Large-scale vegetation restoration projects could spark concerns of water conflicts between humans and ecosystems in water-stressed areas. However, whether these efforts compromise water availability (WA) in humid karst systems, where precipitation is abundant yet rapidly lost, remains debated. Here, we developed and applied a new karst-based model to simulate ecohydrological processes in six typical karst watersheds of southwestern China, a global greening hotspot. Our model performed well in simulating streamflow (Kling-Gupta Efficiency: 0.67-0.79), evapotranspiration (0.70-0.82), gross primary production (0.62-0.77), and leaf area index (0.72-0.86). Our results showed that the simulated WA did not exhibit significant changes during the study period, despite substantial greening areas since the 2000s. Precipitation was identified as the dominant driver controlling WA dynamics (contributing 84.6-117.6 %), while vegetation restoration had a limited impact (-8.0 %-3.8 %) due to energy constraints and repaid subsurface recharge. Our results suggest that afforestation is unlikely to trigger water scarcity in humid karst regions under current climatic conditions. However, projected drying trends may increase future risks, underscoring the need for adaptive strategies. Our model advances understanding of coupled ecology-hydrology interactions and provides a practical tool for guiding sustainable water management and ecological restoration. These insights can help to guide trade-offs between greening projects and water security under future climate change.