Modeling the impacts of plant hydraulics in tropical forest response to drought

In land models, vegetation water stress is heuristically related to soil water matric potential. Since the relationships are not constrained by measurable hydraulic traits, they are not able to mechanistically represent plant response to drought. To improve modeling of tropical forest response to drought, two plant hydraulics schemes have been implemented in the E3SM Land Model (ELM) to represent the hydraulic dynamics for each plant function type. Both schemes explicitly model water transport through the vegetation following Darcy's Law for porous media flow equations. One scheme is a steady state model from the Community Land Model (CLM). The other is a trait-based model from the Functionally Assembled Terrestrial Ecosystem Simulator (FATES), which considers the dynamic change of plant water storage. Vegetation water stress is a function of prognostic leaf water potential and is used to regulate stomatal conductance. The two models are compared at two tropical forest sites in the Amazon and Panama with available measurements. There are apparent differences of simulated fluxes between the root and soil using these two schemes, but the difference of vegetation stress is insignificant. Both models predict that the tropical forest at one site is less stressed compared to that simulated by the heuristic water stress model. Model experiments will be performed to evaluate the sensitivity of plant stress to uncertainties in modeling hydraulic dynamics.