Curve Numbers and Urban Runoff Modeling—Application Limitations

The U.S.D.A. SCS (now the NRCS) Curve Number method has been in continuous use for about 50 years. As originally developed, the method yields a direct runoff depth from the accumulated 24 hour rainfall depth as function of the curve number CN. The method has since been extended to hydrograph generation and is found in commonly used hydrologic models applied to urban drainage design (e.g., WinTR55, SWMM and HEC-HMS). A number of recently published studies, including Curve Number Hydrology - State of the Practice, by the ASCE/EWRI Curve Number Hydrology Task Committee, have warned that it is inappropriate to use the method to generate runoff hydrographs, yet the practice continues with little awareness of this limitation by most users. A common misconception is that the CN method is an infiltration model, which can lead to significant errors in peak discharge predictions. CN values can be converted into equivalent physically based infiltration model parameters used in the Green-Ampt method in SWMM (or HEC-HMS), or in the infiltration component used in the PERLND module of EPA's HSPF model, each of which can produce a more acceptable hydrograph that matches CN method direct runoff depth.