Ecosystem controls on the vertical soil hydrologic complexity required to represent soil-plant-water interactions

Terrestrial ecosystem atmosphere exchanges are governed by soil plant hydraulic processes that link soil water availability to transpiration and surface energy fluxes. Physically based soil hydrology models that resolve vertical soil moisture and matric potential gradients using the Richards equation provide the most complete representation of these processes, but their computational cost and parameter demands limit their use in long-term and multi-site applications. As a result, simplified bucket-type soil moisture models remain widely employed despite their coarse treatment of vertical soil water dynamics. Here, we quantify the level of vertical soil hydrologic complexity required for bucket-based models to reproduce the key soil–plant hydraulic behavior of Richards equation models. Using a unified framework, we couple the Penman–Monteith formulation with (i) a single-bucket model, (ii) a hierarchy of multi-bucket configurations with increasing vertical resolution within a fixed soil column, and (iii) a Richards equation model discretized over the same depth. All configurations share a consistent big-leaf canopy representation and explicitly track soil, root, and leaf water potentials and their effects on transpiration and energy fluxes. Model performance is evaluated using soil moisture and latent heat flux observations from diverse AmeriFlux sites spanning grasslands, shrublands, croplands, and forests. We show that similar soil moisture states can produce markedly different transpiration responses due to differences in soil matric potential. The vertical resolution required for bucket models to emulate Richards-based behavior is strongly ecosystem dependent, reflecting contrasts in rooting depth and plant hydraulic strategies. Our results demonstrate that plant-informed, ecosystem-specific soil discretization provides a computationally efficient pathway to improve the representation of soil plant water coupling in land surface models.