Determination of Watershed Curve Number Using Derived Distributions

Curve numbers (CNs) are developed from measured rainfall (P) and runoff (Q) data, yet there is no standard method for determining CN. The original method of CN determination used maximum annual events. Subsequent development treated measured P and Q data as frequency distributions. In this paper, derived frequency distributions are evaluated as another method for determining watershed CNs from measured data, treating P and Q data as separate frequency distributions. A Monte Carlo simulation showed the derived-distribution method gave fewer variable estimates of CN for a wide range of sample sizes than two other methods for CN estimation. CN estimates using measured data with the derived-distribution method agreed well with CN estimated by the asymptotic method for the watershed types tested. The derived-distribution and asymptotic methods were in agreement, and the other methods gave lower CN estimates for the “violent” watershed type. CN was indeterminate for the derived-distribution and asymptotic methods for “complacent” watersheds. The derived-distribution method has a potential for determining CNs when there are limited P and Q data.