Runoff and water quality responses to future climate–land use–pollution source change scenarios in the upper reach of Xin’an River Basin

Rapid socio-economic development has placed significant pressure on drinking water sources. This study focuses on the upper reaches of the Xin’an River Basin (XRB), establishing a predictive framework for runoff and water quality under integrated climate − land use − pollution source change scenarios. Quantile mapping method was used to correct errors in the simulation results of climate. Based on the historical patterns of land use change and the prediction results of future population and urbanization rate under different scenarios, the future land use was predicted. The optimized Gaussian process regression (GPR) model was adopted to predict the TN and TP input amounts of fertilization and livestock and poultry. Then, a Geomorphology-Based Nonpoint Source Pollution (GBNP) model was employed to simulate the transport and transformation of water, sediment, and pollutants across various future scenarios. The results indicate that, under the Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, and SSP5-8.5) scenarios from 2021 to 2070, multi-year average annual precipitation is projected to increase by 4.8%–7.9%, whereas runoff depth is expected to decrease by 0.7%–5.9%. Total nitrogen (TN) load is projected to decrease by 36.1% and 23.7% under SSP1-2.6 and SSP2-4.5, respectively, but increase by 7.9% under SSP5-8.5. Total phosphorus (TP) load is projected to decrease by 0.3% under SSP1-2.6, while increasing by 16% and 86.7% under SSP2-4.5 and SSP5-8.5, respectively. In the southwestern part of the basin, TP load intensity increases significantly, whereas urbanized areas exhibit reductions. TN concentrations at the basin outlet demonstrate declining trends under SSP1-2.6 and SSP2-4.5, but increase under SSP5-8.5. TP concentrations increase across all scenarios, with the most pronounced rise observed under SSP5-8.5. These findings highlight the critical importance of green development strategies in safeguarding water quality in the upstream reaches of the XRB.