Soil and Water Assessment Tool (SWAT) and Soil and Water Assessment Tool-Variable Source Area (SWAT-VSA) models were employed to predict surface runoff generation in a watershed of the Himalayan landscape in GIS environment. Both the models differed in term of defining hydrological response units (HRUs) that serves as basis in assigning curve number for surface runoff estimation. HRUs in SWAT was derived by combination of hydrological soil groups based on soil types and land use/land cover (LULC) whereas in SWAT-VSA, it was based on soil wetness index derived from digital elevation model (DEM) and LULC. Both models were calibrated to predict surface runoff at watershed scale. SWAT-VSA predicted quite well [Root Mean Square Error (RMSE) = 3.88 = 3.88 , Nash–Sutcliffe coefficient of efficiency (NSE) = 0.75 = 0.75 ] than the SWAT (RMSE = 4.12 = 4.12 , NSE = 0.72 = 0.72 ) model. Paddy (rice) cropland in the watershed generated highest surface runoff. Integration of topographic wetness index derived from DEM with SWAT model helped in estimating spatially distributed surface runoff generation in the watershed. Study revealed that saturation excess as the dominant runoff process in the Himalayan landscape and SWAT-VSA provide more representative results than the SWAT based on infiltration excess.
