Runoff Probability, Storm Depth, and Curve Numbers

Using existing antecedent moisture condition (AMC) Curve Number relationships, a general expression of rainfall‐runoff is developed for AMC I and AMC III. The probability distribution of event runoff exceeding zero is found to be lognormal. Relative storm size is then proposed to be defined on the ratio P/S, where a “large” storm has P/S>0.46, when 90% of all rainstorms will create runoff. Consequent problems arising in the definition of CN from rainfall‐runoff data are discussed.