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<br />falling within each subbasin, or (2) by taking a proportion of the total <br />watershed area based on the number of cells within each subbasin. <br /> <br />A significant amount of bookkeeping relating to land use, computational <br /> <br /> <br />procedures, etc. is performed and displayed as output. <br /> <br />Computational Procedures <br /> <br />Soil Conservation Service (SCS) Methods. The U.S. Soil Conservation <br /> <br /> <br />Service (SCS) developed a technique for determining direct runoff from rain- <br /> <br /> <br />fall based on land use, antecedent moisture conditions and soil type. The <br /> <br />technique (Curve Number technique) is especially attractive to many hydrologic <br /> <br /> <br />planning evaluations since it provides a systematic and consistent method of <br /> <br /> <br />evaluating the effect of alternative land use patterns on surface runoff <br /> <br />within a watershed (U.S. Soil Conservation Service, 1972). <br /> <br />The SCS method uses a relationship in which accumulated runoff is a function <br /> <br /> <br />of accumulated rainfall and the physiographic characteristics, land use, <br /> <br />antecedent moisture and soil type. Curve numbers have been developed to <br /> <br /> <br />represent various combined states of these particular physiographic parameters. <br /> <br />In the HYDPAR program only land use and soil type are ~tilized in determining <br /> <br /> <br />a curve number (CN). Tbe proce.s involves accessing the grid cell data bank <br /> <br /> <br />and determining the land use and soil type for each grid cell. Then based on <br /> <br /> <br />an input relationship between CN and these two particular parameters, a value <br /> <br />(of CN) is assigned to each cell. Based on the number of cells in each sub- <br /> <br /> <br />basin and their respective CN's an average value of CN is then computed for <br /> <br />each subbasin in the study area. <br /> <br />Since the curve number technique is Bsed to determine the amount of surface <br /> <br /> <br />runoff generated by a given storm (or precipitation) event, an additional <br /> <br />technique is required to develope the shape of the discharge hydro graph (dis- <br /> <br /> <br />charge versus time) at a subbasin outlet. In order to do this, the SCS has <br /> <br /> <br />developed a simplified dimensionless unit hydrograph approach to transform <br /> <br />3-4 <br />