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<br />6. SOIL CONSERVATION SERVICE (SCS) METHOD. <br /> <br />A dimensionless unit hydrograph was developed by the Soil Conservation <br />Service (SCS) based on an analysis of measured runoff data. Dimensionless unit <br />hydrographs were developed from unit hydrographs derived for a large number of <br />watersheds. These dimensionless unit graphs were then averaged to develop the SCS <br />dimensionless unit hydrograph (DUH). Characteristics of the DUH include, a <br />curvilinear shape, a time base. equal to 5 times the time-to-peak, an inflection point on <br />the recession limb at about 1.7 times the time-to-peak, and approximately three-eights <br />of the total volume prior to the time-of-peak. The DUH can be defined with the time <br />and discharge ratios in the following table: <br /> <br />SCS DIMENSIONLESS UNIT HYDROGRAPH RATIOS <br />./ .. IT.).. . ..... .............................2\ ... 1/ ....\.,.,.,.,.2\........ <br />TIME (t DISCHAl'lc;~!qtq.r' . . <br /> 0 0 <br />.2 .10 <br />.5 .47 <br />.7 .82 <br /> 1 .0 1.0 <br /> 1.3 .86 <br /> 1.5 .68 <br /> 2.0 .28 <br /> 3.0 .06 <br /> 4.0 .01 <br /> 5.0 0 <br /> <br />where, the time ratio (t I T pI is the ratio of a selected time (t) to time <br />from beginning of rise to peak (T pI and the discharge ratio (q I q p) is the ratio of <br />discharge at time t (q) to peak discharge (q pl. <br /> <br />In order to develop simple mathematical relationships for the DUH, the <br />SCS fit a triangular unit hydrograph to the DUH by matching the peak and the volumes <br />on the rising limb and recession limbs. For the triangular hydrograph, the volume of <br />runoff can be computed as the area within the triangle which can be expressed <br />mathematically as follows: <br /> <br />7-34 <br />