Modeling Stomatal Conductance in ORCHIDEE Land Surface Model and Comparison with FLUXNET Observations

Plant stomata are the privileged place of exchange of water and carbon between the leaves and the atmosphere. Vegetation optimizes it stomatal conductance in order to maximize CO2 uptake from the atmosphere and minimize water loss by transpiration. Stomatal conductance, thus, bridges energy, water and carbon budgets. Several processes are controlling the stomatal conductance such as plant hydraulics, sugar concentration or past damages. Among them, the plant hydraulics seems to be a key process. Leaf water and sugar content are correlated, especially during drought events, and plant damages are tightly correlated to water stress through cavitation and embolism. Modelling the vegetation hydraulic architecture in land surface models is therefore essential to control stomatal conductance and to simulate plant water stress especially in the context of global warming with increased drought stresses. However, only few models are representing stomatal control based on plant hydraulics (Damour et al. (2010)). In this study, we implemented the vegetation hydraulic architecture model (SPAC) developed by Tuzet et al. (2017) into the ORCHIDEE Land Surface Model (LSM) in order to better represent plant stomatal regulation. This process-based model (SPAC) aims at reproducing water transfers in the soil-plant-atmosphere continuum following a hydraulic resistances and water potentials scheme. Soil water absorption is calculated by solving the Richard's equation radially around the roots while tree water storage is modelled thanks to capacitances. Finally, stomatal conductance is coupled to the leaf water potential thanks to an adapted form of Ball-Berry model (Tuzet et al. (2003)). Preliminary tests at local scale performed at the HESSE FLUXNET site (a deciduous forest in Moselle, France) showed that the SPAC model integration is able to better simulate the surface turbulent uxes during drought events in summer 2003, compared to the standard representation used in ORCHIDEE. The comparison has been extended to the other sites documented in the FLUXNET2015 database covering a larger range of forest types and climates. The expected improvements brought by this new way to parameterize stomatal control in ORCHIDEE will be presented.