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<br />e <br /> <br />.. <br /> <br />e <br /> <br />. <br /> <br />e <br /> <br />EM 1110,2,1406 <br />31 Aug 69 <br /> <br />The initalloss, in inches of oC'pth, is estimat('d and converted to inch-square' milf's for each <br />station polygon (columns .5 and 6, part II, plate 1\0. 4). The accumulative rainfall must <br />C'xceed the initial loss befofr the infiltration theory is aSSUffiC(] to apply. <br />A trial value of the infiltration indrx p~v in inehes per hour is assumrd, and thp f'quivalf'nt <br />infiltration loss, in inch-squarp milf's, for each station polygon is tabulat<'d (column 8, part II, <br />plate No.4). Th<' rainfall quantitirs in ('XC{'SS of th(' trial value of Fau after the initial loss has. <br />been satisfied, are added and compan'd with the quantity of surfaee runoff computed ahovt:'. <br />Th(, procedure is repeated until the value of Fau necessary to givp rainfall excess equal to the <br />corred volume of surface runoff is uC'tE'rmineu. The rainfall and rainfall excess data for <br />representative zones are plotted as hyetographs above the runoff hydrograph in the mannpr <br />illustrated in plate !\o. o. <br />c. The method outlined above is not difficult. after a reasonable amount of experiencc in its use has <br />been gained. The basic data necessary are usually required in connection with other phases of hydrologic <br />studies. lnasmueh 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, according to the accuracy of basic data and the nc('ds of the particular project.. <br />In the example citp<l, the infiltration index 'was assumed to be constant oyer the drainage area, but <br />variations may be estimated by consideration of soil characterist.ics if the necessary information is <br />availablp. The probable decr('ase in infiltration capacities \vithin the respective polygons may also be <br />f'stimatf'd to correspond with infilt.ration curves dprived from rxpcrimcntal projects. In some cases, a <br />hpttpr n>prescntation of rainfall-int.ensity variations in various portions of a drainage basin may br <br />ohtained by intrrpolating mass rainfall Cllrves for the various areas, instead of adhering strictly to the <br />Thi,'ssen polygon method. <br />d. V-alurs of infiltration indices for a larg~ number of drainage basins are presente.d in plate ~TO. 5. <br />Th(' indices weI'f' _computf'(l from data relating to modt'ratf' and major floods. The infiltration indices <br />Wf'f(> not derived in exaetly the maJllH'r outlined in the pn>cf'ding paragraphs, but the valuf's are con- <br />sid('rcd as an approximate indication of the rninimum infiltration indices to be pxpectl'd during major <br />storms in the resppctive areas. <br /> <br />UNIT HYDROGRAPHS <br /> <br />15. GENERAL. a. A unit hydrograph, as used herein, is a hydrograph representing I inch of direct runoff <br />from a rainfall of some unit duration and specific areal distribution. The basic premise implies that rainfall <br />excess of 2 inches within the unit of duration will produce a runoff hydrograph having ordinates twice <br />as gn>at as those of the unit hydrograph. It is also assumed that rates of runoff from consecutivf' <br />units of rainfall ,'xcess having the same areal distribution will be proportional to the unit hydrograph, <br />and that ordinat('s of the Sf'veral partial hydrographs obtained by multiplying the unit hydrograph by <br />su('cpssivp rainfall ('xeess amounts of unit durat.ions may be added to obtain the total hydrograph of <br />runoff. Thrsr basil' assumptions are not rigorous, but it has bren found by experience that the unit <br />hydrogruph method gin>s results suffirirntly accurate for most practical problems, if reasonable <br />jmlgmf'Jlt. is used in its applicat.ion.4 <br />IJ. III the earlipr stag('s of development of thC' unit hydrograph method) it was generally assumed <br />I hat l'uIIoff-producing rainfall rrsuJting in it unit hydrograph was uniform over the drainage area <br />illvoh-'('(l. 1-1 ow ('y('l', sueh a COllcPpt grpatl.v rt'stricts the> uspfulness and applicability of the unit hydro- <br />graph proC't>durr. Unit hydrographs rrsulting from rainfall-('x('{'ss quantities uniformly distribut.ed <br />ov(>1' u druinagt' hasin may ht' LlSNI to compute ratps of rUlloff that would result under average rainfall <br />(.onditioIlR, wh('I'(>as Ollf' that rdi('cts the rrgimrll of l'uTloff from precipitation of somewhat higher <br />illt('llsity in thr lower hasin may be wwfuJ in (1stimating critical ratt's of discharge. "-alley storage <br />S('I'Vt>s to pliminate tht-' effp('ts of minor variat.ions ill rainfall distribution, but major variations in dis- <br />trihution aJ'P I'p!lp('u.d ill thp runoff hydl'Ograph. It is praC'tieabl(' to drrive unit. hydrographs to reflect <br /> <br />7 <br />