Quantifying the impact of land use changes on surface runoff in the Loess Plateau area of Northwest China by SWAT

Rapid socio-economic development has precipitated substantial transformations in land use and land cover (LUCC) within the Yanhe River basin, significantly impacting production dynamics, confluence mechanisms, and the basin's runoff response processes. To elucidate the runoff response patterns under varying land use/land cover change conditions, this study analyzed the land use change characteristics from 1980 to 2020. Employed the SWAT (Soil and Water Assessment Tool) model, and simulated the precipitation-runoff dynamics under five distinct land use scenarios to scrutinize the basin's runoff response to varying land use conditions. The results demonstrated the applicability of the SWAT model to the Yanhe River basin, with R-2 and Ens values for monthly runoff at two hydrological stations exceeding 0.6 during both calibration and validation periods. Between 1980 and 2020, the area of farmland decreased by 27.96%, whereas the areas of woodland and grassland by 36.59% and 16.2%, respectively. Scenario analysis revealed that the primary contributors to the increased runoff in the study area, in descending order, were grassland, farmland, and woodland. The results indicated that converting farmland to woodland would reduce the runoff depth by 0.26 mm, while converting farmland to grassland would increase the runoff depth by 0.39 mm in the watershed. The conversions exhibited pronounced seasonal effects, with varying degrees of runoff depth changes observed across different seasons. The contribution order of different hydrological years to runoff depth change rates was median flow year > low flow year > high flow year. Land use conversion, particularly among farmland, grassland, and woodland, exerts diversified impacts on runoff depth across different water periods.