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<br />P -Q=tR dQ . . . (3.11) <br />e dt <br /> <br />The runoff rate, Q, at the outlet of a single cataclunent <br />area is obtained by solving the differential Equation <br />3.11. <br /> <br />Channel Routing <br /> <br />The outflow hydrographs at the discharge poinl <br />of each subzone are generated by applying the urban <br />watershed model. The computed discharge for a par- <br />ticular zone is then routed and combined with the dis- <br />charge from the adjacent downslream zone, and Ihe <br />procedure is continued until the outlet of the waler- <br />shed is reached. A channel routing technique was de- <br />vised by Evelyn et aI. (1970) to combine subzone dis- <br />charges to produce the oulflow hydrograph from the <br />entire basin. The method is based on the assumption <br />that the channel or storm drain is a linear storage res- <br />ervoir. Hence, <br /> <br />Q. Q = dSc <br />I' 0 dt <br /> <br />. . (3.12) <br /> <br />and <br /> <br />lnterc.ptlon <br />Storage <br /> <br /> <br /> <br />O."..lIon <br />StorOie Sd <br /> <br />Sc = T L Qo' . . . . . <br /> <br />. (3.13) <br /> <br />in which <br />Qj = <br />Qo <br />Sc = <br />TL <br /> <br />rate of inflow into Ihe section at the <br />upslream boundary between subzones <br />rale of oulflow from the section at the <br />boundary with the adjacent downstream <br />subzone <br />instantaneous volume of channel stor- <br />age <br />proportionality faclor between Sand <br />Qo which represents the time lag of <br />water flowing belween upstream and <br />downstream channel sections <br /> <br />Substituting Equation 3.13 into Equation 3.12 <br />gives: <br /> <br />Qj-Qo=TL dQo . . . (3.14) <br />dt <br /> <br />Use of the linear storage system analogy for chan- <br />nel routing necessitated derivation of an expression for <br />the characteristic lag time, T L> in Equation 3.14. TIlls <br />lag time represents the time required for flow to move <br /> <br />Precipitation <br />(Storm Dohl <br />Input) <br /> <br />S,.. <br />Storage <br /> <br /> <br />OR <br /> <br />IHal O(t<t <br /> <br />'.1 t <t(t <br /> <br />d'd <br />CTqadi"'"'" . 12 <br /> <br />"<'''. <br /> <br />YES <br />P: at (I4i{p.-F)/Sd) <br />o 2 <br />fl'3 <br /> <br />Figure 3.5. Schematic [low chart for obtaining hYdrograph of rainfall excess. <br /> <br />28 <br />