Elsevier

Journal of Hydrology

Volume 375, Issues 1–2, 30 August 2009, Pages 128-142
Journal of Hydrology

Rainfall regime across the Sahel band in the Gourma region, Mali

https://doi.org/10.1016/j.jhydrol.2009.03.007Get rights and content

Summary

The Sahel 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 for the Gourma region located in Mali, Central Sahel (14.5–17.5°N and 2–1°S). The rainfall regime is described using two datasets: the daily long term raingauge records covering the period 1950–2007, and the high frequency raingauge records collected under the African Monsoon Multidisciplinary Analysis (AMMA) project between 2005 and 2008. The first rainfall dataset was used to analyse the interannual variability and the spatial distribution of the precipitation. The second dataset is used to analyse the diurnal cycle of precipitation and the nature of the rainfall. This study is complementary to previous analyses conducted in Sahelian areas located further south, where the influence of the continental Sahara heat low is expected to be less pronounced in summer.

Rainfall regimes in the Gourma region present a succession of wet (1950–1969) and dry decades (1970–2007). The decrease of summer cumulative rainfall is explained by a reduction in the number of the rainy days in southern Gourma, and a decrease in both the number of rainy days and the daily rainfall in northern and central Gourma. This meridional difference may be related to the relative distances of the zones from the intertropical discontinuity, which is closer to the 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 in the daily rainfall, which suggests an intensification of rainfall events in the more recent years.

High-frequency data reveal that a large fraction of the rainfall is produced by intense rain events mostly occurring in late evenings and early mornings during the core of the rainy season (July–September). Conversely, rainfall amounts are less around noon, and this mid-day damping is more pronounced in northern Gourma. All these characteristics have strong implications for agriculture and water resources management.

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/

References (45)

  • M.A. Bell et al.

    Integration of weather system variability to multidecadal regional climate change: the West African Sudan–Sahel zone, 1951–1998

    J. Climate

    (2006)
  • A. Dai et al.

    The recent Sahel drought is real

    Int. J. Climatol.

    (2004)
  • N. D’Amato et al.

    On the characteristics of the rainfall events in the Sahel with a view to the analysis of climatic variability

    Int. J. Climatol.

    (1998)
  • P. Hiernaux et al.

    Les parcours du Sahel

    Sècheresse

    (2006)
  • R.A. Houze

    Stratiform precipitation in regions of convection: A meteorological paradox?

    Bull. Am. Meteor. Soc.

    (1997)
  • M. Hulme

    Rainfall changes in Africa: 1931–1960 to 1961–1990

    Int. J. Climatol.

    (1992)
  • S. Janicot

    Spatio-temporal variability of West African rainfall. Part I: regionalizations and typings

    J. Climate

    (1992)
  • P.J. Lamb

    Persistence of subsaharan drought

    Nature

    (1982)
  • K.-M. Lau et al.

    Detecting trends in tropical rainfall characteristics, 1979–2003

    Int. J. Climatol.

    (2007)
  • L. Le Barbé et al.

    Rainfall climatology during the years 1950–1990

    J. Hydrol.

    (1997)
  • L. Le Barbé et al.

    Rainfall variability in West Africa during the years 1950–1990

    J. Climate

    (2002)
  • T. Lebel et al.

    Seasonal cycle and interannual variability of the Sahelian rainfall at hydrological scales

    J. Geophys. Res.

    (2003)
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