A REGIONAL SAHELIAN GRASSLAND MODEL TO BE COUPLED WITH MULTISPECTRAL SATELLITE DATA .1. MODEL DESCRIPTION AND VALIDATION

An approach to combining remote sensing spectral measurements with an ecosystem process model is presented. In this approach, the ecosystem model is not bound by the sole use of satellite data, but integrates t he latter in an explicit formulation of the main processes of vegetati on functioning. A close analysis of the relationships between processe s described by the model and spectral measurements can therefore be ca rried out, and the capability of the model to be driven by remote sens ing can also be investigated. This first article presents a regional e cosystem process model for Sahelian regions. The model describes a her baceous layer composed of only annual species. The processes of the so il-plant-atmosphere system, such as water fluxes in the soil, evaporat ion, transpiration, photosynthesis, respiration, senescence, bitter pr oduction, and litter decomposition at the soil surface, are modeled. M oreover, structural parameters such as vegetation cover fraction, LAI, and canopy height, which are essential parameters for coupling with p hysical models of reflectivity, are also simulated. Comparison with ab oveground biomass measured between 1976 and 1992 at a regional scale i n two different regions of the Sahel, namely, Ferlo in Senegal and Gou rma in Mall, shows that the model is able to simulate the temporal evo lution of the aboveground biomass components.