WATER-TABLE FLUCTUATION AND RECHARGE IN SEMIARID CLIMATE - SOME RESULTS OF THE HAPEX-SAHEL HYDRODYNAMIC SURVEY (NIGER)

Groundwater level measurements taken over a 4-year period from an exte nsive network of wells and boreholes within the HAPEX-Sahel (Hydrologi c Atmospheric Pilot Experiment in the Sahel) degree square (south Nige r), together with existing data, have provided an insight into infiltr ation and recharge processes taking place in the porous phreatic aquif er of the Continental Terminal formation. Despite high spatial and tem poral variability of aquifer response to rainfall (rises of between 0 and 9 m are recorded), a pattern of recharge can be recognised. Aquife r responses vary from site to site, but the type of response at any si ngle point tends to be consistent from year to year. Recharge is domin ated by infiltration from temporary drainage networks (pools and strea ms) and aquifer response depends to a large extent on aquifer hydrauli c characteristics and distance from the nearest infiltrating zone. In many wells, for which data extending back to 1987 is available, water levels show a consistent year by year rise. This is interpreted as a p rocess of aquifer recovery following the severe drought of the 1970s a nd early 1980s, though part may also be attributable to changing patte rns of land management (e.g. woodland clearance). Initial estimates of regional recharge are from 50-60 mm year(-1), or in other words about 10% of annual rainfall. The figure is supported by other methods of i nvestigation (hydrochemical analyses; water budgets of pools).