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<br />EM 11111-2,1405
<br />31 Aug 59
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<br />various portions of the hasin in order to df'termirw, at least. approximatf'I:'lT, thr arpa eov(':I'f'c! by l'fff'ctiv('
<br />runoff-producing rainfall.
<br />d. During actual Hood-producing storms, thr rainfall in various, portions of a drainage basin is rnOff'
<br />or less intprmittent. Sueh interruptions in prpcipitatioll modif~ the' onl('r of dt'('T('USC in infiltration
<br />capacity that might othel'wisf\ he expt>etf'cI according to thl' infiltration thpory.
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<br />13. INITIAL LOSS. a. Initial loss is defined as th(' maximum amount of pr('cipitation that ('fin oc('ur
<br />und('f specifie conditions without producing runoft'.6 Initial loss valuC's for hasins in humid an'as of tll{'
<br />Cnited States may rangl:' from a minimum valu(' of a fpw tenths of an inch during rdativply w('t, srRsons
<br />to appl'Oximatt>ly 2 inches during dry. summr!' and faU months. Thl' init.inlloRs for ('oudit.ions usually
<br />prec{,lJing majol' floods in humid n'gions Ilormall~. rang{'s from about 0.2 to 0..1 ineh and is rl'llltivd~'
<br />small in comparison with the flood runoff volume. Consequrntl.y, in computing' infiltration indicl's
<br />from data for major floods, allowancf's for initial loss may be neglPct.pd or l'stimat('d approximatl'ly,
<br />without introducing important ('rrors in the ff'SUltS.
<br />b. Franklin F. ~nyder 6 has suggpst('d a prOCf'(JufP for rstimating initial loss valtH's corrrsponoing
<br />to firld moisturp ('onditions as rpfl('d('d by thl' groundwater diseharg<'. Th(, proe(,lhll'f' is rl'l'Omml'IH!rd
<br />for considl'ration in ('onlleetion with problems requiring estimnt('s of nllloff fl'om mOdl'I'lltp storms.
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<br />14. INFILTRATION INDEX. a. In view of th.. approximations inYOIVl'd ill infiltration ('stimates,
<br />the' avrragf' loss ratr l'omputpd from rainfall-runoff datn. for nutural dl'lLillag(' basins \villlw t'l'ferrpc! to
<br />hpt'pin as "infiltration ind(.x," rathl'l' than as I'infiltration ('apa('ity." Infiltration ind('x Fav is drfilwd
<br />as :lJl aVl'l'ng(' ratp of loss sueh that thr VOIUIlW of rainfall in l'XCPSS of that ratp will pqual thl' volump
<br />of dir('ct runoff.
<br />b. Thl' follO\ving procedure is r('comm('nd(,d for th(' ('oInoutation of infilt.ration indieps to 1)(' us{'d
<br />in pstimnting thr volume of I'unoff from major storms ill ldl'gl' dl'uinagl' bll~ills:
<br />Hydrographs of thp major floods of I'('('ol'd in tilt' basin nl"(' ~'H'l(,(.t('d fOl' st.udy: th(' ~'olllnll'
<br />of dirt,ct rUllOff is computf'd hy :mhtl'u\'ting thl' pstimatptl hasp flow, alli\ runoff from ('xtrallPOUS
<br />rainfall, from thp rp('ordpd gross ,"Ohlffi(' of I'ulloff. (Sc'p platt' Xo. 4, column 2!l, lillPs 2:{,
<br />:12, an,l :14.)
<br />Isohyrtal maps, similar to platp :\'0. 1 UI'(' drU\Vll for ('ueh principal rain fiLII Jwriod, alld
<br />mass rainfall curvps (platC' Xo. 2) are ('onstruet('d.
<br />The drainngl" arpfi is subclivit\l.t\ by a sprips of Thiessf>n polygons dpflning arpftS that an'
<br />lll'arrst the various prpcipitation stations (platt' ;\0. 1). In dt'vploping th!' Thipsst'1\ polygons,
<br />constructionlinps arp drawn bptw<'l'1l Ilt'ighboring stations and JH'r[wndieular bispet.ors of thpsp
<br />linrs arp el'rdrd as rpquirf'd to obtain clost'd polygons surrounding t.hp ff'Spl'etivt' precipitation
<br />stations. The area Ap within (lach polygon and within t.hl' dl'ainnge basin is mpusurrd and
<br />tabulat..d (('olumn 4, plate :\"0.4).
<br />Thp aVPfagc drpth of rainfall Pa~ within pach station polygon is pstimat.pd from th('
<br />total,storm isohyetal map and tabulated as shown in ('olumn 6, part I of plate 1\"0. 4.
<br />Rainfall quantities, in inehl's, for suc('('ssivp units of t.ime afP scalpel from mass rainfall
<br />purves for the rrspC'ctivp prpripitation stations. In sdN'tillg thp 1f11nit." of time to ht, ust'd, th('
<br />cl ('nsit~T of obSPfvation stations and l'harader and accuraey of l'('l'o1'ds should be eonsidprNI.
<br />A unit period of :~ hours is ~mitabl(' for most studies rplating to large drainage basins.
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<br />A . f L' A /'", . d fl'" . (I 8
<br />conversIOn actor, 1" ,= P -/..> ' IS compute Of ('ac 1 pr(,(,lpltatlOn station co umn ,
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<br />part I, plate 1\"0. 4), and the volume of rainfall within eaeh station polygon, ('xprcssed in inch-
<br />square miles, is computed by multiplying the :l-houl" rainfall quantiti('s by r,. (Si'e columns
<br />1O~24, part I, plate Xo. 4.)
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