Improved runoff curve numbers for a large number of watersheds of the USA

This study presents a procedure to determine improved curve numbers (CNImp) for application of the enhanced Soil Conservation Service - Curve Number (SCS-CN) model developed by Verma et al. for improved runoff prediction. To this end, five different mathematical relationships, i.e. linear, exponential, power, logarithmic and polynomial, are developed for 57 US Department of Agriculture - Agricultural Research Service (USDA-ARS) watersheds to compute CNImp with the aid of CN derivable from the National Engineering Handbook, Section-4 (NEH-4) table (CNT) and from the enhanced SCS-CN model (CNV) with significant R (2) values. Using these relationships, CNImp values are computed for another 57 watersheds for validation. Based on common performance criteria, all five developed relationships for CNImp are found to be superior to the original model in runoff computation. When compared, CNImp are close to CNV and very different from CNT. Despite the fact all the relationships performed equally well, with similar performance statistics, the exponential relationship performed the best of all and the power relationship was the worst.