Interactive vegetation modelling at a regional scale: application to the adour basin

The ISBA (Interactions between Soil Biosphere and Atmosphere) SVAT model was modified to include a representation of interactive vegetation. A parameterization of CO₂ assimilation was introduced, the photosynthesis process was parameterized as a function of vegetation physiological properties and environmental conditions. Thereby, the canopy stomatal resistance is described in a more realistic way than in the initial version. A growth model was used to compute Leaf Area Index (LAI) evolution instead of prescribing it from satellite data without taking into account possible climate changes and their consequences on plant development. In this paper, the “ISBA-A-gs” modified model is tested at the regional scale in the Adour basin. For this purpose it was necessary to calibrate three parameters for each land cover type (mainly coniferous forest, summer and winter crops): the mesophyll conductance, the maximum turnover time of leaves and an effective biomass per LAI ratio. This work has been followed by a long term simulation (1986–1995), where the surface scheme was coupled with a distributed hydrological model. The results were compared to those obtained with the “standard version”, i.e ISBA without A-gs. For the whole period, the simulated LAI was lower than the prescribed one, which decreased evaporation (10 to 15 % less with A-gs) and led to a better simulation of streamflow on both crop and forest areas.