Climate change fundamentally reshapes regional hydrology, with profound implications for water security and ecosystem sustainability. While the direct impacts of altered meteorological conditions on water resources are well-established, the indirect effects caused by climate-driven vegetation dynamics remain poorly understood and inadequately quantified. Focusing on the upper Yellow River Basin of China, this study performed scenario experiments using the Variable Infiltration Capacity (VIC) distributed hydrological model to investigate the effects of climate-induced changes in leaf area index (LAI) on the regional water resources during 1985-2018. Our results demonstrate that direct climate effects are dominant, with direct contributions exceeding 80% for all hydrological variables. Specifically, for regional water availability (precipitation minus evapotranspiration), the direct impact accounts for 93.3%, while the vegetation-mediated indirect effect contributes -6.7%. This reduction occurs as vegetation growth enhances evapotranspiration and partially counteracts climate-driven water increases. Moreover, these effects manifest with significant divergence across different hydrological components. Increased LAI strongly enhances canopy interception (+11.1%) but has a negligible indirect impact on vegetation transpiration, indicating that transpiration changes are primarily controlled by direct climate effects. Concurrently, climate-induced vegetation growth exerts negative contributions to water yield components, reducing surface runoff and baseflow by -2.7% and -9.4%, respectively. Spatially, these effects are most pronounced in the densely vegetated southeastern area of the basin. Our findings reveal a fundamental trade-off in semi-arid ecosystems where ecological greening, despite its environmental benefits, may compromise water resource availability under changing climate conditions.