Runoff response to terrace construction, vegetation restoration, and urban expansion: Case studies in the Yiluo and Upper Wei River basins

Study region This study investigates two typical semi-arid loess watersheds in China, the Yiluo River Basin and Upper Wei River Basin, where terrace renovation, vegetation restoration, and urbanization have substantially altered watershed hydrological processes. Study focus Conventional SWAT-based land-use studies often apply extreme land-use scenarios and oversimplify landscape heterogeneity. Terraces are treated as static BMPs with fixed Curve Number (CN), ignoring rainfall-driven CN variations and lowering simulation accuracy in terrace-dominated loess regions. This study uses realistic spatial land-use configurations instead of extreme scenarios and improves SWAT via slope-based terrace delineation and rainfall-intensity-dependent dynamic-CN calibration to evaluate land-use hydrological impacts and the conditional applicability of dynamic CN. New hydrological insights for the region The 2000 land-use dataset yields the best model performance (NSE = 0.8837, R² = 0.8865, PBIAS = −7.83%), superior to 2010 and 2020 datasets. Grassland expansion reduces runoff by 12.97%, much higher than the 2.49% from forest expansion, and urban expansion produces unstable interannual runoff responses. The dynamic-CN method improves daily-runoff simulation under drought and heavy-rainfall conditions, whereas static CN works better under mild stable rainfall. Terraces redistribute surface runoff into subsurface flow and baseflow with little impact on annual total runoff.