Substantial root-zone water storage capacity observed by GRACE and GRACE/FO

Abstract. Root-zone water storage capacity (S_r) – the maximum water volume that can be held in the plant root zone – bolsters ecosystem resilience to droughts and heat waves, influences land-atmosphere exchange, and controls runoff and groundwater recharge. However, S_r is difficult to measure, especially at large spatial scales, hindering accurate simulations of many biophysical processes, such as photosynthesis, evapotranspiration, tree mortality, and wildfire risk. Here, we present a global estimate of S_r using direct measurements of total water storage (TWS) anomalies from the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On satellite missions. We find that the median S_r value for global vegetated regions is at least 220 ± 40 mm, which is over 50 % larger than the latest estimate derived from tracking storage change via water fluxes, and 380 % larger than that calculated using the soil and rooting depth parameterization. Parameterizing a global hydrological model with our S_r estimate improves TWS and evapotranspiration simulations across much of the globe. Furthermore, our S_r estimate, based solely on hydrological data, correlates realistically with an independent vegetation productivity dataset, underscoring the robustness of our approach. Our study highlights the importance of continued refinement and validation of S_r estimates and provides a new pathway for further exploring the impacts of S_r on water resource management and ecosystem sustainability.