The root zone is the upper part of the unsaturated zone, where water and nutrients are accessible to plants, controlling hydrological responses, vegetation dynamics, biogeochemical processes, and land-atmospheric interaction. The root zone storage capacity (<em>S</em><sub>umax</sub>) represents the maximum subsurface moisture volume that can be accessed by the vegetation’s roots, controlling the partitioning of precipitation into storage, runoff and percolation. Previous work has illustrated that <em>S</em><sub>umax</sub> varies spatially, largely responding to climatic conditions. It can be therefore expected that <em>S</em><sub>umax</sub> varies temporally as well in response to climate change. However, this hypothesis has not been tested. In this study, we utilized a conceptual hydrological model and a dynamic parameter identification analysis method, to quantify the temporal trends of <em>S</em><sub>umax</sub> for 497 catchments in the USA. We found that 423 catchments (85 %) showed increasing <em>S</em><sub>umax</sub>, which averagely increased from 178 to 235 mm between 1980 and 2014. The increasing trend was also validated by multi-sources data and independent methods. Our results suggest that ecosystems dynamically adapt their root zone in response to climate change, which significantly affects hydrological processes and water resources availability. Moreover, the increase of <em>S</em><sub>umax</sub> significantly correlates to hydroclimatic indicators and vegetation dynamics. These results highlight the importance of considering the co-evolution of climate, ecosystems, and hydrology.