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<br /> <br />q pR = (q p* tp )1 tpR <br /> <br />(7) <br /> <br />Qp = qpA <br /> <br />(8) <br /> <br />The distance to the center of area may be taken as the mean distance off <br />a cumulative area-distance curve or estimated as the distance to the center of gravity <br />of the drainage area above a particular station obtained by use of a digitizer or <br />estimated in the following manner: <br /> <br />Trace the outline of the drainage basin on a piece of cardboard and trim <br /> <br />to shape. <br /> <br />Suspend the cardboard before a plumb-bob by means of a pin near the <br />edge of the cardboard and draw a vertical line. In a similar manner, draw a second <br />line at approximately a 90' angle to the first. The intersection of the two lines is the <br />center of gravity of the area. Transfer the point to the original map for use in <br />determining Leo' <br /> <br />The distance L co is measured along the principal stream channel to a ' <br />point approximately opposite the center of area. The distance L is equal to L co' plus <br />the remaining distance to the upper basin limit, following approximately the principal <br />stream channel. <br /> <br />The average value of the product 640 C p is approximately 400, as <br />determined by Snyder, for basins in the fairly mountainous Appalachian Highlands, and <br />the corresponding average value of C t is 2.0. Analyses of unit hydrographs for <br />various sections of the country have indicated an extreme range in values of 640 C <br />p and C t of from 600 and 0.4, respectively, for southern California to 200 and 8.0, <br />respectively, for sections of states bordering on the eastern Gulf of Mexico. If <br />hydrologic records are available for a representative portion of the drainage area under <br />study, or for nearby basins of similar characteristics, the coefficients 640 C p and C <br />t should be computed therefrom. Special field observations to determine the lag may <br />be practicable and desirable in many cases. If the drainage area is formed by.two or <br />more important tributarilils of different characteristics, unit hydrographs should be <br />computed separately for each and the results combined to obtain a graph for the total <br />area. Reference is made to Snyder's original paper for additional details regarding the <br />derivation and use of the equations given above. <br /> <br />5. CLARK METHOD. <br /> <br />As discussed previously, there are many possible unit hydrographs for the <br />same basin, since the shape of the hydrographs vary with different unit durations and <br />other factors. To define a unique unit hydrograph for a basin that can be adjusted to <br /> <br />7-30 <br /> <br />. <br /> <br />