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<br />EM 1110-2,1405
<br />31 Aug 59
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<br />b. The runoff from rainfall ant(>(~('(l{'nt to tllp rise undrr stud,Y is ('xelmlcd h,Y C'xtending the r('cession
<br />curve of the antecedent risp to an intc'fsection with the rstimated base flow linr, as indicalrd by CUl'v('
<br />D-C in plate No. :L
<br />c. The groulldwatf'f flow is assumed to d('('re8se in the onlpt" illdicatc.d hy thf' llnol'mal depletion
<br />('urv('" for a ppriod of approximat<{y 12 to l~ hOlll'S aftd' tllp tlPgilltlillg of the flood rls(', (See Curve
<br />A-B, plate 1\0. ;t) It is pl'obabl{' that during tlH' parl:\-' periods of a suddC'Il l'i8{, a rpVPl'sr head is
<br />impospd on thr groundwater tahle rlf'ar tll!' strpam whirh must hp oV('t"comp brfon' fin incl'rtlse in hasp
<br />Aow will oceul'. The first accretion to basp flO\v probably I'psu]ts from l'Pchal'gf' by infiltration near
<br />the stream channel wlH're the distanee to the \V3:t(>r table is a minimum. Thp position of the base flow
<br />line must be estimated, as w('II as possib](', h,'- rdprt'rlce to tht, last point ,dlpJ"(' it is known that runoff
<br />was substantially unaffN,tcd by din'ct runoff from rainfall.
<br />d. A straight linc is drawn from the Io\v point of tlH' base flow line to intN'scrt tlU' reccssion curve
<br />of the given risc at a point wlH're it is estimated that discharge resulting from ('hanll{'l storage o(Tasioned
<br />by surfaee runoff harl suhstantially ended. (See Curv(' B-C-E, plate :.\"0. :() It is int.ended in this
<br />pro('('ss to indudc the major portion of subsurfa('e storm flO\v as n part of hasp flo....v. Ac,tually, the
<br />suhsurfacc storm flow probahl.y rcaches a maximum shortl.....- aft('J' the rainfall l'IHls, and tC'nds to recede
<br />tIH'rpaft('r. Howevcr, in ....-icw of the many uJl('('rtaillti('s involv('d, thr simplp pn)('rdure suggested for
<br />pstinmtillg the basr flow apppars to b(' satisfaetory. SubsUl'fu('f' storm flow may normally he expected
<br />to appt'ar in the later portions of a hydrograph, and frequentl.\" ('olltinups in r('cognizable proportioIls
<br />for a ppriod ('onsid(~['abl,\T longer than f'('quirpd for surfacp rUlloff to pu:;s through the chanIlel storage
<br />phasp. I n some basins a ('('lativpl...... sharp hn-'3:k occurs in the rec('ssioll ('urv('. when plotU'fl on logarithmic
<br />paper, lhat appears to indi('ate tlH' point \dIPI'P the raLp of outf1O\,,- from ('haTllld storagt' lwcoffies rela-
<br />tivply small in proportion to subsurface storm flow. Ll IJowever, such {'haruelpristics appur('ntl~T vary
<br />with diff('rPIIl basins and must bp as('t'rtained, as w('ll as Play hr frasible, from studies of several h,vdro-
<br />graphs that rcprespnt runoff from U1(' basin undpr a \'ariety of conditions.
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<br />INFILTRATION INDICES
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<br />12. GENERAL. It has been demonstrated that the capacity of a given soil to absorb rainfall applied
<br />continuously at an excessive ratf. rapidly decrease's until a fairly drfinite' minimum rate of infiltration is
<br />rrachf'd, usually within a prriod of a few hours.L,2.3,s The ordpl' of decrease in infiltration capacity
<br />and the minimum rate attainrd arc primarily d<'pendent upon t.he size of soil pores within the zone of
<br />af'rll.tion and the conditions aff('cting the rate of removal of capillary water from the zone of aeration,
<br />Thf' infiltration theory, \",-ith certain approximations, offers a. pra.ctical means of estimating the volume
<br />of surfacf' runoff from intcnst' rainfall in humid regions. However, in applying the method to natural
<br />drainage hasins, the following factors must be considered:
<br />a.. The infiltration capacity of a given soil at the beginning of a period of rainfall is related to ante-
<br />cedcnt field moisture and the physical condition of the soil. Accordingly, the infiltration capacity for
<br />thf' samp soil varies appreciably.
<br />b. Inasmuch as the infiltration capacity of a soil is normally highest at the beginning of rainfall,
<br />whf'n'as rainfall fr('quf'ntl,v hf'gills at t't'lativ('ly modrrate ratf's, a substantial pt.riod may elapse bdore
<br />thp rainfall intpDsity exc-reds the prc\"ailing infiltrati9n capacity. Actually, studies have indicatf'rl that
<br />a fairJ,v definite quantity of water loss by infiltration is required to satisfy initial field moisture deficiencies
<br />hdorf' rllnoff will occur, the amount of loss depending upon antecedent conditions. A practical applica-
<br />tion of th(' infiltration tht'ory in estimating runoff from moderate I'\1infall int{'nsities ordinarily includes
<br />an allowance for "initiallossC's" corr(,spollding to various antecedent field moisture conditions.
<br />r.. Rainfall ovrr rrlativ{'l,v large drainage basins frequently does not covel' the {,Iltire basin during
<br />all pf'riods of prrcipitation with intensities exceeding infiltration capacities. Tht'refol'c, a rat.ional appli-
<br />cation of thf' infiltration th(>ory to large drailla~e arras requirE'S consideration of rainfall intensities in
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