Hybrid modeling of the active root zone storage and its capacity

The active root zone storage (aS_rz) is a critical yet unobservable quantity in the water cycle. It represents the dynamic component of subsurface water that can be accessed by ecosystems for evapotranspiration (ET), directly linking water, energy and carbon exchanges across the land-atmosphere interface. In this study, we propose a catchment-scale, ecosystem-oriented hybrid model to understand the spatiotemporal dynamics of aS_rz. We introduce aS_rz as a latent soil moisture state in the model; without explicitly prescribing soil layers or providing rooting depth information, the model diagnoses aS_rz (and its capacity) by relying on first-order ecohydrological principles and multi-source observational constraints (ET, runoff, snow and terrestrial water storage). We train the model across hundreds of U.S. catchments over the period 1985–2020 and then upscale to a 0.25° grid, finding that the inferred root zone storage peaks in transitional regions. We explore the interplay of vegetation, atmospheric demand and water supply, seasonality and topography in modulating the root zone storage dynamics. Furthermore, aS_rz capacity reveals the long-term ecosystem adaptation to hydroclimate and substrate conditions. By investigating the differences between aS_rz dynamics and its capacity across catchments, we uncover divergent ecosystem strategies for managing water resources, especially along the aridity gradient. Overall, our parsimonious hybrid model structure provides a physically consistent and observationally constrained roadmap for diagnosing ecosystem processes that cannot be directly observed.