The role of the Sahelian biosphere on the water and the CO2 cycle during the HAPEX-Sahel Experiment

The HAPEX-Sahel experiment was organised to investigate the impact of water, energy and CO2 fluxes at the soil-vegetation-atmosphere interface on climate processes in the Sahelian region. Measurements of the energy balance components, CO2 flux and soil moisture were conducted over a savanna area at the East Central Supersite of the one degree square during a 3 month period in 1992. The aim of this particular investigation was to understand the role of surface conditions (i.e. vegetation and moisture) in the partitioning of available energy at the surface into sensible and latent heat flux. It also aimed to improved the understanding of how water and carbon cycles are affected by vegetation functioning, soil water avaibility and atmospheric demand. The analysis presented in this paper showed that the relative contribution of the soil and the vegetation to latent heat flux varies intimately with the temporal rainfall distribution of precipitation than to its amount. Finally, semi-empirical parameterizations were developped to formulate (1) the daily evapotranspiration rate of the savanna 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.