The hydrology of inland valleys in the sub-humid zone of West Africa: Rainfall-runoff processes in the M'bé experimental watershed

Inland valleys with wet lowlands are an important water source for farming communities in the sub-humid zone of West Africa. An inland valley and surrounding contributing watershed area located in the sub-humid zone near M'bé in central Côte d'Ivoire was instrumented to study surface runoff and base flow mechanisms. Four flumes at different distances down the main stream and more than 100 piezometers were installed. Measurements were taken during two rainfall seasons in 1998 and 1999. Under initial wet conditions, a typical single-peak hydrograph was observed. Under low antecedent moisture conditions, however, runoff was characterized by a double-peaked hydrograph. The first peak, which occurred during the storm, was caused by rain falling on the saturated valley bottom. The second peak was delayed by minutes to hours from the first peak and consisted of rain flowing via the subsurface of the hydromorphic zone that surrounds the valley bottom. The duration of the delay was a function of the water table depth in the hydromorphic zone before the storm. The volume of the second peak constituted the largest portion of the stream flow.