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<br />e <br /> <br />e <br /> <br />e <br /> <br />EM 1110-2-1405 <br />31 Aug 59 <br /> <br />b. The runoff from rainfall anlf'ccdptlt to tfH' risr under study is <,xcluc1rd hy rxtending the recession <br />eurv{' of the antecedent ris{' to an inlcrscction with the C'stimatcd buse Aow lillC', us indicated by CUl'VC <br />D-C in plate No. :I. <br />c. The grotlndwatf'f How is assumed to drcreuse ill the ol'd(.1' indiculNl hy t1](~ "normal d{'plction <br />eurv('" for a period of approximately ]2 to lB hours aftpI' tfl(' lH'g-illlling of the naod rise. (See Curve <br />A-B, plal(~ i\'o, :L) It is prohahle that during tJH' parly periods of a suddcll rise a reverse head is <br />impos('d on Un' groundwah'r' tablt, J)('ar tlIp strpam whi('h must [Ho overcome brfoI'e an incrrasc in basC' <br />flow will ocellf. The first acerction to hnsp flow probabl.y I'psults from J'(~("harg-(' by infiltration near <br />the stream channel whl're the diRtance to thp wnl<'r tablc is a minimum. The position of the base flow <br />line must be estimated, as wPlI as possiblf', by l'pfpl'pnre to tlw last point whl'rp it is kilO""'! that runoff <br />was suhstantially unaffpclcd by dirpct runoff from rainfalL <br />Ii, A straight line is drawn from the low point of the base ftO\'l line to intprspf't tht' recession curve <br />of the giVNl ris(' at a point wlH're it is estimated that discharge resulting from channel storage occasioned <br />by surfaee runoff had substantially ended. (See Curve B..C"E, plate ,",o,:l.) It is intended in this <br />pro('r-ss to indudc the major portion of subsurfacE' storm flow as u part of basI" flo\\l. ArtuaII.v, the <br />subsurfacp storm flow probably fpaefH'S a maximum shortly after the rainfall ('nds, and tends to recede <br />Owrpuftpr. However, in view of the many U1)("('rtaintics invoh'pd, tll(' simplp pro('pdul'e suggested for <br />estimating' the basp flow apppars to bp satisfadory, Suhsurfu('(' storm flow may normaII,\' be expected <br />to appC'ar in the later portions of a h:vdrograpb, and frequently ('olltinues in r{'cognizable proportions <br />for a JH'riod considerably long-f'r than n'quirrd for surfacl~ runoff to pass through the channel storage <br />phase, I It some hasins a relativply ~harp break occurs in the re(~pssion curvp, whcn plotted on logarithmic <br />paper, t.hat apppars to ihdieale Ow point \vhC'rr- the ratc of outflO\1l from chanllrl storage bN'omes reIa- <br />tivply small in proportion to subsurface storm flow. II However, such charaderisties apparently vary <br />with different hasins and must be ascf'rtainedl as well as Play he feasible'l from studies of sevcral h.vdro. <br />graphs that reprcsC'nt runoff from the hasin under a variety of conditions. <br /> <br />INFILTRATION INDICES <br /> <br />12. GENERAL. It has been demonstrated that the capacity of a given soil to absorb rainfall applied <br />continuously at an excessive rate rapidly decreasf's until a fairly definite minimum rate of infiltration is <br />rPRchNl, usually within a ppriod of R few hours,I.2,3,5 The order of decrease in infiltration capacity <br />and the minimum rate attained are primarily dependent upon the size of soil pores within the zone of <br />aeration and the conditions affecting the rate of removal of capillary water from the zone of aeration. <br />Thr- infiltration theory, with cf'rtain approximations, offf'rs a practical means of estimating the volume <br />of surfacC' runoff from intense rainfall in humid regions. HO\vever, in appl}ing the method to natural <br />draina.ge basins, the following factors must be considered: <br />a. The infiltration eapaeity of a given soil at the beginning of a period of rainfall is related to ante- <br />e('(lent field moisture and the physical condition of the soil. ,\ecordingly, the infiltration capacity for <br />th(' same soil varies appreciably. <br />b. lnasmueh as the infiltration capaeity of a soil is normally highest at the beginning of rainfall, <br />wh(,ff'as rainfall frf'quently begins at relatively moderate rates, Ii substantial period may elapse before <br />the rainfall intensity exeeeds the prevailing infiltration capaeity. ' Aetnally, studies have indicated that <br />a fairly definite quantity of water loss by infiltration is required to satisfy initial field moisture deficiencies <br />hrfofp runoff will occur, the amount of loss depending upon antececlC'nt conditions. A practical applica- <br />tion of thC' infiltration theor.v in estimating runoff from moderate nlinfall intensities ordinarily includes <br />an allowane(' for Hinitialloss{'s" corresponding to various antecedent field moisture conditions. <br />c. Rainfall ovrr rrlativC'l,v large drainagt' basins frequently does not cover the entire basin during <br />all p{,l'iods of prpcipitation with intensities excepding infiltration capacities. Therefore, a rational appli- <br />cation of the infiltration tht'ory to la'rge drainage arras require'S consideration of rainfall intensities in <br /> <br />5 <br />