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<br />Unit Hydrograph. The unit hydro graph corresponding to the <br />appropriate duration of rainfall excess can be supplied directly or may <br />be computed from coefficients of two synthetic techniques; the Clark <br />procedure and the Snyder procedure. The Clark procedure develops a <br /> <br /> <br />unit hydrograph from a coefficient describing the subbasin time of <br /> <br /> <br />concentration and a coefficient describing the subbasin natural storage <br /> <br /> <br />characteristics. A time-area diagram (sometimes termed a time delay histo- <br /> <br /> <br />gram or translation hydrograph) with a base time equal to the time of <br /> <br /> <br />concentration is routed through a linear reservoir characterized by <br /> <br /> <br />the storage coefficient. The time-area relation may be derived from <br /> <br />subbasin physiographic data and supplied directly or the program may <br />be permitted to compute and use an idealized relationship that consists <br />of a simple reverse parabola symmetrical about the center of the time <br /> <br />base. <br /> <br />If a unit hydrograph that conforms to specified Snyder coefficients <br />is desired, it is established by successive approximations for the corres- <br />ponding Clark coefficients. <br />Base Flow and Recession. The program assumes that the flood to be <br /> <br />computed occurs after a previous flood and therefore begins at a flow <br /> <br />on the recession limb of the previous flood hydrograph. The recession <br />is assumed to be described by an exponential function. The parameters <br />required to describe the base flow and recession are the starting flow, <br /> <br />the flow at which recession begins, and the recession constant. Figure <br /> <br />J shows the base flow separation concept used by the program. <br /> <br />13 <br />