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3~Y <br /> <br /> <br /> <br /> <br /> IncL - -- <br /> L_~:; thatl l,~.j i.~ ~., L!::I L' _. <br /> 1T_ a.j .~i _. _ _ .-. ~l x_.- <br /> 111 Od~x' 1.1 G: n' ~'. _ <br />FiCure 4. t0 i].lnstrates a typical nat.rx:~.l sto:2~ Ccr w!iieh the stox^^ du- <br />rntiun ctu~t be arbitrarily dcfineci. 'i^.te ii~u_:: :}:o~•:s t::~ acctumLlated <br />runoff occurring ,Jiren tl:z runoff curv^ nut::bers of lOJ, '.0, and 70 are <br />applicable. Alote tl.~t the duration of excessive minia , which is the <br />r:~infall producing iltc iltnoff, is al~:ra;yo less than fire sto:ri duration ex- <br />ctpt crhen rwxoff is 100 perc:nt. mince the duration of excce;;ive rain- <br />fall is t}rz correct duration to use with peat: rat; ~q-rations, such equa- <br />tion:- will be more successful with natural stcx-tnc. l}:at arc brief and in- <br />tet:ee. The h~droiogic desi~ methods of chapter _'1 !rave been dzveloped <br />to account for the initial abstraction,so t!:at d.rrntior. of excessive raiu- <br />fail is used. <br />• <br />Effective duration (De <br />41h~n standard gaffe data are used in a watersL~_i prof ct evahtation, the <br />stox'm durations will usually bz unhlro,m. M ^pprcxirratz duration for <br />use with all the stox2ns can bz estimated usin` Fi,•,urc 4.11, which Dhows <br />the relation between average annual rainfall and xm "effective duration". <br />The ga€~ rainfalls pre used as if they lead iallzn in De hours. The <br />plotted points on figure- 4.11 were obtained by differznt methods. Ttra <br />tecthod used in obtaining the St. Louis, too. ::Dint still be described, <br />since it best illustrates the significance of D . <br />The hourly records of precipitation at Wit. Louis, [1~. were used to rsti- <br />^iate, to the nearest hour, the duration et each rain in `he };eriod hi<•xrch <br />].920 through December 1y29. The fornr sham in table }~ . j i:as used to tall;,r <br />the durations. If there was no rain for one cr '1o~•z i~oul•s, thn.•d-oration <br />was decrea:•~d by the sane number of hourr.. T'nc iu•c?ondcrat.t ntuubc•r of <br />rains iJas wntinuous. After completing t!te t'.:xlation, the numbzx• of <br />tallies i;an accumtLlated as shotm in column !:. Tlu' mrdiar. nwnber of items <br />is 1C2, j (t!tc brand t~t.al is an even r.~u:bcr, ,.?x i L}.:' ^.~d i an is obT.e.ined <br />~h- J <br />/` <br />