Ecosystem mobilization of subsurface water revealed by global active root-zone storage

Abstract. Subsurface water sustains vegetation between precipitation events and shapes ecosystem responses to hydroclimatic variability. Yet existing observations capture either near-surface soil moisture or bulk terrestrial water storage, leaving the dynamic component of subsurface water that vegetation actively mobilizes poorly understood at the global scale. Here we analyze a global reconstruction of active root zone water storage (aS_rz), defined as the depth of subsurface water ecosystems actively mobilize for evapotranspiration, from 2001 to 2020. The global area-weighted mean aS_rz is about 100 mm, with the largest values concentrated where precipitation supply and atmospheric demand are climatologically comparable. Trends in climatic water balance are most strongly reflected in aS_rz in seasonally driven systems such as croplands, savannas, and seasonal forests, and more weakly in evergreen tropical forests and tundra. The turnover time of the active component distinguishes rapidly cycled storage in warm and seasonally dry regions from slowly cycled storage in boreal and Arctic regions. Comparison with satellite gravimetry shows that aS_rz co-varies with bulk terrestrial water storage at monthly scales, but the coupling weakens and becomes more regime-dependent at interannual scales, especially in snow-dominated and dry regions. These patterns identify where bulk storage provides information on subsurface water accessed by ecosystem and where changes in bulk storage mainly reflect changes in other water stores. Overall, these findings establish ecosystem-accessed water storage as an observation-based dimension of the terrestrial water cycle, revealing patterns of ecosystem water access and change that bulk storage and climatic wetness indicators do not resolve, and providing a foundation for assessing ecosystem water vulnerability under hydroclimatic change.