EXTENSION OF THE SOIL CONSERVATION SERVICE RAINFALL-RUNOFF METHODOLOGY FOR UNGAGED WATERSHEDS

The estimation of direct runoff for ungaged watersheds is a common problem in\nengineering hydrology. The method of the Soil Conservation Services (SCS) is widely used due to its ease of application. Runoff estimates are based upon the soil types and land use practices occurring within a watershed. This report presents an extension of the SCS method employing modern physically based infiltration equations. Problems with the SCS equations are discussed and contrasted with the physically based equations. The requirement of equal amounts of total·abstraction from constant rainfall events by both methods is taken as the criterion for establishing an equivalence between the SCS curve number (CN) and hydraulic soil parameters. A corresponding CN is found for each of nine major soil textural classes. Regression equations are used to generalize these results and form a table of correspondence. This table permits the estimation of direct runoff by physical infiltration equations for any ungaged watershed for which a CN may be determined by knowledge of soil types and land use. However because the CN tables of the SCS were established for actually observed storms which are not uniform the correspondence based on uniform storms is biased. The bias is removed by comparing the prediction of excess rainfall for pairs of storms of equal duration and total precipitation for the case of a uniform rainfall rate versus the case of a variable intensity pattern typical of extreme storms. On the basis of these comparisons a relationship, which eliminates the bias, is established between the CN based on soils and land use and the CN to be used to obtain the proper corresponding hydrau1ic soil parameters.\n