RAINFALL MONITORING DURING HAPEX-SAHEL .1. GENERAL RAINFALL CONDITIONS AND CLIMATOLOGY

The HAPEX-Sahel experiment took place in the midst of the most severe drought that has ever plagued the region since rainfall records have b een available in the Sahel, The aim of this paper is to describe the r ainfall 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 drough t of the past 20 years has not ended, More detailed statistics point t o the high intermittency of the Sahelian rainfall, both in space (each year the ratio between the maximum and minimum recorded seasonal rain fall 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 ev ents. The point event rainfall is nearly exponentially distributed wit h a mean of about 14 mm, while the average areal event rainfall over t he HAPEX-Sahel 1 degrees x 1 degrees square is of the order of 10.5 mm . The probability of zero rainfall is thus close to 1/4. Rain rates an often heavy, with half the annual rain falling at rain rates higher t han 35 mm h(-1) and one third of it falling at rain rates higher than 50 mm h(-1). The year to year statistics of both the event rainfall an d the rain rates vary little, indicating that most of the inter-annual rainfall variability is due to the variation of the number of rainfal l events, rather than to variations in rainfall intensities or in the mean event rainfall.