Observation of the impacts of both geology and vegetation environment on evapotranspiration regime : a case study under sudanian climate

Soil-Vegetation-Atmosphere interactions are difficult to predict because they are the combination of many hydrological, biological and atmospheric processes. It results in an interface composed of imbricate soil, vegetation and atmospheric layers and not only a simple superposition of these layers. Therefore, it is necessary to study the compartments all together to understand the hydrological behaviour of a watershed. More over lateral variability in each water compartments add to the complexity of natural landscapes. To address this complexity, we have instrumented in the framework of the AMMA-CATCH experiment, a 12km2 watershed situated in a rocky context in north Benin. It is part of a larger hydrological device to understand the water cycle in West Africa and the feedback of the continental surface on the monsoon system. The Ara watershed has been instrumented with several ground stations to monitor the water table, the vadose zone, the precipitations and the river discharge. We also implement instrumentation that gives aggregated characteristics such as geophysical prospecting methods. Electromagnetic and MRS (magnetic resonance sounding) have been used to characterize the hydrodynamic ground properties. Also, a Large Aperture Scintillometer together with a flux station are installed on a 2.4km transect over the catchments,from which are measured aggregated sensible and latent heat flux at a km2 scale. At catchments scale, it has been shown from water table level and scintillometry measurements that the AET (Actual Evapo-Transpiration) regime lasts the entire dry period from November till the next wet season in June. As no runoff is observed during this period, a simple pattern is considered and consists in a direct link between water table and transpiration. However we first can’t conclude on the average porosity necessary to drain the water table through transpiration because it varies during the season. In this study, a detailed footprint analysis is carried out together with a MRS drainage porosity characterisation within the same area. These combined information are then used to explain the AET time series in agreement with water table drainage. Methods involved like scintillometry and MRS, have been implemented on the same experiment field for the first time. They will be first rapidly summarized. In particular a fine scale geological and drainage porosity maps have been obtained from an electrical conductivity prospection of the regolith measured with electromagnetic EM34 equipment and MRS measurements. Then, the study will focus on AET time series during the dry seasons 2006 and 2008 derived from scintillometry observations. When the wind shifts from Harmatan (north-east) to monsoon (south-west), it allows to differentiate AET regimes from different vegetation and geological context. This study shows how instrumentation from various disciplines can support, when they are used together, our understanding of hydrological patterns and processes related to soil-vegetation-atmosphere interactions.