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<br />area. <br />C, and C, = coefficients depending upon units and drainage basin characteristics. <br /> <br />The following equations (Chow. 1964) are nJe most frequently used <br /> <br />t p 0 C, ( L L,,") " 3 <br /> <br />(IV 3) <br /> <br />t, <br /> <br />t, <br />5. 5 <br /> <br />(IV4) <br /> <br />640 Co <br />qp -' " <br />, p <br /> <br />(IV5) <br /> <br />t pR ' t c ; O. 25,: t H t,) <br /> <br />(1\16) <br /> <br />q,R <br /> <br />(Clpt p) <br />t pF. <br /> <br />(lV.l) <br /> <br />q, qpA <br /> <br />(IV8) <br /> <br />The distance to the center of area may be taken as the mean distance ojf a <br />cumulative area-distance cu rye or estimated as the distance to the center of gravity of the <br />drainage area above a partiwlar station obtained by use 01 a digitizer. <br /> <br />The distance L ~ i3 measured along the principal stream channel to a point <br />approximately opposite the center of area. The distance L is equal to L w plus the <br />remaining distance to the upper basin limit, followin9 approximately the principal stream <br />channel. <br /> <br />The average value of the product 640 C , is approximately 400, as determined by <br />Snyder, for basins in the fairly mountainous Appalachian Highlands, and the corresponding <br />average value of C t is 2.0. Analyses of unit tlydrograptls for various sections of the <br />country have indicated an extreme range in values ot 640 C panel C t of from 600 and 0.4. <br />respectively. for southern California to 200 and 8.0, respectively, for sections of states <br />bordering 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 and C , 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 dilferent <br />characteristics, unit hydrographs should be computed separately for each and the results <br />combined to obtain a graph for the total area. Rel'erence 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 />CFW-T <br />