Moisture availability and delivery to the site of transpiration acts as a primary control on vegetation carbon and energy exchange with the atmosphere. Water flow through the plant system is a complex process, and simulating the impact of water flow through a plant canopy on stomatal dynamics requires resolution of plant-level hydraulic function, scaled to the canopy-level. Here we present a novel approach to the direct numerical simulation of the hydraulic controls on vegetation mass and energy exchange through the coupling of two models characterizing the biophysical (MLCan) and hydrodynamic (FETCH2) functioning of vegetation canopies. MLCan is a model of vegetation biophysics, linking below-ground root uptake to above-ground biochemical and ecophysiological processes controlling mass and energy fluxes through vertically-resolved radiation transfer and micro-environmental processes. FETCH2 uses a Richard’s Equation approach to simulate water potential gradients through a plant canopy as a function of dynamic xylem conductivity and capacitance through consideration of the plant system as a porous medium. This approach allows vegetation water content (VWC) and water potentials to be resolved vertically through the canopy. MLCan and FETCH2 thereby provide the required prognostic components to simulate the impact of plant hydraulic traits on vegetation productivity, water use and energy exchange.
Results/Conclusions
This newly coupled model is validated against an extensive dataset from the Michigan Biological Station Ameriflux site outside of Ann Arbor, Michigan. The dataset includes tower-based observations of sub-hourly carbon, water and energy fluxes, meteorological forcing, and in-situ observations of vegetation sap flow, water content and soil moisture availability. Here we demonstrate that through the incorporation of highly resolved (sub-hourly in time, vertically resolved in space) hydrodynamics within the context of a sophisticated vegetation biophysical model, canopy-atmosphere fluxes of water vapor and sensible heat are improved with respect to both magnitude and timing. FETCH2 is able to capture the observed diurnal hysteresis in water flux as a function of the loss of stored water through evapotranspiration in the morning, resulting in a reduction of available water, and water potentials, in the early and late afternoon periods when atmospheric demand is greatest. These reduced water potentials through the canopy result in reductions in stomatal conductance in the afternoon period on days when atmospheric demand for moisture (vapor pressure deficit) is high, and propagate in MLCan to reductions in carbon uptake and transpiration, with increases in sensible heat flux and longwave radiation emission by the canopy.