Impact of land use changes on surface feedbacks in sudanian region of West Africa

Friday, 19 December 2014
Sylvie Galle1, Ossenatou Mamadou2, Jean-Martial Cohard3, Christophe Peugeot4, Josiane Seghieri4, Basile Kounouhewa2 and Norbert Cosi Awanou2, (1)IRD Institute for Research and Development, Marseille Cedex 02, France, (2)université abomey Calavi, Cotonou, Benin, (3)Univ. Joseph Fourier, Grenoble, France, (4)IRD Hydrosciences, Montpellier, France
In West Africa, surface atmosphere exchanges have been found to impact both regional and local features of the Monsoon. At local scale the spatial patterns of Evaporative Fraction can drive the trajectories of mesoscale convective systems. Under Sudanian climate a mean of ~80% of the precipitation return to atmosphere through evapotranspiration but this important amount and its dynamics may vary with the vegetation cover. In consequence, any land use or climate changes can lead to modifications on the surface feedbacks and thus on both the atmospheric and the continental water cycle. In West Africa, Sudanian regions are submitted to a ~3% demographical increase per year, which leads to regular deforestation to the benefit of cultivated areas. This study aims at quantifying the changes in evapotranspiration regime caused by such a land use change under Sudanian climate.

Within the framework of The AMMA-CATCH observatory, energy and water vapor fluxes were investigated in west Africa since 2007. Herein, a pluri-annual (2007 – 2010) energy budget of a clear forest and a cultivated area located in northern Benin are analysed. Results show that evapotranspiration rates over the sudanian forest are higher than those of cultivated area, because of agricultural practice and water availability for trees. After harvest, the residual vegetation is burned to bring nutriment to soil and to clean the landscape around the villages. Thus, during the dry season, the cultivated areas are bare. At the same time, a significant evapotranspiration is measured over the forest area despite the lack of precipitations. The deep root system of such vegetation allow the trees to get access to water during the dry season. During the rainy season, a significant difference in evapotransiration rates are also observed. These differences lead to a large deficit of water vapor that returns to the atmosphere and will significantly change the continental water cycle when forests will be replaced by cultivated areas