Evolution of the SCS Runoff Curve Number Method and Its Application to Continuous Runoff Simulation

The Natural Resources Conservation Service (NRCS) [previously Soil Conservation Service (SCS)] developed the SCS runoff curve-number (CN) method for estimating direct runoff from storm rainfall. The NRCS uses the CN method for designing structures and for evaluating their effectiveness. Structural design is usually based on a single event of a certain probability of occurrence. During the years when many floodwater-retarding watershed projects were planned and constructed (1950–1980), the CN equation was used in a continuous mode to evaluate the projects. To operate CN in a continuous mode, runoff was estimated from a daily rainfall record of approximately 30 years. For each day of recorded rainfall, the five-day antecedent rainfall was used to assign a CN1 (dry condition), CN2 (average condition), or CN3 (wet condition), and runoff was estimated with the appropriate CN. With the development of continuous hydrologic simulation models, CN was related directly to soil water content or estimated using rainfall and potential evapotranspiration (PET) to drive an index. Several methods were attempted and used with different degrees of success over a period of many years. The purpose of this study is to describe the evolution of the continuous CN method and its recent developments. Test results on the basis of the direct-link soil-moisture approach and the revised soil-moisture index method are presented for demonstration purposes. The results indicate that the revised soil-moisture index method is robust and produces realistic runoff estimates over a wide range of soil properties.