Water stored in bedrock is crucial for ecosystem water supply, sustaining plant function, and drought resilience. However, conventional ecohydrological frameworks focus on soil moisture, often neglecting bedrock water and lacking adequate methodologies for its quantification. Here, we introduce a dynamic bedrock water evapotranspiration model and estimate the daily bedrock-derived evapotranspiration across China using multi-year meteorological and remote sensing datasets. Temporal and spatial patterns of evapotranspiration from bedrock-stored water reveal that over half of shallow bedrock ecosystems use bedrock water, contributing 10.84% of total evapotranspiration and 7.99% of vegetation transpiration. In arid/semi-arid and seasonally dry regions, seasonal water-carbon decoupling increases dependence on bedrock water, particularly in southwestern and northwestern China. This occurs when transpiration demand rises before the peaks of precipitation and/or photosynthesis, necessitating deeper water sources. Causal machine learning shows that in water-stressed climates, bedrock water enhances ecosystem productivity and carbon stocks by sustaining transpiration (FDR < 0.05). These results challenge the prevailing soil-centric paradigm and underscore the critical role of bedrock-stored water in enhancing ecosystem productivity and drought resilience. By linking hydrology, plant physiology, and carbon cycling across climatic gradients, our study supports improvements to land surface models and the prediction of ecosystem responses to intensifying drought.