HYDROLOGY OF THE HAPEX-SAHEL CENTRAL SUPER-SITE - SURFACE-WATER DRAINAGE AND AQUIFER RECHARGE THROUGH THE POOL SYSTEMS

The hydrology of the Sahel is characterised by the degradation of the drainage network, resulting in the lack of large watersheds over which the spatial integration of the hydrological processes could be studie d. The main hydrological units are small endoreic areas, measuring a f ew hectares to a few square kilometres and the surface runoff is colle cted into pools. A detailed investigation of the role of these pools i n the hydrology of the HAPEX-Sahel Central Super-Site was carried out from 1991 to 1993. The first results of this investigation are present ed. A typology in three classes of the endoreic systems (valley bottom s; sinks; plateaux) is proposed. The behaviour of one representative p ool in each class is analysed, showing that the partition between evap oration and deep infiltration depends on the level of filling of the p ools, The bottom of the pool is clogged by clay deposits, which preven t infiltration. Above a threshold varying between 1 and 2 m most of th e water stored in the poor after runoff infiltrates, contributing to t he recharge of the aquifers. On a seasonal basis, deep infiltration ac counts for less than 50% of the water collected by the plateau pool, a nd more than 80% for the valley bottom pools. Almost all the water run ning off to the sink pools infiltrates rapidly and deeply into the gro und. The valley pools (both valley bottoms and sinks) appear to be the major contributors to the recharge of the upper aquifer, The proporti on of the HAPEX-Sahel Central Super-Site water balance that is taken b y the deep infiltration from the pools varies greatly depending on the temporal distribution of rainfall. Whereas similar seasonal rainfalls were recorded in 1991 and 1992, it is estimated that 5% of the water precipitated over the valley pool watershed infiltrated towards the aq uifer in 1991 and 20% in 1992, This difference is explained by a very irregular time distribution of precipitation in 1992, most of the majo r rainfall events being observed over a short period during the intens ive observation period. In conclusion some preliminary figures are giv en regarding the importance of recharge from the pools as compared wit h in situ recharge.