Transboundary basins increasingly face coupled pressures from climate variability and land-use change, yet their combined impacts on water-ecosystem dynamics in humid regions remain poorly constrained. Here, we investigate these interactions in the humid temperate Tumen River Basin of Northeast Asia during 2001-2024. We integrate GRACE-based terrestrial water storage (TWS), MODIS land-cover, vegetation, and land-surface temperature products, ERA5-Land climate reanalysis, and topographic information, and combine these datasets with a Delta VHI-Delta ILR framework, ridge-regression decomposition, random forest analysis with SHapley Additive exPlanations (SHAP), and GeoDetector. We show that basin-mean TWS has increased significantly since 2002 under a wetter climate, while vegetation health has improved mainly where cropland was converted to shrubland or forest, but shows weak or inconsistent gains after cropland-to-grassland conversion. Interannual TWS anomalies are dominated by climate variability, yet land-use/cover change exerts a persistent secondary control: a characteristic "cropland -> shrubland -> forest" succession, with the most pronounced signals occurring on steep, erosion-prone slopes, is associated with enhanced water retention, soil moisture and greening, without the strong water-vegetation trade-off reported for semi-arid afforestation. Together, these results support a climatedominated, LUCC-enhanced mechanism of water-ecosystem co-evolution in humid temperate basins and highlight the potential of well-designed restoration programmes to deliver concurrent benefits for terrestrial water storage and vegetation resilience.