Assessing the applicability of conventional and modified SCS–CN methods under variable slope and land use conditions

This study experimentally evaluated runoff and hydrological responses at an agricultural research farm in Uttarakhand, India, using nine plots (12 m × 3 m) with varying crops and slopes (8%, 12%, and 16%). Maize and finger millet were grown on selected plots, while one plot per slope remained fallow. Daily rainfall was monitored using a standard rain gauge; among 32 rainfall events, 19 produced measurable runoffs, with one extreme event excluded due to flooding. Additional observations included soil moisture, sediment yield, soil texture, and hydrologic soil group. Laboratory analysis showed predominantly sandy soils. Double-ring infiltrometer tests classified most plots as Hydrologic Soil Group A, except one fallow plot (8% slope), categorized as Group B. Curve Numbers (CN) estimated using frequency matching and optimization methods revealed that the modified Soil Conservation Service’s Curve Number (SCS-CN) approach generated CN values higher than National Engineering Handbook (NEH-4) table values but lower than the conventional SCS-CN method. Optimized initial abstraction ratios (λ) ranged from 0.00 to 0.05, below standard NEH-4 recommendations. Model evaluation using NSE (70–94%) and RMSE (4.11–13.97) demonstrated the superior performance of the modified method. CN values increased with slope and antecedent soil moisture, emphasizing the need for site-specific calibration for reliable runoff and flood prediction in agricultural watersheds.