Rainfall regime across the Sahel band in the Gourma region, Mali
Introduction
The arid and semi-arid regions of Africa are characterized by low and unreliable rainfall, which strongly affects water resources and food security (Nicholson, 1989). The largest of these regions, the Sahel, runs 3800 km from the Atlantic Ocean in the west to the Red Sea in the east, in a belt that varies from several 100–1000 km in width, covering an area of 3,053,200 km2. This semi-arid area is bordered to the north by the Sahara Desert and to the south by Sudanian savannas. The Sahelian climate is characterized by a unimodal rainfall regime controlled by the west African Monsoon – WAM (Nicholson, 1981, Todorov, 1985, Morel, 1992, Hiernaux and Le Houérou, 2006). During the 20th century, the Sahel experienced a multidecennial drought that started at the end of 1960s, with two sequences of extremely dry years, in 1972–1974 and 1983–1985 (Hulme, 1992, Le Barbé and Lebel, 1997, D’Amato and Lebel, 1998, L’Hôte et al., 2002, Lebel et al., 2003). This is, indeed, the strongest measured climatic event of rainfall variability at these time and space scales (Hulme, 2001). The substantial changes in the climate conditions obliged Sahelian farmers and pastoralist communities to adapt to the decrease in water resources (Mortimore and Adams, 2001, Tarhule and Lamb, 2003, Pedersen and Benjaminsen, 2008).
The few recent studies of Sahelian rainfall regimes using raingauge data were carried out over large areas located further south (Le Barbé et al., 2002, Lebel et al., 2003, Bell and Lamb, 2006), whereas the rainfall regimes of the drier central and northern Sahel, which experience a strong climatic influence of the Sahara heat low in summer, remain poorly described.
This study is the first to focus on the rainfall regime of the northern AMMA–CATCH (African Monsoon Multidisciplinary Analysis–Couplage de l’Atmosphère Tropical et du Cycle Hydrologique) mesoscale site (14.5–17.5°N, 1–2°W), located in the Gourma region, in Mali. Ground-based measurements covering a range of complementary scales are used.
This study focuses on an analysis of rainfall variability over the Gourma region during the years 1950–2007, using time series of daily precipitation data. The rainy season is characterized in terms of length, distribution of precipitation, and number and intensity of rainy days. Historical trends affecting West Africa and the Sahel over the past century are presented and discussed for the Gourma region specifically.
Smaller scale modes of rainfall variability are investigated using high frequency measurements collected by the AMMA–CATCH raingauge network during the AMMA Enhanced Observations Period (EOP), 2005–2008. Over the Sahel, and in the Gourma region, rainfall is of convective origin and organized Mesoscale Convective Systems (MCS), which accounts for most of the rainfall as shown by Mathon et al. (2002). Therefore, the distribution of rainfall rates associated with rainfall events over the Gourma region is analysed and compared with existing studies. The diurnal cycle, a major feature of convection and of rainfall over land (Wallace, 1975, Yang and Slingo, 2001), is also analysed and discussed for the Gourma region.
Section snippets
The study region
The Gourma region belongs entirely to the Sahel zone and extends over 90,000 km2, from the south of the large loop made by the Niger River between Timbuktu and Gao – 17°N – down to the border with Burkina-Faso – ∼14.5°N (Fig. 1). This region, mainly pastoral, is bracketed by the 150 and 500 mm annual isohyets, with interannual coefficients of variation of the mean annual rainfall ranging from 15% to 30% (Sivakumar, 1989). A comprehensive presentation of the Gourma AMMA mesoscale site can be found
Major features of rainfall in the Gourma region
The mean annual rainfall over the Gourma mesoscale site is characterized by a south–north decreasing gradient, at a rate slightly larger than 1 mm km−1 (Fig. 2), a characteristic value found elsewhere in Sahel (Lebel et al., 2003). It involves bioclimatic zones distributed latitudinally with a slight southward inflexion from west to east (Hiernaux and Le Houérou, 2006). However, isohyets positions have varied considerably over the past 60 years in the Gourma region (Fig. 3) and more generally in
Conclusion
This study provides the first characterization of the rainfall regime since the 1950s in Gourma region, Mali. Common features and differences from studies undertaken in other Sahelian regions have also been underlined. The results of this study concerning the nature of rainfall, rainfall patterns and their changes during the last 60 years, are essential for policy makers and government agencies in order to manage water resources, and to elaborate agricultural policies.
The rainfall regime in the
Acknowledgements
This work was performed within the framework of the AMMA project. Based on a French initiative, AMMA has been constructed by an international group and is currently funded by large number of agencies, especially from France, the UK, the US and Africa. It has been the beneficiary of a major financial contribution from the European Community’s Sixth Framework Research Programme. Detailed information on the scientific coordination and funding is available on the AMMA international web site (//www.amma-eu.org/
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