SoVegI: a new and efficient model coupling photosynthesis and hydraulic transport within the soil-plant continuum

Climate change impacts on forests cannot be understood without representing the hydraulic functioning of forests. In this work we present SoVegI (Soil-Vegetation Interaction model), a numerically efficient, process-based model of the soil-plant-atmosphere continuum, developed to represent groundwater-forest interactions under drought conditions for broadleaf and deciduous forests of Europe.

The model includes (1) a single layer sun/shade model of mass and energy fluxes at the canopy scale, (2) a stomatal conductance model depending, among other things, on leaf water potential describing the direct link with soil water availability, (3) a process-based soil-root-xylem hydraulic transport scheme assuming the hydraulic transport to be analogous with water transport in a porous media, and (4) a root water uptake model representing the direct coupling between the soil and the vegetation.

The novelty of the model is to present a fast and efficient numerical implementation of the hydraulic transport process within the whole soil-plant-atmosphere continuum that allows coupling with large spatial models. The porous media analogy results in a set of three coupled nonlinear partial differential equations similar to Richards’ equation for porous media in soil. The system is solved using a finite volume, time-implicit approach and an advanced iterative scheme is used to treat the non-linearity of the system.

This new numerical model was successfully tested at the tree and the forest scales at two sites in northern France after being calibrated against sap flow measurements and eddy covariance data, respectively. At the tree scale, the model was able to reproduce the mid-day partial stomatal closure showing the availability of the model to catch the dynamic feedback between the atmospheric and soil water conditions. The model was also capable of reproducing the water storage pool drainage during day time and the night time replenishment, opening interesting perspectives to investigate forests’ risks to hydraulic failure. At the forest scale, the model was able to reproduce the transpiration response to the 2003 soil drought and heat wave in northern Europe with limited computational efforts. These preliminary steps open interesting research perspectives where SoVegI will be coupled with a physically-based integrated hydrologic model to assess the impact of extreme events on groundwater-forest relations.