Semi-distributed hydrological models that incorporate three-layer soil structures with explicit representation of evapotranspiration, infiltration, and surface runoff processes demonstrate considerable capability for watershed-scale hydrological simulation. This study evaluates the performance of the Variable Infiltration Capacity model (VIC-3L) in simulating daily streamflow and water balance components for the Qareh-Sou mountainous watershed in Kermanshah Province, Iran (drainage area: 5,534 km²), characterized by semi-arid climate and complex topography. The assessment utilized hydrometeorological observations from 1993 to 2008, with streamflow routing performed using the Route model.

Model evaluation yielded a coefficient of determination (R²) and Nash-Sutcliffe efficiency (NSE) of 0.75 and 0.73, respectively, during calibration, and 0.72 and 0.74 during validation, indicating satisfactory accuracy in reproducing observed daily hydrographs. To identify influential parameters, sensitivity analysis was conducted using the one-at-a-time (OAT) approach in both pre-calibration and post-calibration phases. Of the 30 parameters examined, 11 were identified as sensitive, including the variable infiltration curve parameter, subsurface flow coefficient, maximum baseflow parameter, soil moisture fraction, and soil layer depths. Results showed that parameters controlling subsurface soil behavior exerted the greatest influence (sensitivity ratios: 0.85–0.95) on runoff generation and baseflow simulation.