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<br />where: <br /> <br />t.t = time period of computation interval <br /> <br />R = attenuation constant l1aving the dimension of time <br /> <br />It can be shown that when inflow into the principle storage reach has ceased: <br /> <br />- Q <br />R-- <br />dQ <br />df <br /> <br />(IV-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 abeve ratio <br />decreases to a minimum at the point of inflection and remains constant thereafter. <br /> <br />The hydrograph that resuits from routing these flows from the incremental areas <br />is the instantaneous unil hydrograph. The IUH can be converted to a unit hydrograph of <br />unit-rainfall duration, t.t, by simply averaging two instantaneous unit hydrographs spaced <br />an interval t.t apart as follows: <br /> <br />0; = 0.5 (0 ; + O. ) <br />,., <br /> <br />(IV-13) <br /> <br />The IUH can be converted to a unit hydrograph of some unit-rainfall duration other <br />than t.t (provided that it is an exact muitiple of t.t) by the following equation: <br /> <br />1 <br />q = ,,((.5) q-,,+q-n.'+'" +q_,+(.5) q) <br /> <br />(IV-14) <br /> <br />where: <br /> <br />Q; = ordinate at time i of unit grapl1 of duration D and tabulation <br />interval t.t <br /> <br />n = ~ <br />t.t <br /> <br />D = unil graph duration <br /> <br />t.t = tabulation interval <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />7-4:3 <br /> <br />a=w=r <br />