328 - Africa’s monsoon challenge
The monsoon sets the pattern of life of 300 million West Africans. The intensity and duration of the rains it brings govern all crop cultivation and water resources – and hence food security. In a period of less than four months, from June to September, it brings the greater part of the rainfall for the whole year. However, it has lost its intensity over the past decades, plunging the Sahel into a succession of famines.
As part of the international programme AMMA1, IRD research scientists and their partners have been making long-term observations on this erratic monsoon system in order to predict the consequences. The observation system AMMA-CATCH (Couplage de l’Atmosphère Tropicale et du Cycle Hydrologique ), launched in 2002 to study the coupling between the tropical atmosphere and the hydrological cycle, has brought to the fore certain characteristics and paradoxes of the water cycle associated with the African monsoon: a significant change in the season cycle and a decrease in the number of substantial rainfall events, but an increase in surface runoff and flooding in a greener Sahel.
The term ‘monsoon’ comes from the Arabic mawsim , meaning ‘season’. From June to September, this period of strong rainfall brings in over 80 % of West Africa’s annual precipitation in. However, over the past 40 years, it has been less intense. The communities of the Sahel are suffering the effects of this prolonged drought, including series of disastrous famines. The AMMA(1) programme aims to improve overall understanding of this trend. More especially, IRD researchers are studying the variability of the water cycle, using the long-term observation system AMMA-CATCH which has led to the identification of major modifications of some of its components.
Three sites, three eco-climatic zones
AMMA-CATCH is the hub of AMMA’s terrestrial data capturing system. Observations made are concentrated on three densely instrumented mesoscale sites, each covering 15 000 to 25 000 km² and representative of different ecosystems and climates of this vast region of the African continent: the Gourma in Mali, the Niamey square degree site in Niger and the Oueme basin of Benin.
Work on these sites consists of long-term studies to trace surface processes involving the vegetation and the water cycle and also assess their interaction with the climate. The research teams are producing local and regional-scale models of the hydrology and ground-atmosphere exchanges in order to assess their role in the overall system and predict the rainfall and vegetation dynamics. In the present situation of climate change and current land use patterns, the body of information obtained is vital for developing water management policies and for aiding decision-makers in planning the region’s future economic development.
Water cycle disruptions
In the 1980s and 1990s, the whole Sahel zone and practically all of West Africa saw their rainfall diminish by 20 to 40 %. This rainfall deficit stems primarily from a reduction in the number of rainy episodes, not from a shortening of the monsoon season. However, since the beginning of the XXIst Century, this general reduction has been giving way to a more patchy distribution. The west of the continent is still experiencing severe drought whereas the centre is registering a moderate rainfall deficit, albeit still a significant one (between 10 and 25 %), and the south has seen the return of conditions similar to those which prevailed before the great drought. The season cycle has also undergone profound changes, with a strong reduction in the August peak of rainfall.
The paradoxes of the Sahel
In spite of the reduction in precipitation, surface runoff and discharge have increased significantly over the past 40 years throughout the Sahel and a greener vegetation cover is appearing. In the basins of the great Rivers Volta, Niger and Senegal, for example, discharges have doubled in 40 years, owing to an increase in runoff since 1968, at the onset of the drought. The number, volume and life-span of pools and lakes have also increased: the AMMA-CATCH (Couplage de l’Atmosphère Tropicale et du Cycle Hydrologique ) observation system has brought out evidence for a doubling of their surface between 1975 and 2002. These water bodies represent a sizeable water resource in the pastoral Sahel and one of the principal types of area for water-table recharge, which generates a continuous rise in groundwater levels. At the scale of the small drainage basins, these phenomena result from the loss of plant cover brought on by drought conditions and farmers’ tree and bush clearance in the savannah a practice which accelerated during the last century(2).
A further paradox is that, in spite of locally negative impacts of human activity on vegetation, the larger-scale vegetation dynamics shows an overall trend towards a re-greening across West Africa, resulting in a larger plant biomass. This change-around seems primarily due to changes in the water cycle.
For 40 years now, the ecosystems of West Africa have entered a phase of rapid complex transitions, of a size and intensity unequalled in human history. The IRD and its African partners have undertaken to maintain the long-term hydrological and environmental observations through the use of specific instrumentation arrays. Documentation of these changes and their impacts is necessary for devising and applying management practices adapted for pastoral and arable agricultural areas, in one of the regions of the world that ongoing climatic and environmental changes make most vulnerable.
- AMMA (African Monsoon Multidisciplinary Analysis) is an international research programme which studies atmosphere-ocean-continent interactions involved in the West African monsoon system. The programme was launched in 2001 by French research scientists, and now brings together 140 African, American and European laboratories. In August 2009 it held its third international conference in Ouagadougou, and 500 scientists attended.
- See Scientific bulletin 303: Aerial photographs further Sahel land change observation
What is the monsoon?
The monsoon is generated by the difference in de temperature between the air above the African continent, superheated in summer, and the air over the Atlantic Ocean, which is cooler and laden with moisture. The moist air masses are brought from the Gulf of Guinea towards the continent, in a South East-South West flux. These bodies of air then collide with hot dry air masses blown by the wind from the North East, the Harmattan. Storm systems then form in this intertropical convergence zone which then send their rain to fall over the whole of the Sahel.
Rédaction DIC – Gaëlle Courcoux
Lebel T., Cappelaere B., Galle S., Hanan N., Kergoat L., Levis S., Vieux B., Descroix L., Gosset M., Mougin E. et al. AMMA-CATCH studies in the Sahelian region of West-Africa: an overview. Journal of Hydrology , p.1-34, 2009
Lebel T., Cappelaere B., Vieux B., Galle S., Hanan N., Kergoat L., Levis S. Surface processes and water cycle in West Africa, studied from the AMMA-CATCH observing system . Special issue of Journal of Hydrology , 375 (1-2)