THE ROLE OF THE SAHELIAN BIOSPHERE ON THE WATER AND THE CO2 CYCLE DURING THE HAPEX-SAHEL EXPERIMENT

The HAPEX-Sahel experiment was organized to investigate the impact of water, energy and CO2 fluxes at the soil-vegetation-atmosphere interfa ce on climate processes in the Sahelian region, Measurements of the en ergy balance components, CO2 flux and soil moisture were conducted ove r a savanna area at the East Central Supersite of the one degree squar e during a 3 month period in 1992. The aim of this particular investig ation was to understand the role of surface conditions (i.e. vegetatio n and moisture) in the partitioning of available energy at the surface into sensible and latent heat flux. It also aimed to improve the unde rstanding of how water and carbon cycles are affected by vegetation fu nctioning, soil water availability and atmospheric demand. The analysi s presented in this paper showed that the relative contribution of the soil and the vegetation to latent heat flux varies intimately with th e temporal rainfall distribution and the growth of the savanna grass s pecies, which is more sensitive to the distribution of precipitation t han to its amount. Finally, semi-empirical parameterizations were deve loped to formulate (1) the daily evapotranspiration rate of the savann a in terms of available energy at the surface and soil water content, and (2) the instantaneous carbon uptake in terms of photosynthetically active radiation received at the surface and soil water availability.