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<br />where: <br /> <br />L\t = time period of co mputation irterval <br /> <br />R " attenuation constant Ilaving ':he dim9nsion of time <br /> <br />It can be shown that when inflow into tile principle storage reach has ceased <br /> <br />,'1 <br /> <br />Q <br />dQ <br />di' <br /> <br />(IV-12) <br /> <br />and the time of this basin characteristic is depicted by the pOint of inllection of the <br />recession limb of tho observed hydrograph after base flow separation. Ttle above ratio <br />decreases to a minimum at the point of inflE'ction a.1d remains constant thoreafter. <br /> <br />The hydrograph that results from routing these flows from the incremE,ntal areas <br />is the instantaneous unit tlydrograph. The IUH can be converted to a unit hyd'Ograph of <br />unit-rainfall duration, L\t, by simply averaging two instantaneous unit hyclroqrapl'ls spaced <br />an interval L\t apart as follows: <br /> <br />() <br /> <br />" 0.5 (0. + 0 ) <br />,.' <br /> <br />(IV-13) <br /> <br />The IUH Can be converted to a unit rlydrograph 01 some unit-rainfall duration other <br />than L\t (provided ttlat it is an exact multiple of L\t) by the following equation: <br /> <br />q <br /> <br />1 <br />,,(( 5) qn 1 qc.'.. + q, ' ( . 5) q . <br /> <br />(IV-14) <br /> <br />where: <br /> <br />Q , ordinate at time i of unit graph of duration [) and tabulation <br />interval tit <br /> <br />n ..Q. <br />i'1t <br /> <br />D unit graph dUl'2ltion <br /> <br />tit tabulation interval <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />~'-43 a::w:r <br />