Rainfall regime over the Sahelian climate gradient in the Gourma, Mali

Frédéric Frappart⁽¹⁾, Pierre Hiernaux⁽¹⁾, Françoise Guichard⁽²⁾, Eric Mougin⁽¹⁾, Laurent Kergoat⁽¹⁾, Marc Arjounin⁽³⁾, François Lavenu⁽¹⁾, Mohamed Koité⁽⁴⁾, Jean-Emmanuel Pature⁽⁵⁾, Thierry Lebel⁽³⁾

This study provides a characterization of the rainfall regime in a poorly described region of Central Sahel, Gourma (14.5° to 17.5° N and 2° to 1° S) in Mali, for the long term precipitation daily time series during the period 1950-2007 and for a high time resolution dataset between 2005 and 2007. First, we analyse the raingauges data collected since the middle of the 20th century both in terms of interannual variability and spatial distribution. Second, we investigate the diurnal cycle of the precipitation distribution and the nature of the rainfall using automatic raingauges during the period 2005-2007.
Gourma rainfall presents a sucession of wet (1950-1969) and dry decades (1970-2007). The annual decrease of the rainfall is explained by a reduction of the number of the rainy days in the South part of Gourma and by both a decrease of the number of rainy days and of the precipitation amount per rainy day in the North and the Centre. The results presented in this study, complete those previously reported on other Sahelian regions located further South. The length of the rainy season has varied since the 1950s with two episodes of short rainy season during the drought of the 1980s and since 2000 associated to an increase in rainfall per rainy days suggesting an intensification of the rain events.
Higher time resolution data available between 2005 and 2007, allowed to document that most of the rainfall was produced by intense rain events occurring mostly in the late evening and the early morning for the core of the rainy season (July/August/September).