Hydrological process and origins of the runoff in a granitic basement: the case of the Kolondieba watershed in the south of Mali

The Kolondieba Watershed covering an area of 3050 km² takes place in the southern granitic basement of Mali. It's used as part of the international program RIPIECSA (Interdisciplinary and Participatory Researches on Interactions between Ecosystems and Climate in Africa) to study the hydrological process occurring in order to determine the origin of the runoff at the outlet. The method used is based on rainfall, hydrometric, piezometric and geochemical monitoring over three hydrological cycles (2009 to 2012). The results show that the runoff starts at the outlet when groundwater rise to high level and stop when it's lower. Water resource mineralization monitoring based on the electrical conductivity [EC (μScm⁻¹)] and [TDS (mgL⁻¹)] showed that the superficial Runoff (Rs) is very close to precipitations with EC=18.54 ± 9.56. The groundwater provided by the wells (shallow aquifers) and the drillings (deep aquifers) are more mineralized than precipitations. Their EC values are respectively: 126.10 ± 95.60 in the wells and 134.40 ± 84.91 in the drillings. The seepages during the drying up is assigned to hypodermic Runoff (Rh) because of their EC = 47.29 ± 22.88 far from the groundwater's. At the outlet, the EC is nearest the Rs and the Rh ones during the rainy season and the drying up with EC = 42.97 ± 18.89. On the other hand, the mineralisation of the runoff at the outlet comes closer to the shallow aquifer during the baseflow, with a maximum value of 90 μScm^−1. This value is assigned to a depth goes from 2 to 6 meters. So this mineralization is considered like superficial groundwater drainage (Ns) from shallow aquifer because the lithostratigraphic cross drilling showed that the deep aquifers are in 40 meters in average. In these conditions deep aquifers don't contribute to the runoff at the outlet. Therefore, they are three origins of the runoff in Kolondièba watershed such as: superficial and hypodermic runoff during the wet season and the beginning of dry season plus groundwater drainage from shallow aquifers during the baseflow on the subsurface.