Stability assessment of the curve number methodology used to estimate excess rainfall in forest-dominated watersheds

The credibility and wide acceptance of the curve number (CN) method are rooted in its simplicity, convenience, authoritative agency support, absence of a strong competitor, and versatility to estimate runoff especially from ungauged watershed. Numerous studies have reported that the application of the tabulated CN (CN Tab ) and the fixed initial abstraction ratio ( λ = 0.20) to estimate the runoff volume is inferior. In the current study, we investigated the applicability of λ = 0.20 and its successful modification to λ = 0.05 using the CN Tab and other calibrated CNs based on the measured rainfall-runoff data. In addition, with a fixed CN Tab , the value of λ was reduced to 0.01 and 0.0 in order to evaluate the runoff variability response for 587 storm events from 10 South Korean forest-dominated mountainous watersheds. Both for λ = 0.20 and λ = 0.05, the application of CN Tab exhibited poor results in all of the watersheds based on quantitative statistics using four performance evaluation indicators. The application of the calibrated CN increased the runoff prediction capability of the CN method both for λ = 0.20 and λ = 0.05. Using λ = 0.05 and its adjusted calibrated CN depicted a modest improvement in the results. Among the different calibrated CN variants, the asymptotic method was comparatively appropriate. However, the method still needs improvement for a consistent CN. Interestingly, for the unaltered CN Tab , the λ = 0.01 and λ = 0.0 exhibited very consistent results for the estimation of runoff from watersheds. Therefore, it is more tempting for application in ungauged watersheds without deterioration of the CN model’s versatility and simplicity.