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<br />COlllPosite Imperviousness and Snyder Methods. A common method utilized <br /> <br /> <br />in urban hydrilogy studies is to determine a precipitation loss rate for an <br /> <br /> <br />observed flood event (discharge hydrograph) or events associated with a par- <br /> <br /> <br />ticular estimated value of impervious surface. The ~ype of loss rate func- <br /> <br />tion is not important but is frequently the simple initial plus uniform loss <br /> <br />formulation commonly used by personnel of the Army Corps of Engineers (U.S. <br /> <br /> <br />Army, Corps of Engineers, 1973). Estimation of the effect of changing land <br /> <br />use on runoff is performed by recomputing the proportion of imperviousness <br /> <br /> <br />for an alternative land use plan and adjust4ng the loss rate accordingly. <br /> <br />The HYDPAR program computationally implements this basic concept by assigning <br /> <br /> <br />percent values of imperviousness to esch grid cell based on input land use <br /> <br />and then computes an average percent imperviousness for the subbasins under <br /> <br /> <br />investigation. Any change in land use contempiated for a particular grid <br /> <br /> <br />cell would necessitate recomputation of the subbasin (containing that grid <br /> <br /> <br />cell) imperviousness. <br /> <br />Several investigators have attempted to capture the rainfall-runoff response <br />to land use change by performing regression studies of unit hydrograph para- <br />meters (Gundlach, 1976). Typically, regression expressions were derived in <br />which a parameter, such as basin lag, was expressed as a function of sub- <br />basin area, imperviousness, and perhaps other physiographic characteristics. <br />The HYDPAR program computes a Snyder's lag (U.S. Army Corps of Engineers, <br />1973) based on a generalized equation of this type in which <br /> <br />tp = crxC1; (JOC~) <br /> <br />(3-3) <br /> <br />where t <br />P <br />C <br /> <br />- Snyder's lag in hours <br />= regression constant <br /> <br />x <br /> <br />L * LeA <br />= <br />{S <br /> <br />, <br /> <br />L = characteristic stream length in miles <br /> <br />LCA = length from subbasin outlet along stream channel to a point <br />opposite the centroid of the subbasin area in miles <br /> <br />S = characteristic stream slope in feet per mile <br /> <br />C1 = regression coefficient <br /> <br />C2 = regression coefficient and <br /> <br />I = imperviousness in percent <br /> <br />3-7 <br />