HAPEX-Sahel

Groundwater level measurements taken over a 4-year period from an extensive network of wells and boreholes within the HAPEX-Sahel (Hydrologic Atmospheric Pilot Experiment in the Sahel) degree square (south Niger), together with existing data, have provided an insight into infiltration and recharge processes taking place in the porous phreatic aquifer of the Continental Terminal formation. Despite high spatial and temporal variability of aquifer response to rainfall (rises of between 0 and 9 m are recorded), a pattern of recharge can be recognised. Aquifer responses vary from site to site, but the type of response at any single point tends to be consistent from year to year. Recharge is dominated by infiltration from temporary drainage networks (pools and streams) and aquifer response depends to a large extent on aquifer hydraulic 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 process of aquifer recovery following the severe drought of the 1970s and early 1980s, though part may also be attributable to changing patterns of land management (e.g. woodland clearance). Initial estimates of regional recharge are from 50-60 mm/y, or in other words about 10% of annual rainfall. The figure is supported by other methods of investigation (hydrochemical analyses ; water budgets of pools). (Résumé d'auteur

HAPEX-Sahel

The HAPEX-Sahel experiment took place in the midst of the most severe drought that has ever plagued the region since rainfall records have been available in the Sahel. The aim of this paper is to describe the rainfall conditions that were observed during HAPEX-Sahel by a network of 100 recording raingauges, and to analyse them in the perspective of the long-term statistics at the Niamey station. Globally it is found that the average rainfall over the HAPEX-Sahel study area, as well as over Niger as a whole, during the experiment has been moderately (1991 and 1992) to markedly dry (1990 and 1993), indicating that the drought of the past 20 years has not ended. More detailed statistics point to the high intermittency of the Sahelian rainfall, both in space (each year the ratio between the maximum and minimum recorded seasonal rainfall was of the order of 2) and in time (half the annual rain falls in 5 h). Particular attention is paid to the climatology of the rainy events. The point event rainfall is nearly exponentially distributed with a mean of about 14 mm, while the average areal event rainfall over the HAPEX-Sahel 1° x 1° square is of the order of 10.5 mm. The probability of zero rainfall is thus close to 1/4. Rain rates are often heavy, with half the annual rain falling at rain rates higher than 35 mm/h and one third of it falling at rain rates higher than 50 mm/h. The year to year statistics of both the event rainfall and the rain rates vary little, indicating that most of the inter-annual rainfall variability is due to the variation of the number of rainfall events, rather than to variations in rainfall intensities or in the mean event rainfall. (Résumé d'auteur