AN APPROACH TO COUPLE VEGETATION FUNCTIONING AND SOIL-VEGETATION-ATMOSPHERE-TRANSFER MODELS FOR SEMIARID GRASSLANDS DURING THE HAPEX-SAHEL EXPERIMENT

This paper presents a model which has been developed to simulate the m ajor land surface processes occurring in arid and semiarid grasslands. The model is composed of a hydrological submodel which describes the water and energy budgets, and a vegetation growth submodel which group s the processes associated with biomass production, Emphasis has been placed on developing a realistic representation of the interaction bet ween these subprocesses taking account of the different time scales in volved. The hydrological submodel couples the energy balance of the so il/canopy with the soil moisture and thermal dynamics. It interacts wi th the vegetation growth submodel by exchanging information needed to account for the influence of plant water status and canopy temperature on photosynthesis, and the influence of the vegetation canopy on the boundary layer within which transport processes are taking place. The model has been tested with meteorological, biomass and energy flux mea surements made on a grassland site during the HAPEX-Sahel experiment, Niger, in 1992. Model simulations of biomass over the growing season a re all found to be within a 15% error margin allowed on biomass measur ements, Hourly values of net radiation, as well as latent and sensible heat fluxes, are simulated with an RMSE of less than 50 W m(-2). Give n the relative simplicity of the model and the long period of uninterr upted simulation, these results are considered satisfactory, Overall, the results show that the model behaves consistently at different stag es of vegetation growth, and satisfactorily reproduces the interdepend ence of vegetation growth with the physical processes giving rise to t he water and energy balances.