Rainfall Regime over the Sahelian Climate Gradient in the Gourma Region, Mali

Abstract

"The Sahelian zone is characterized by low and highly variable rainfall, which strongly affects the hydrology and the climate of the region and creates severe constraints for agriculture and water management. This study provides the first characterization of the rainfall regime in a poorly described region of Central Sahel, Gourma region (14.5° to 17.5° N and 2° to 1° S) in Mali, as obtained from long-term daily precipitation time series covering the period 1950-2007 and from high-frequency dataset between 2005 and 2008. First, raingauge data collected since the middle of the 20th century were analysed both in terms of interannual variability and spatial distribution. Second, we investigate the diurnal cycle of precipitation and the nature of the rainfall using automatic raingauges during the period 2005-2008. This study also completes previous analyses conducted in Sahelian areas located further South, where the direct continental influence of the Saharan heat low is expected to be less pronounced in summer. The Gourma region rainfall presents a succession of wet (1950-1969) and dry decades (1970-2007). The decrease of the summer cumulative rainfall is explained by a reduction of the number of the rainy days in the southern Gourma region and by both a decrease of the number of rainy days and of the mean precipitation amount per rainy day in the North and the Centre. This difference may be related to their respective distances from the intertropical discontinuity, which is closer to northern stations. The length of the rainy season has varied since the 1950s with two episodes of shorter rainy seasons: during the drought of the 1980s and also, since 2000. However, this second episode is characterized by an increase of the mean rainfall per rainy day, which suggests an intensification of the rain events in the more recent years. High-frequency data reveal that most of the rainfall is produced by intense rain events occurring mostly in the late evening and the early morning during the rainy season (July/August/September). Conversely, rainfall amounts are the weakest at noon, and this mid-day damping is more pronounced in the North. All these characteristics have strong implications for agriculture and water resources management."