The Soil Conservation Service (now known as the Natural Resources Conservation Service) has developed a synthetic unit hydrograph procedure that has been widely used in their conservation and flood control work. The unit hydrograph used by this method is based upon an analysis of a large number of natural unit hydrographs from a broad cross section of geographic locations and hydrologic regions. This method is easy to apply. The only parameters that need to be determined are the peak discharge and the time to peak. A standard unit hydrograph is constructed using these two parameters.
For the development of the SCS Unit Hydrograph, the curvilinear unit hydrograph is approximated by a triangular unit hydrograph (UH) that has similar characteristics. Figure 3-7 shows a comparison of the two dimensionless unit hydrographs. Even though the time base of the triangular UH is 8/3 of the time to peak and the time base of the curvilinear UH is five times the time to peak, the area under the two UH types is the same. The area under a hydrograph equals the volume of direct runoff QD which is one millimeter or one
inch for a unit hydrograph. The peak flow is calculated as follows