The Merguellil catchment (central Tunisia), which is typical of the Mediterranean situation, has undergone rapid hydrological changes over the last decades. The most visible signs are a marked decrease in surface runoff in the upstream catchment and a complete change in the recharge processes of the Kairouan aquifer downstream. Fluctuations in rainfall have had a limited hydrological impact. Much more important have been the consequences of human activities, such as soil and water conservation works, small and large dams, and pumping for irrigation. Several independent investigations of the catchment were implemented, based on hydrodynamics, thermal surveys, and geochemistry including isotopes. These helped to identify the different terms of the regional water balance and to characterize their changes over time. However, major uncertainties remain and our results may contradict previous interpretations or calculations. Conservation works, now covering more than a quarter of the upstream catchment, drastically reduce the runoff production from rain events of less than 40 mm. Wadi Merguellil now ends in the big El Haouareb Reservoir, which loses more than half of its water by infiltration through karstic fissures and 30% by evaporation, the rest being pumped or released. El Haouareb Dam was built in 1989 and the reservoir has often dried up in the last decade. The major modifications in groundwater flow resulting from construction of the dam are observed in the geochemical tracers in the first seven kilometres downstream from it. Temperature measurements confirmed the recent invasion of new water. The rest of the Kairouan plain aquifer retains the signature of older recharge, but the whole aquifer is affected by the decrease in the water table (about 1 m per year), the consequence of the ever increasing pumping for irrigation.