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<br />EM llJ(I--2--1405 <br />31 Aug 59 <br /> <br />C. ::\0 definite procedure' can be' sugg-('stL'd at this time' for estimating tlH'- volume or I'ah~ of runoff <br />from subsurfacr storm flow whrH' a larg<' proportion of rUlloff is of thut typt'. A dt'tn.ih'd st,lId.'" of basin <br />characteristics dnd uyailablp hydrologic datu will usually [pvpul practical nH'thods of pst.imating sub- <br />surfac(' flow conditions ill a parti('ulnr an'a. TIll' limit{'{{ studi('s bf such problt>JIls now fl.viLilablt' in- <br />dicate that th(' propP!" approach in dl'tl'l'minillg both VOllllllt' llnd Niti' of runoff ill basills having pro- <br />nounced subsllrfu('(' flow ('haractpristics n'quin's a ('oIT('}atioH of rUIl-ofl' ph('llOnWlloll \vitll initial fipld <br />moisture conditions and grolllldwau'f sloragC'. <br />d. 1n the majority of the an'us classified as humid ill tllp Cnitpd Statf's, suhsurface storm How eOIl- <br />stitutC's a small proportion of major flood runoff yoI1ll0('5, although thp ('ontributiolls of sUhsUrf:H'{' <br />rIow to minor' flood I'isps an' frNIlH'lltly l'('('og'nizahl(' and may ('onstitutp It signilieltnt proportion th('rpof. <br />In thp following discussions of hydrogr'aph uIlalysps l'<'iat('d to ilw <!n,"dopnwllt of unil li,nlrographs 01' <br />computation of infiltration indiecs for drainag<' basins in humid n.'gions, slIhslIrfu('P storm flow will lw <br />treated as part of hase flow. <br /> <br />9. NORMAL RECESSION CURVES, In the anah'ses of hydrog-raphs, it is usuall.\' rwe('"ary to eorred <br />the r{'cord('d hydrograph to rliminat(. I"lmofl' from rainfall ftlltp('pd('nt, or suhst'quPllt, t.o the Iwriod of <br />rainfall ulHlp[' consideration. In studies of rUllofl' from lurg(' ha~ills, flood nmtillg ('_omput.ations mn,V b(>. <br />rcquin'd to associatE' runoff with rC'lntNl rainfall. How{'vp}", ill basins ol'(lirmrily involvf'd in unit hydro- <br />~raph allulysrs, runoff from distinct fwriods of inl('t!sp rainfalluslIu.Ily may 1)(' isolah,tJ salisfact.oril.v hy <br />use of linonnnl n'cessioIl ('un'('s." A normal !'('('Nw.ioll ('UI"V(' IlHl)" bp ('omput('d from spg'ments of hydro- <br />graphs that J'('prrsPllt dischurgr from llutllra.I vallpy or chullrw! storagp, after hasp flow has hel'1l suh- <br />trad{'d, as suggrstpd by '\V. B. Lnllgbpill.9 SpgmPllts of spvl'l'H1 hydrogTlLphs may IH' IH'('('Ssary to {.ov('r <br />a satisfaetor.y range in dis('burg-ps. Haying sp]P('t('d some' ('oIlv('nipllt. unit of time, sa,Y () hours, tbp <br />discharge a,t SlH'C('ssiv(' timp int('('\'als is [('ad from thp sqrHH'nts of hydl"Ographs lhlLt. apppur to 1)(. 1111- <br />affectC'd by ('urr('nt rainfall. Thp discharge Ylthws at tllp lH'gilllling of ('n('h unit I'llllofl' JH'riod an' plol tpd <br />against values for the t'nds of thr rpsIwcti\'(' Iwriods. A ('urVI' is th('Jl drawn as all approximaf,l' IIwan <br />of the hig}ll'st points, llrglrcting a fpw points that plot so far to tllp Ipfl as 10 apIH'aJ' to {'(',H'('s('1l1 I'('- <br />cession of rllnoff from a partial-area storm, Vallles ::-wal('d from tll(' IlWltll ('ur\'(' aI"(' r('plot tpd to tll(' <br />scale of the natural hydrographs to be studiC'(1. 11 should !H' o})s('I"\,(,<I that f'('('pssioll ('lII'VPS dpriv('d ill <br />the mannrr described above apply to 1I0rmal 01' fivpragp storm distributioTls. It may l)(' ('xpt'etpd that <br />runoff from rainfall eOTleentrat('(1 in tlIp UppPI' portion of a hasin will I'('("pd(' mol'(' gradually than ill- <br />dicated by the norma) recession ("urve and ('onyprspl.v for rainfall ('oncentrat<'d in the lower hasin, <br /> <br />10. GROUNDWATER DEPLETION CURVES. The rat" of diseharg-e from g-[,olllldwater slorag-e may <br />be estimated by means of a "ground''v'atpr deplPlion cuI'vp" d('l'iv('d from spgnH'nts of re('orded hydro- <br />graphs that are not appreciably affected b~' direct rlllloff or disclmrgC' from ehalllH'1 storagp. The <br />method suggested in the pre('('ding paragraph for eonstrueting normal {'('cession eurv('s may be used in <br />computing ground \\'ater dt'pktion eurvps, ('xeppt that a unit time interval of approximatC'ly 2 to 5 days <br />would bC' us('d instt'ad of ollr of u frw hours, The dist.indioll hrtw{'en n "nol'mal r('cession curve," <br />whieh reprC'sC'nts disellarge from ehanlH'} and vall(',\" storagr, and "groundwatpr depletion curve," whi('h <br />repregC'tlts outfiow from groundwater storage, should not 1)(' overlooked. <br /> <br />II. SUBDIVISION OF HYDROGRAPHS. A eOllsid"rabl" amoullt, of p"J'Solla.l jlldg-mcllt is involvcd <br />in the subdivision of obsC'rvet! h~'drographs into the threC' eOmpOll('nts of surfu,('c runoff, subsurface <br />storm flow and groundwatpr flow, The following g('lleral procC'dul'c is suggestcd for pl'ohlpms in which <br />the volume of subsurfa('(' storm fio,"{ is rC'lativeIy small in proportion to surfaec runoff from major floods <br />and hence may be satisfactorily considered as a part of base flow: <br />a. Rainfall dnta. are analyzed by means of isoh,vetal maps a,l}d mass rainfall curves for the entire <br />period of the givPll flood risc and several da~'s antcf'pdellt thereto, Hyctographs for representative <br />areas un' plotted above the hydrograph in propf'r time rdation for use in ('stimating the timing of <br />surfaee runoff, (See plate X 0, :l.) <br /> <br />4 <br /> <br />e <br /> <br />e <br /> <br />e <br />