Impact of Deforestation on Water Budget in Sudanian Climate (Benin)

Abstract:

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. Within Sudanian climate, ~80% of the precipitation returns to atmosphere through evapotranspiration. However, this amount and its seasonal dynamic may vary with the vegetation cover. Consequently, one might expect that any land use or climate changes could lead to the modification of the surface feedbacks, and, thus on both the atmospheric and the continental water cycle. The sudanian region of West Africa is submitted to a 3% demographical increase per year, which induces a drastic expansion of crops areas. In the Upper Oueme basin, the natural forest cover reduces from 70% to 25% in 40 years. This study aims at quantifying the changes in evapotranspiration regime caused by such a land use change under sudanian climate.

The AMMA-CATCH observatory documents evapotranspiration flux in West Africa since 2007. A pluri-annual energy budget of a forest and a cropland area are analysed. It is shown that sudanian forest evapo-transpirated always more than cropland areas because of agricultural practice and water availability for trees. Thus, during the dry season, the cropland areas are bare while the forests do not completely lose their leaves. Their deep root systems allow the trees to get access to water. Observed evapotranspiration is significant over forests. During the rainy season, vegetation is fully developed and well-watered. Nevertheless, lower but significant differences in evaporative fraction are also observed. At annual scale these differences lead to a 13% to 30% reduction of evapotranspiration with deforestation.