TT3: EOP Integrative studies on the Gourma meso-scale site (Mali)

TT-leaders: Eric Mougin, Josiane Seghieri, Lassine Diarra

Detailed information: TT3 document: EOP Integrative studies on the Gourma meso-scale site (Mali) (latest update: 29.07.2005, pdf, 3.8 MB)

Scientific justification and objectives

In Mali, EOP focuses on quantification of water, CO2 and energy fluxes that occur between the surface and the atmosphere. Among the surface processes under consideration, emphasis is put on evapotranspiration which is the most important process coupling the physical, biological and hydrological processes occurring at the continental surface. It is also the term of the water budget responsible of the surface retroaction to the atmosphere, since evapotranspired water then enters the regional atmospheric circulation. It is also the most poorly measured and understood term of the water budget. The main scientific objective is therefore to better understand, model and predict temporal and spatial variations of evapotranspiration in a bottom up approach integrating all the physical, biological and hydrological processes involved.

Understanding, modelling and predicting plant phenology (i.e. the seasonal cycle of LAI and biomass) is the key issue to correctly predict the evapotranspiration cycle along the year. Most process studies focus on integrating the numerous ecological processes (carbon assimilation, water uptake and release, vegetation growth and decay, coupled C/H2O/N cycles,…) responsible for the phenological cycle. Moreover, water transpiration is closely linked to CO2 assimilation through the stomatal control by plants. This coupling is simulated in most current SVAT models. The water extraction strategy and the phenological cycle of plants are used to define major plant functional groups.

The question of the long term dynamics of sahelian vegetation is also addressed since the Gourma vegetation sites have been monitoring for about 20 years. The vegetation monitoring sites are part of the AMMA-CATCH observing system (ORE: Observatoire de Recherche sur l'Environnement) and of the 'GLOBALSAV' network. Furthermore, they are used as validation sites for satellite products including vegetation parameters (LAI, Cover) within the frame of the VALERI project, and soil moisture within the SMOS activities.

Besides, sahelian ecosystems contribute a significant fraction to global biogeochemical C and N cycling. Biogenic emissions of NOx by soils as well as wet and dry surface deposition control tropospheric ozone, particle formation (aerosols) and gas concentrations. Biogenic emissions are linked to meteorological and surface characteristics such as soil moisture and temperature (NOx), phenology and plant water stress (BVOC) whereas dry deposition fluxes are largely dependant on land use covers. We also have the objectives to provide a better parameterization linking biogenic emissions and surface characteristics at local and meso scale.

The overall methodology is as follows:

• Analysis of the spatio-temporal variability of soil moisture and vegetation in relation with the variability of atmospheric variables (mainly rainfall),
• Analysis of the spatial-temporal variability of water and energy fluxes from local to mesoscales,
• Investigate how biogeochemical cycles are controlled by land surface processes,
• Estimation of meso-scale surface fluxes, vegetation phenology, soil moisture and water budget through the assimilation of remote sensing data into coupled vegetation/SVAT models.