Incorporating plant access to groundwater in global root zone storage capacity estimate

Abstract

Plant-available groundwater water is well-documented. However, the impact of groundwater flux on root zone storage capacity (S_r), the maximum water storage within the subsurface root zone available for plant transpiration, remains poorly understood. In this study, we present a more conservative, lower-bound global estimate of S_r, incorporating groundwater for the first time into the deficit-based calculation of S_r using a novel conceptual method that accounts for groundwater contribution fraction. Our findings reveal widespread plant reliance on groundwater, with a mean use of at least 20 mm, equivalent to a water volume of 1700 km³. In western United States, this use can exceed 100 mm. Distinct spatial patterns in S_r emerge globally, with higher values in mountainous forests and lower values in boreal grasslands. Comparisons with observed rooting depths confirm that the deficit-based method, when incorporating groundwater, effectively predicts underground root traits. Biotic and abiotic factors critically influence S_r values, with irrigation and topographic convergence exacerbating this reduction. Groundwater-dependent ecosystems rely heavily on root-zone water storage, utilizing an average of 2151 km³ of water. Despite inherent uncertainties in input data, our study provides the first systematic evaluation of groundwater’s role in shaping S_r estimates, offering key insights for water resource management and ecosystem sustainability.