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<br />EM 1l1()..2-1405
<br />31 Aug 59
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<br />various portions of the hasin in order to determint', at lrust approximatd~-, thp arC'lL eoverf'd b,v f'ff(lCt-iv{'
<br />runolT-produ('ing rainfall.
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<br />d. During actual t1ood-produC'ing storms, the rainfall in various, portions of a drainage basin is marC'
<br />or l('s8 intf.'rmiUf'nt. Sueh interruptions in pl'peipitation modify th(' orel{'r of df'en'asc in infiltration
<br />capacity that might othrf\\risp be exp('('ted according to th!' infiltration th('ol')'.
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<br />13. INITIAL LOSS. a, Initial loss is defined as thl' maximum amount of prl'cipitation that can oc('1lI'
<br />under specific conditions without producing runoff.6 lnilialloss vahlf's for basills i.n humid n.n'RS of tIlt'
<br />United States may range from a minimum valut' of a few l('nths of an inch during n'lativC'ly w('t srasolls
<br />to approximately 2 in('hes during dry. summrr and fall months. Th(, initial loss for ("onditions usually
<br />pn'cNling major floods in humid f('gions normally rangf'S from ahout 0.2 to o.!) inch and is rPlativC'I:--
<br />small in comparison with the flood runoff volumf'. ConsC'qllC'ntly, in computing infiltration indices
<br />from data for major floods, allowanc('s for initial loss may be rH'gl('ctrtl or pstimat,N! npproximll.t('Iy,
<br />without introducing important rfrors in the rrsults,
<br />b. Franklin F. Snyder 6 has suggrstpd a proc('dur(' for f'stimating initial loss vahlf's corrrsponrling
<br />to fipld moistllrp conditions as 1't'f1C'eteu by tht' gl'Oundwatr-f dischargf'. Tht, proe('(hlr<' is f('('ommt'nd{,d,
<br />for C'onsid{'ration in conn{'ctioIl with probh'ms requiring r-stimatps of I'llllofl' from modt'fat.e stOl'IllfL
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<br />14. INFILTRATION INDEX. a. In vil'W of Ih" approximatiolls illvolVl'd ill illHltratioll ('stimatl's,
<br />th(' av('ragp loss ratr eomputr-d from rainfall-runoff data for natural draillagl' basins willlH' l'('ferrNI to
<br />hpf('in as "infiltratioIl imlC'x," rathpr than as "infiltration capacity." lllfilll'atioll indt,x FafJ is clt'filwd
<br />as ;lIl avpragt' ratt' of loss su('h that thp volump of raillfall ill rxcpss of that nill' \vill ('qual tlw vohnnf'
<br />of dirrcl runoff. .
<br />b. Th{, follmving proccdure is rr('ornmpndNI for th(' ('omoutation of infiltrat.ion indief's to 1)(' lisen
<br />in f'stimnting th(' volume of runoff from major storms ill l.ugP drninagp hasins:
<br />Hydrographs of thr major floods of rr('01'41 in (ll(' hasin ltn' spl(,(,tpd for study: thp voluI1H'
<br />of dir('d rU'lloff is comput('d h.\T suhtl'uding tlw pstimntt'd basp flow, and rUlloff from ('xtrUIlf'OUS
<br />rainfall, from th(' r('coniPd gross volunw of runoff, (;-';p(' pInt<' :'\0. 4, column 29, lin('s 2:1l
<br />;32, and :34.)
<br />IsohYf'tal maps, similar to platf' .\'"0. 1 art' drawn for (,Il('h pl'itlriJlal I'uinfnll Jwriod, ntld
<br />mass rainfall curves (plate 1\0. 2) ar(' constl'uctt'd.
<br />The drainAge arra is suhdividt'd by a s('!'ips of 'fhipssl'n polygons dt,fining un'as that an'
<br />neare'st the various precipitation stations (platr Xo. 1). In d('vrloping thp Thipss('n polygons,
<br />construction lines are drawn fH'tw('pn fH'ighhoring statioIls and pt'qH'ndicular his{'ctors of t.hpsp
<br />line'S arr errcte(( as rC'quirpd to obtain elos('() polygons surrounding t.hp I'('sppetiv(' precipitation
<br />stations. The arpu Ap ,'.;ithin raeh polygon and within th{' drainagt' basin is mpaslll'cd and
<br />tahulatl'd (column 4, plate :\0.4).
<br />Thp avprage drpth of rainfall PrH within ('arh statioll polygon is rstimat.rd from thp
<br />total-storm isohyrtal map and tabulat('d as shown in eolumn 6l part I of plate No.4.
<br />Rainfall quantitit's, in in('ht,s, for su('r('ssivp units of timp an' sealed from mass rainfall
<br />curv('s for the resppctiv{' precipitation stations. In sdt'cting the "unit" of time to.bp us('d, th('
<br />d ('nsity of ohsPl'vation stations and ('haratt('r and accuracy of rp('ords should be considt'rNI.
<br />A unit period of :3 hours if; suitablf' for most studies fplating to large drainagf' basins.
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<br />A conversion factor, }?l=Ap -If:~, is computed for paeh prN'ipitation station (column 8,
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<br />part Il plate 1\0, 4), and the volume' of rainfall within ('arh station polygon, rxprcs.~f'(l in ineh-
<br />square miles, is C'omputed by multiplying the :~-hour rainfall quantit.ies by Fe. (Si'e columns
<br />10-24, part I, platl' Xo. 4.)
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