Active root zone water storage dynamics reveal changes in ecosystem access to subsurface water

Changes in the terrestrial water cycle reflect not only atmospheric forcing but also how terrestrial ecosystems store and use subsurface water, a component that remains difficult to quantify at large scales. Most observation-based studies rely on surface soil moisture or total water storage anomalies (TWSA) as indicators of ecosystem water availability, although surface soil moisture does not fully represent the water accessed by vegetation, and TWSA integrates multiple storage components that are not necessarily involved in vegetation water use. Here we diagnose active root zone water storage (aSrz), defined as the dynamically operated water volume associated with vegetation water use, using a hybrid ecohydrological framework constrained by precipitation, evapotranspiration, runoff, and terrestrial water storage anomalies. We estimate aSrz and its temporal envelope across the continental United States, and examine its spatial structure and long-term trends. We show that aSrz isolates the vegetation-related component of subsurface water storage and reveals where changes in ecosystem water use are not captured by surface soil moisture or TWSA. Across climate regimes, aSrz remains closely coupled to gross primary productivity anomalies, while surface soil moisture and TWSA often decouple. Regional trends in aSrz further identify where vegetation water access has intensified or weakened relative to ecosystem structure under similar climatic conditions. By explicitly isolating actively exchanged storage, this work provides a new diagnostic for assessing changes in the pace of the terrestrial water cycle.