Streamflow generation in humid West Africa: the role of Bas-fonds investigated with a physically based model of the Critical Zone

Abstract:

In West Africa, the drought initiated in the 70’s-80’s together with intense land-use change due to increasing food demand produced very contrasted responses on water budgets of the critical zone (CZ) depending on the lithological and pedological contexts. In Sahel, streamflow increased, mostly due to increasing hortonian runoff from soil crusting, and so did groundwater storage. On the contrary, in the more humid southern Sudanian area, streamflow decreased and no clear signal has been observed concerning water storage in this hard-rock basement area. There, Bas-fonds are fundamental landscape features. They are seasonally water-logged valley bottoms from which first order streams originate, mostly composed of baseflow. They are a key feature for understanding streamflow generation processes. They also carry an important agronomic potential due to their moisture and nutrient availability.

The role of Bas-fond in streamflow generation processes is investigated using a physically-based coupled model of the CZ, ParFlow-CLM at catchment scale (10km²). The model is evaluated against classical hydrological measurements (water table, soil moisture, streamflow, fluxes), acquired in the AMMA-CATCH observing system for the West African monsoon, but also hybrid gravity data which measure integrated water storage changes. The bas-fond system is shown to be composed of two components with different time scales. The slow component is characterized by the seasonal and interannual amplitude of the permanent water table, which is disconnected from streams, fed by direct recharge and lowered by evapotranspiration, mostly from riparian areas. The fast component is characterized by thresholds in storage and perched and permanent water tables surrounding the bas-fond during the wet season, which are linked with baseflow generation. This is a first step toward integrating these features into larger scale modeling of the critical zone for evaluating the effect of precipitation intensification and land use changes scenarios in the area.