AN INTERACTIVE VEGETATION SVAT MODEL TESTED AGAINST DATA FROM 6 CONTRASTING SITES

The interactions between soil, biosphere, and atmosphere scheme (ISBA) is modified in order to account for the atmospheric carbon dioxide co ncentration on the stomatal aperture. The physiological stomatal resis tance scheme proposed by Jacobs (1994) is employed to describe photosy nthesis and its coupling with stomatal resistance at leaf level. In ad dition, the plant response to soil water stress is driven by a normali zed soil moisture factor applied to the mesophyll conductance. The com puted vegetation net assimilation can be used to feed a simple growth submodel, and to predict the density of vegetation cover. Only two par ameters are needed to calibrate the growth model: the leaf life expect ancy and the effective biomass per unit leaf area. The new soil-vegeta tion-atmosphere transfer (SVAT) scheme, called ISBA-A-g(s), is tested against data from six micrometeorological databases for vegetation ran ging from temperate grassland to tropical forest. It is shown that ISB A-A-g(s) is able to simulate the water budget and the CO2 flux correct ly. Also, the leaf area index predicted by the calibrated model agrees well with observations over canopy types ranging from shortcycled cro ps to evergreen grasslands or forests. Once calibrated, the model is a ble to adapt the vegetation density in response to changes in the prec ipitation distribution. (C) 1998 Elsevier Science B.V. All rights rese rved.