Theoretical Justification of SCS Method for Runoff Estimation

Abstract

The Soil Conservation Service (SCS) method is widely used to estimate runoff from small- to medium-sized watersheds. The most critical assumption of the SCS method is that the ratio of the actual retention to the potential retention is the same as the ratio of actual runoff to potential runoff, but this assumption has not been theoretically nor empirically justified. This paper shows that the exact relationship between rainfall and runoff in the SCS method can be derived theoretically if two simple but reasonable assumptions are made: (1) The spatial variation of infiltration capacity has an exponential distribution; and (2) the temporal variation of rainfall rate also follows an exponential distribution. A theoretical basis for the SCS method allows an independent validation of the method by testing how rainfall intensity and infiltration capacity actually vary in time and space, respectively.