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<br />2R + Clt <br /> <br />where: <br /> <br />Clt = time period of computation interval <br /> <br />R = attenuation constant having the dimension of time <br /> <br />It can be shown that when inflow into the principle storage reach has <br /> <br />ceased: <br /> <br />R = .sL <br />dQ/dt <br /> <br />(12) <br /> <br />and the time of this basin characteristic is depicted by the point of inflection of the <br />recession limb of the observed hydrograph after base flow separation. The above ratio <br />decreases to a minimum at the point of inflection and remains constant thereafter. <br /> <br />The hydrograph that results from routing these flows from the <br />incremental areas is the instantaneous unit hydrograph. The IUH can be converted to <br />a unit hydrograph of unit-rainfall duration, Clt, by simply averaging two instantaneous <br />unit hydrographs spaced an interval Clt apart as follows: <br /> <br />o = 0.5 (0. + 0 ) <br />I I j-1 <br /> <br />(13) <br /> <br />The IUH can be converted to a unit hydrograph of some unit-rainfall <br />duration other than Clt (provided that it is an exact multiple of Clt) by the following <br />equation: <br /> <br />Q; = 1 [(.5) 0 ;-n + 0 i-n+l + ... + Oi_l + .5 (0 ;)] <br />n <br />where: <br /> <br />(14) <br /> <br />Q; = ordinate at time i of unit graph of duration D and tabulation <br />interval Clt <br /> <br />n = JL <br />Clt <br /> <br />D = unit graph duration <br /> <br />Clt = tabulation interval <br /> <br />7-33 <br />