Hydrogeochemical processes and hydrograph separation in the Donga catchment in Benin
The international and multidisciplinary AMMA project (African Monsoon Multidisciplinary Analysis) was designed to better understand the West African climatic variability and its hydrological impact. To attain this goal, the upper Oueme river (10,000 km2) was instrumented (rainfall, runoff and groundwater) in 1997. In 2003, the Donga subwatershed (586 km2) was selected in order to characterize the major processes and quantify components of the water budget. Specifically, the objective was to identify and quantify the major components of the surface flow using a ydrogeochemical approach. The transient character of the surface water, the very low mineralization of the river as well as the asynchronism between the drying up of rivers and the water table height, suggest a shallow origin for the river flow and an unimportant exchange between that water table and the hydrographic network. River flow at the outlet, limited to the rainy season, appears to be essentially made of a rapid runoff (Hortonian runoff and flow over saturated soils) and a subsurface delayed flow without significant contribution of the water table. In agreement with this assumption, a hydrograph separation for three floods was achieved.