A Conceptual Model of the SCS Runoff Method

The SCS (or NRCS) method, arising in the 1950’s out of analysis of real data from large numbers of catchments across the USA, is one of the most commonly applied flood estimation methods. It is used in several regions within New Zealand and is the preferred method for the Auckland Region. The original approach was empirical and this paper presents a concept of the physical rainfall-runoff processes for the SCS method. It is based on the variable source area concept for promoting runoff. The variable source concept is based on part of the catchment contributing to runoff at an increasing rate with increasing rainfall. It can be demonstrated that a unique storage function can be defined across a catchment representing the catchment type (land use, soil type etc.). The concept provides a physical meaning for the runoff number (CN), and demonstrates that the storage (S) in the SCS method can be taken to represent the average storage across the catchment.

It is anticipated that this concept will aid an understanding of SCS applications and perhaps its use in continuous rainfall-runoff modelling.