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<br />EM 1110--2,1405
<br />31 Aug 59
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<br />c. ~o definite proC'E'dur(' can b(' suggpstpd tlt this timp for t.stimating till' voluffir or ratt' of runoff
<br />from sUbSUrfUf{, storm flow wh(>l'(' a lnl'g-l' proportion of runoA' is of that typ(', A t!<.taih'd study of husill
<br />characteristics .;wd uyuilllbIr hydrologic datu will usually I'('v('ul praetieal illPthods of l'stimatillg sub-
<br />surfac(' flow ('oIlditiOIlS ill a parti('ulal' arpa. Till' limitpd studit's bf such probll'ms !lOW availahl(' in-
<br />dicate that tilt' propPl' appt'Ofieh ill dl'tpl'millillg both volultH' and rntc of nlllofl' ill basins having pl'O-
<br />nouI1cf'd suhsul'fuc(' flow t'iwra('f('l'isties n'quin'8 a. cOI'l'l'latioll of J'l1Il-Ofr plll'1l0mPIIOIl \vith initial tipld
<br />moisture conditioIls and groundwau,l' storag(',
<br />d. In the majority of tl1(' un'us dassifipd as humid ill t/w enitl'd Statps, suhsurfacr storm flow eon-
<br />stitutr-s a small proportion of major flood I'unofl' yolllmps, although tlH' l'olltl'ibutiorls of suhsUrf~H'('
<br />flow to minor' Hood ri:ws are fn'qu('ntIy l'('('of!.'llizahl(' and may COllstitutp it signili('u,nt propol'tion tlw['('of.
<br />In the following discussions of hydrogi'uph ullitlys('s t'('}at('d to tlH' d('V(.lopnwlll of ullit flydrographs 01'
<br />computatioll of infiltration indic('s for drainage basins in humid regions. suhsllrfac(' storm !low will fW
<br />trratNI as part of hus(' flow.
<br />
<br />9. NORMAL RECESSION CURVES. In the analyses of hy'drographs, it is us,mll,\' nr<'cssa,ry to correct
<br />the J'(>('oI'(lpd h,nlrograph to plimitlut(' I'unoff from rainfall fillt('cl'dpllt, OJ' subs('flll('llt, 1,0 tl1(' (wriod of
<br />rainfall ulldpr cOllsid('l'utiOIl. In studi('s of rUllofl' fmIll large' basins, flood routing" comput.ations may bp.
<br />rrquin>d to assodatp rUlIofl' with I'(,lated rainfall. Ho\\,pv('r, in basins ol'dinarily iflvolvl'd ill unit hydro-
<br />graph analysps, runon' from distinct [wl'iods of iIlU'llSP raillfallusuaIly may 1)(' isoJal<.d sa1isfadoril.v by
<br />usp of "normal I'{'ccssion cun'l's." A Ilormal t'('('('s~ioll ('Ul'\'(' may he l'OInJHllf'd from spgrrH'llts of h,nlro~
<br />graphs that I'Pprrsc>nt disdwl'g'C' from natural nlllpy 01' elllltlllP! storagl', after hasp nO\V hus h(,(,1l sub-
<br />traded, us suggcslrd hy \V. B. Lung/win,o S('gm('rl1s of sevl'l'ul hydrogmphs muy !)(> IH'C'('SSUI'Y to ('OV('I'
<br />a satisfactory rUIIgp in dischul'gps. Ha,"ing S('!<'dcd BOInP ('onvPllipllt unit of timl', say () hOIlI's, Ihp
<br />discharge at SlH'('('ssive tinw illt(>l'\'als is rC'ud from tlH' sq~nH'nts of hydrogrllphs thu,t. npp('ur to bp 1111-
<br />afrr('iC'd by CUf]'Pllt rainfall. Th<' dischurgp "altlC's at the' lH'gillllillg of <,aeh ullit t'unoff JH'riod H.I'P plottNJ
<br />against VUltH'S fof' the <'Tlds of thr f'('SIH,(,tiV{' pPl'iods. A ('lJrvC' is tlH'1l dl'llWIl as all upproxirnatp mpl1ll
<br />of the highest points, lIPglllctillg u. fl'w points tllat plot so far 10 the Idt as to apIwu.1' to !'pprpsPllt I'P-
<br />cession of rUTloff from a partial-area storm. Values scnh'd from the mc'utl ('UfVP un' rpp(ol1pt! 10 IIIl'
<br />scale of thr natural bydrographs to tH'studipd. 1t should bp ohsPl'vl'd that !'('('l'ssioll CllrVI'S l/Priv('d ill
<br />the mannpl' dpscrihr-c! abovc apply to Jlormal or il.VPI'Ug'P stonn distributions. It may Iw ('xlwdpd that
<br />rUllolr from rainfall cOll('pntratl'd in thp UpP<'I' portion of it basin \viII n'('('d(' IlHH'(' gnlCluully t IUUl ill-
<br />dicatcd by tlll' normal rcccssion curvp and ('on\rrnwly for rainfall conccntrat<'d ill tlH' !o\Vpr hasill.
<br />
<br />10. GROUNDWATER DEPLETION CURVES. The ratl' of disl'har~(' from grolllldwntl'r stOnt~l' mlty
<br />be estimated by means of a "ground\vatrr dl'pl(>tion ('UI'VP" dl'rivpd from ~H'gm(>lIts of re('onl('(I hydro-
<br />graphs that aI'€' not appI'c('iably aff(>ct<'d by -dircet runoff or dis('harg(' from dULTHH'( stoJ'agp. The
<br />method suggcstrd it! thc prcl'Nling paragraph for constructing nOl'mal I'('('('ssion (;urvps may he used ill
<br />computing ground watl'r df>pktiotl curvps, ('xcrpt that a unit time intpl'val of approximat.ely 2 t.o ,!) days
<br />would he uSNl instead of Ol1l' of a f('w hours, The distinctioll hd,,,r('cn a llnormal recession curve,"
<br />which represents discharge' from challll('l alld vall('y stOl'ugf', aJld ilgrounch'v'atPJ' d('plelion eUl've," which
<br />reprE'srnts outflow from groundwater storag<" should not hr ovPrlOOkl'd.
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<br />11. SUBDIVISION OF HYDROGRAPHS. A {,ol1sidrrubll' amollllt of persollal jud~ment is involved
<br />in the subdivision of obs('rv('d hydrogl'aphs into tll(' thr('p eompollPIIts of surfaec runoff, subsurface
<br />storm flow and grolllldwakr flow. Tllf~ followillg gPHt'ral pro('('durf' is suggested for prob}('ms in whie-h
<br />the volume of subsurfac<' storm flow is rrlativE'ly small in proportion to surfaee runoff from major floo<ls
<br />and hpTlcr may be, satisfactorily considel'l'd as a part of hasp flow:
<br />a, Rainfall datu arc analyzed by ffiPUllS of isohyetal maps and mass rainfall curves for the entirt'
<br />period of the given flood fisc and sev('ral da,vs antN'p(Icllt thpI'pt.o. Hyetog-raphs for representative
<br />areas ar(' plotted abo,rc the hydl'ograph in prop<,r time rrlution for use in pst.imating- the timing of
<br />surfaee fUlIoff. (See plate ;\;0. :J.)
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