Future Runoff and Sediment Dynamics in a Highly Erodible Loess Plateau Watershed Across Alternative Climate and Land-Use Pathways

Runoff and sediment regimes in highly erodible watersheds are increasingly affected by concurrent changes in climate and land use. However, runoff and sediment do not necessarily respond synchronously, making future sediment evolution and erosion-control planning difficult to assess. This study integrated Coupled Model Intercomparison Project Phase 6 (CMIP6) projections, the Patch-generating Land Use Simulation (PLUS) model, and the Soil and Water Assessment Tool (SWAT) to investigate historical and future runoff–sediment dynamics in the Wuding River Basin on the Loess Plateau. Meteorological, land-use, runoff, and sediment records for 1961–2021 were used for historical diagnosis and model development. Future simulations for 2022–2045 considered three Shared Socioeconomic Pathway (SSP) climate pathways and four land-use scenarios, including natural development, farmland protection, ecological protection, and urban development. Historical analysis identified asynchronous regime shifts, with runoff changing markedly in 1996 and sediment transport in 2002, respectively. Future runoff and sediment responses also varied among climate pathways. Mean annual runoff changes by +0.15, +0.34, and −0.65 × 108 m3 under SSP1-2.6, SSP2-4.5, and SSP5-8.5, respectively, while the corresponding sediment changes are −0.01, +0.04, and −0.06 × 108 t, showing that increased runoff does not necessarily lead to increased sediment export. Ecological protection and urban development produce the largest streamflow increases, whereas all land-use scenarios reduce sediment export relative to the 2020 baseline. Strong seasonal concentration persists, with July–October contributing more than 40% of annual runoff and June–September accounting for over 70% of annual sediment transport. These findings provide a quantitative basis for adaptive water resources management and ecological restoration in erosion-prone watersheds.