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<br />e <br /> <br />e <br /> <br />e <br /> <br />EM 1110-2-1405 <br />31 Aug 59 <br /> <br />The initalloss, in inches of drpth, is ('stimated and ('onverted to inch-square miles for each <br />station polygon (columns 5 and 6, part, II, plate ~o. 4). Thp accumulativc rainfall must <br />excecd the initial loss belore tbe infiltration theory is assumed (0 apply, <br />A trial value of the infiltration inf}('x Fa~ in inches ppr hour is assumed, and thr equivalcnt <br />infiltration loss, in inch-squarp milf's, for each station polygon is tabulated (column 8, part II, <br />plate No.4). The rainfall quantitips in ,'xcess of th,> trial value of Po. after the initial loss has <br />been satisfied, are added and compared with t.he quantity of surface runoff computed ahovp. <br />The procedure is repeated until the value of Fa~ necessary to give rainfall excess equal to the <br />correct volume of surface runoff is determined. The rainfall and rainfall excess data for <br />representative zones are plotted as hyctographs above the runoff hydrograph in the mRnnf'r <br />illustrated in plate Xo. ~, <br />c. The method outlined above is not difficult after a reasonable amount of expprience in its use has <br />hpcn gained. The basic data necessary are usually required in eonnection with other phases of hydrologic <br />studies. Inasmue-h as the procedure takes into account the areal distribution of rainfall and variations <br />in intensity, it is suitable for application to large drainage ba,sins. The method is susceptible to refine- <br />ment or simplification, aecording to the accuracy of basic data and the ncC'ds of the partie-ular project. <br />In the example titpo, the infiltration index 'was assumed to be constant over the drainage area) but <br />variations may be estimatNl by consideration of soil characteristics if the necessary information is <br />availabl('. The,probable decrease in infiltration capacities within the respective polygons may also be <br />{'stimated to correspond with infiltration eurvps derived from experimental projects. In some eases, a <br />hett.('r I'('presentaLion of rainfall-intf"nsity variations in various portions of a drainage basin may be <br />ohtained by interpolating mass rainfall curves for the various areas, instf"ad of adhering strictly to the <br />Thi,'ssen polygon method. <br />d. Values of infiltration indices for a large number of drainage hasins are presented in plate No.5. <br />The' indi('.('s wcrC' computrd from data relating to modpratc and major floods, The infiltration indices <br />W('r<' not derived in cxa(~t.l.Y the manner outlined in the precE'(ling paragraphs, but the values are con- <br />sidprcd as an approximate indication of the minimum infiltration indices to be f>:x:pectcd during major <br />storms in the rcsppctive areas. <br /> <br />U.1\IT HYDROGRAPH8 <br /> <br />15, GENERAL. a. A unit hydrograph, as used herein, is a hydrograpb representing 1 inch of direct runoff <br />from a rainfall of somp unit duration and specific areal distribution. The' basic premise implies that rainfaH <br />excess of 2 in(~hf's within thp unit of ouration will produce a runoff hydrograph having ordinates twice <br />as grrat as those of t.he unit hydrograph. It is also assumed that rates of runoff from consecutive <br />units of rainfall execss having the same areal dist,rihution will be proportional to the unit hydrograph, <br />and that ordinatl's of tbe sevpral partial hydrographs obtained by multiplyiug the unit hydrograph by <br />Sll('('rssivl' rainfall px('f'ss amounts of unit duratioIls may be added to obtain the total hydrograph of <br />rUlloff. Tbt's(' hasie assumpt.ions arf' not rigorous, but it has been founel by expenence that the unit <br />hydrograph mpthod giv{'s results suffieirntly accurat.e for most practical problems, if reasonable <br />jlldgmrJlt is used in its applicat.iol),4 <br />h. ln th(' purlipr sta-g('S of d('vC'lopnwnt of the unit. hydrograph mpthod, it was geof>rall,Y assumed <br />that rU!loff-produeing rainfall rt'slllting in a unit hydrograph was uniform over the drainage area <br />illvolvpd, Ho,wevpr, such a rOllcppl gn'lltly rt'strids thf' Utwfulrwss and applicabilit.y of the unit hydro- <br />graph IH'O{'Nlurl'. Cnit hydrographs J'esultin~ from l'aillfall-px('Pss quantitif>s uniformly distributed <br />OVPI' fl draillag(l hasill may lw liSP!! to {'omputt' rate's of runoff that would f{'slIlt under average rainfall <br />('oIlditioIlS, wh{'n'as 011(' that n'f1(-.('ts thr rC'ginH'tl of rUlloff from prp('ipitation of SOffif\.{hat hight'r <br />illt(,tlsity ill thp lower hasin may }H' llsdul.ill estimating criti('u.l ratl's of discharge. \I~alley storagp <br />sprvps to pliminal<' till' (.ffpets of miIlor variations ill r;llinfall distrihution, but major yar::ations in dis- <br />tril)lltioll llf'(' n.fll'l'j(.d in t.ht' rUlloff hydrog-raph. It i::-: prlH,ti('ahL, to deriv(' unj!', h,vdrographs to reflect. <br /> <br />7 <br />