Résumé
The hydrodynamic and isotopic characteristics of the two largest phreatic aquifers in Niger have been studied. Even though the climate, geomorphology and vegetation seem similar, these aquifers differ. In the Iullemmeden basin, a seasonal fluctuation of the water table during the rainy season is combined with a significant long-term rise due to a change in land use; isotopes ( (super 18) O, (super 2) H, (super 3) H, (super 14) C) show a median renewal rate of between 0.1% and 0.2%. In the Lake Chad basin, seasonal fluctuation is much rarer and the long-term trend is of stability or even a small decline of the water table due to the drying up of the Lake Chad; isotopes indicate a median renewal rate of between 0.1% and 0.06%. In both cases, the large difference between the extreme values underlines the great variation in time and space of the recharge.