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<br />area. <br />C t and C p = coefficients depending upon units and drainage basin characteristics. <br /> <br />The following equations (Chow, 1964) are the most frequently used: <br /> <br />tp~c;(LLca)O.3 <br /> <br />(IV-3) <br /> <br />t , ~ .!J.... <br />5.5 <br /> <br />(IV-4) <br /> <br />qp ~ 640 Cp <br />tp <br /> <br />(IV-5) <br /> <br />tpR~tp+O.25(tR-t,) <br /> <br />(IV-6) <br /> <br />qpl; ~ (q/ p) <br />t pR <br /> <br />(IV-7) <br /> <br />q, = qpA <br /> <br />(I-V8) <br /> <br />The distance to the center of area may be taken as the mean distance off a <br />cumulative area-distance curve or estimated as the distance to the center of gravity of the <br />drainage area abeve a particular station ol:)tained by use of a digitizer. <br /> <br />The distance L co is measured along the principal stream channel to a point <br />approximately opposite the center of area. The distance L is equal to L co' plus the <br />remaining distance to the upper basin limit, following approximately the prillcipal stream <br />channel. <br /> <br />The average value of the product 640 C p is approximately 400, as determined by <br />Snyder, for basins in the fairly mountainous Appalachian Highlands, and the corresponding <br />average value of C I is 2.0. Analyses of unit hydrographs for various sections of the <br />country have indicated an extreme range ill values of 640 C p and C t of from 600 and 0.4, <br />respectively, for southern California to 200 and 8.0, respectively, for sections of states <br />berdering on the eastern Gulf of Mexico. If hydrologic records are available for a <br />representative portion of the drainage area under study, or for nearby basins of similar <br />characteristics, the coefficients 640 C p aM C I should be computed therefrom. Special <br />field observations to determine the lag may be practicable and desirable in many cases. <br />If the drainage area is formed by two or more important tributaries of different <br />characteristics, unit hydrographs should be computed separately for each and the results <br />combined to obtain a graph for the total area. Reference is made to Snyder's original <br />paper for additional details regarding the derivation and use of the equations given above. <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />7-40 <br /> <br />fFAq <br />