Effect of a coupled soil water–plant gas exchange on forest energy fluxes: Simulations with the coupled vegetation–boundary layer model HIRVAC

Long-living plant communities such as forests reduce their transpiration by closing and opening the leaf stomata as a common strategy to save water in dry periods. Meteorological models including vegetation should consider this mechanism to simulate realistic water transport from the plant to the atmosphere. Results of the German network project VERTIKO showed that commonly used meso-models such as Lokalmodell (German Weather Service) often overestimate evapotranspiration of vegetated surfaces during dry periods. This is, among other things, due to the insufficient plant-specific coupling between the soil water content and the physiological reactions of leaf stomata in the implemented SVAT modules. This study presents an approach to describe the above-mentioned coupling mechanism by upgrading the coupled vegetation boundary layer model HIRVAC. A stomatal reaction on soil moisture change, which is a part of HIRVAC, is parameterised in the included mechanistic photosynthesis model for C3 plants (PSN6).In the new HIRVAC version several parameters of the PSN6 model were scaled by a power function of the matrix potential to consider the stomatal reaction to changes in soil water content. This leads to an adaptation of the additional humidity source term in the basic equation of HIRVAC. As a result the humidity profiles in the canopy air, the latent heat flux and the energy balance of each canopy model layer are changed.The new parameterisation in HIRVAC was applied for the VERTIKO special observation period in May and June 2003 for vegetation parameters adapted to the Tharandter Wald Anchor Station (experimental site of the Department of Meteorology, TU Dresden).The HIRVAC modification leads to a realistic decrease in latent heat flux for dry soil conditions. Without coupling, latent heat flux increases continuously due to an increase in the atmospheric driving parameters of vapour pressure deficit (vpd) and temperature in the canopy. Despite some differences during the night the simulated and measured sensible heat flux agree very well, especially under dry soil conditions, while the correlation between measured and simulated latent heat flux is only moderate. The best agreement between simulated and measured latent heat flux was reached for the vpd and crown air temperature under moderate soil moisture conditions.