Résumé
Rainfall decreased for 35 years in West Africa, leading to a reduction of runoff of sometimes >60%. This amplified reduction of runoffs is explained by a reduction of the baseflow, consequence of the cumulative deficit of annual rainfall. All the Sub-Saharan Africa is concerned, except north of the 700-800 mm annual precipitation line, where runoff increased since 1970 for the Sahelian rivers, as in Burkina-Faso and Niger (+35 to +108%). We assume that it is a continental scale phenomenon, primarily influenced by climate change, then accelerated by the anthropogenically driven land cover degradation. The consequences are extreme: very low flows and reduction of the groundwater table level in the humid regions; and paradoxal: increased runoff coefficients in Sahel, causing higher peak flows and damaging under-dimensionned equipment. The hydrological normal in Sahel needs to be urgently revised.