Plant Hydraulic Architecture for a Mechanistic Representation of Soil‐Plant‐Atmosphere Water Transfer in the Land Surface Model ORCHIDEE (r9107)

Abstract Land surface models (LSMs) typically represent soil moisture control on stomatal conductance through an empirical sensitivity function, without considering plant hydrology. This study proposes integrating water transfer representation within the soil‐plant‐atmosphere continuum in the ORCHIDEE land surface model. This new configuration includes vegetation hydraulic architecture and a stomatal control based on leaf water potential , along with a mechanistic representation of water absorption by roots via radial diffusion around the roots. An adaptive numerical scheme is implemented to prevent numerical instabilities during hydric stress, reducing hourly instabilities by a factor of 2. The implementation and the standard configuration of ORCHIDEE are calibrated and evaluated at FLUXNET sites with eddy‐covariance flux measurements. A detailed assessment is carried out at two well‐documented forest sites (FR‐Hes and FR‐Pue), where both configurations perform similarly regarding the seasonal dynamics of latent heat flux (RMSEs of 16.0 W/ for the potential‐based configuration and 15.8 W/ for the standard configuration at FR‐Hes). An evaluation of leaf water potential at FR‐Pue shows correlations of 0.87 and 0.72 for predawn and midday respectively. A second evaluation across 135 sites from the FLUXNET2015 database highlights similar performances for both configurations. Finally, a global assessment of the differences between the two schemes emphasizes the good performance of the hydraulic architecture model. Overall, the new hydraulic architecture provides a more mechanistic description of stomatal conductance response to soil water stress and paves the way for incorporating physiological processes controlling tree mortality and using in situ observations to calibrate plant responses to water stress.