Résumé
Hydrological processes are rapidly changing in semi-arid areas under the double constraint of climate change and anthropization. Such changes were investigated at different scales of time and space in the Trarza aquifer (SW Mauritania), a flat region of about 40 000 km2 with a low rainfall (100 to 250 mm a−1). All old sources of hydrogeological information were exploited in complement with the more recent surveys. The geochemical information shows the importance of Quaternary eustatic variations in the present groundwater mineralization. In spite of very great variations of rainfall, the piezometric stability over the last 50 years shows that infiltration (about 4 mm a−1) did not vary significantly. Since 1989, the big Diama dam has raised the mean level of the Senegal River, which brings an additional inflow to the alluvium and to the Trarza aquifer (about 34 hm3 a−1). Groundwater recharge from the river and rainfall infiltration over the whole area have the same order of magnitude. In the Nouakchott area, the new supply of water to the capital (45 hm3 a−1) has led to a significant groundwater rise and the submersion of several urban districts.