Infiltration Formulas by Curve Number Procedure

The Soil Conservation Service (SCS) curve number procedure for estimating runoff volume is examined in terms of the validity and applicability of the derived infiltration equations. Two different sets of the runoff and infiltration equations can be formulated from the water balance equation and one of the two SCS hypotheses postulating the basic proportionality among the relevant variables. The infiltration rate formula derived based on the SCS hypothesis with the potential maximum retention or infiltration defined to include the initial abstraction proves to be as general as, if not more general than, the Holtan-Overton equation. If the initial abstraction results mainly from infiltration that occurs before runoff begins, the amount of initial abstraction can be reasonably estimated from the derived infiltration rate formula; therefore, the SCS curve number procedure can be generalized without sacrificing its simplicity. The proposed method, however, may not be valid for large watersheds, where the surface interception and depression storage dominate the initial abstraction.