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

HAPEX-Sahel

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 studied. The main hydrological units are small endoreic areas, measuring a few hectares to a few square kilometres and the surface runoff is collected into pools. A detailed investigation of the role of these pools in the hydrology of the HAPEX-Sahel Central Super-Site was carried out from 1991 to 1993. The first results of this investigation are presented. A typology in three classes of the endoreic systems (valley bottoms ; sinks, plateaux) is proposed. The behaviour of one representative pool in each class is analysed, showing that the partition between evaporation and deep infiltration depends on the level of filling of the pools. The bottom of the pool is clogged by clay deposits, which prevent infiltration. Above a threshold varying between 1 and 2 m most of the water stored in the pool after runoff infiltrates, contributing to the recharge of the aquifers. On a seasonal basis, deep infiltration accounts for less than 50% of the water collected by the plateau pool, and more than 80% for the valley bottom pools. Almost all the water running off to the sink pools infiltrates rapidly and deeply into the ground. The valley pools (both valley bottoms and sinks) appear to be the major contributors to the recharge of the upper aquifer. The proportion of the HAPEX-Sahel Central Super-Site water balance that is taken by 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 aquifer in 1991 and 20% in 1992. This difference is explained by a very irregular time distribution of precipitation in 1992, most of the major rainfall events being observed over a short period during the intensive observation period... (D'après résumé d'auteur

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