Exploring trade-offs, synergies and driving pathways between water availability and ecosystem services in critical...

Water availability (WA) is a key ecohydrological indicator linking regional hydrological conditions with ecosystem functioning and constraining ecosystem service supply. This study examined trade-offs, synergies, and driving mechanisms between WA and carbon storage (CS), soil conservation (SC), habitat quality (HQ), and food production (FP) in the Qinling-Daba Mountains during 2001-2020. Pixel-wise Pearson correlation analysis, XGBoost-SHAP, and piecewise SEM were used to identify WA-ES relationships, dominant drivers, nonlinear thresholds, and direct and indirect pathways. WA showed a slight increasing trend at the regional scale, but the magnitude and direction of change differed markedly among aridity zones. WA increased significantly in the humid zone at 4.05 mm yr^-1, whereas it decreased significantly in the semi-arid zone at -4.41 mm yr^-1. WA-SC was characterized by widespread synergy, while WA-CS and WA-HQ showed stronger trade-off tendencies in the semi-arid zone. The WA-FP relationship was generally weak. Threshold analysis indicated that WA-CS synergy was enhanced when the precipitation trend exceeded 1.26 mm yr^-1, whereas WA-CS trade-off risk increased markedly when the potential evapotranspiration trend exceeded 0.16 mm yr^-1. WA-FP became more likely to change from synergy to trade-off when PM_2.5 concentration exceeded 48.78 μg m^-3. Path analysis indicated that climate factors exerted significant direct effects on WA-ES trade-offs, whereas synergies were jointly regulated by topography, soil, human activities, and habitat quality. Human activities mainly affected WA-ES relationships through indirect pathways associated with PM_2.5 and LAI. This study provides a scientific basis for threshold-oriented water-resource management and zoned ecological governance in ecological function areas.