Spatiotemporal Evolution and Multi-Scenario Simulation of Ecosystem Services in the Core Water Source Area of the South-to-North Water Diversion Project’s Middle Route

Inter-basin water transfer source areas must sustain local habitat quality while ensuring downstream water security, yet the spatial differentiation mechanisms, driving mechanisms, and scenario-dependent responses of their ecosystem services have not been statistically tested. This study aimed to assess historical changes and future trajectories of ecosystem services in the core water source area of the Middle Route of the South-to-North Water Diversion Project, and established a historical assessment–spatial statistics–driver attribution–scenario projection analytical framework. Five ecosystem services—water yield, soil conservation, nitrogen export, carbon storage, and habitat quality—were quantified using InVEST from 2005 to 2020 and projected to 2050 under SSP126, SSP245, and SSP585 via the PLUS–InVEST coupled model. Spatial patterns were analyzed using Getis–Ord Gi* hot–cold spot analysis, hexagon-based spatial statistics (4.5 km bins), Mantel tests, and the optimal-parameter-based geographical detector. Results show that slope was the dominant spatial driver, exhibiting highly significant Mantel correlations with all five services (p < 0.001), and two-factor interactions consistently exceeded individual factor effects (e.g., population density × temperature: q = 0.527 for habitat quality; slope × temperature: q = 0.516 for soil conservation). Hexagon-based statistics revealed that carbon storage declined substantially from 2005 to 2020 (−3.23%, from 98.75 t/ha to 95.56 t/ha), while water yield and soil conservation showed no pronounced temporal trends. Scenario projections revealed divergent trajectories: relative to the 2020 baseline (267.84 mm), water yield increased by 20.1% under SSP126 (321.86 mm) but declined by 56.3% under SSP585 (117.13 mm); nitrogen export increased under all scenarios; and habitat quality declined continuously to 0.557, 0.546, and 0.539 under SSP126, SSP245, and SSP585, respectively. Within this scenario framework, SSP126 and SSP245 may better support the maintenance of water source ecosystem functions, whereas SSP585 may be associated with greater potential ecological pressures, reflected in lower water yield, higher nitrogen export, and lower habitat quality.