Asymptotic Determination of Runoff Curve Numbers from Data

Background, general instructions, and examples are given for determining runoff curve numbers (CN) from small watershed event rainfall and direct runoff data sets. The technique matches rank‐ordered (i.e., sorted) rainfall and runoffs, which preserves the return‐period matching between the rainfalls and the runoffs. Usually a secondary trend of CN with the storm rainfall itself emerges, and three different patterns are observed; complacent, standard, and violent. The standard and violent cases lead to a constant CN with increasing storm size, but the complacent case does not lead to a stable determination of curve number. Some measures of asymptotic attainment to the fitting equations and to the hydrologic definition of the watershed are also given.