Curve Number Determination Methods and Uncertainty in Hydrologic Soil Groups from Semiarid Watershed Data

Four curve number (CN) determination methods are evaluated from 16 watersheds in the southwestern U. S. using 1,284 events that satisfy rainfall and runoff criteria. The use of ordered pairs versus natural pairs of rainfall and runoff data has a larger effect on the CN, whereas the difference from using a partial duration series versus an annual series was not significant. The best-available USDA soil series data were obtained for 20 Arizona watersheds and 10 groups of New Mexico natural runoff plots. The hydrologic soil groups (HSG) were determined from either direct USDA assignment or textural properties and compared with the HSGs required by the CNs and the cover condition. This study showed a standard error of about one HSG, resulting in an error in CN of approximately seven units when using the best-available data. Compared with USDA handbook table values, the CNs found from rainfall and runoff data were higher for 21 of the 30 semiarid watersheds. DOI: 10.1061/(ASCE)HE.1943-5584.0000452. (C) 2012 American Society of Civil Engineers.