'"ter exel'''L'S for surfal'r anrl subsurface runofl' may be roukd through basin stora;.;f', f'itlH'r by ullit hydrogr;lph or stonlgf' routing pro- ccduri's, as descrilwd in parngrapll ';'-05. Storagc' rouling constants or unit hydrograph shapes can be derivL'<1 by trial-anLl-error techniquL's throu~h rl'constitution of historical data. Inasmuch as snowlllrlt is morE' or less continuous over fi long period of timp, it is impractical to derive unit hydrograpbs by analysis of isolated short periods of intense l'ulloff. 9-09. EXAMPLE OF' SNOWMELT RECOXSTITUTION. In connl'l'tion with the denlopment of the maximum probable flood for th(' Salmon Rin'l' at. "-bite'bird, ldulto, basin snowmelt, Ilnd Touting coefficients IH'l'e testpel by synthesizing strt-':l111flmy for two snowmelt seasons, 1955 and 1956. The method of computation aurl rrsults of this '''lalysis arc prt'srnted in ehaptl'r II. 32 e . . e e