The latest figures on soil and climate help explain the enigmatic ups and downs observed in Sahelian hydrology for decades. Knowledge of the mechanisms involved paves the way for practical solutions to adapt agriculture to new environmental conditions.
Floods during droughts and run-off despite the development of vegetation: experts were puzzled by the dual paradox of Sahelian hydrology for a long time. This paradox has finally been clarified and, even better, concrete solutions are coming to light to enhance soil absorption while optimising water management. “Combined with the extensive knowledge gained on the subject, the latest figures1 on soil and rainfall patterns help explain how and why this West African region has experienced unexpected hydrological phenomena for decades2”, points out hydrologist Luc Descroix.
Drought and river flow
Sub-Saharan Africa was plagued with a severe drought from the late 1960s to the early 1990s. There was a sharp 30% if not 40% drop in rainfall, over millions of km² for 25 years. Against all odds, the flow of rivers and the volume of ponds did not follow the same dynamic, far from it. They substantially increased despite the water shortage, leaving the experts puzzled by this strange paradox. “Field observations show that the unexpected flow increase at a time when rainfall was down results from increased volumes of water run-off”, explains the researcher. “The partial loss of vegetation associated with the drought and human land use intensification, driven by demographic pressure, affected the absorptive capacity of soil.” As a result, much of the water from the scarce rainfall started flowing down everywhere into water courses without seeping into the soil and water tables like it used to.
Similarly, the dramatic increase in river flow from the 1990s is partly due to water run-off. At the time, rainfall started to rise again and the region, which was once parched by drought, grew green again. While this should have facilitated infiltration into the soil, the river flow continued to rise. In some rivers, it even reached levels five times higher than those of the wetter period of the 1950s, taking scientists aback once again …
“With the increase in rainfall, the vegetation is flourishing again, but the situation is not uniform”, points out the researcher. “In dry pasture areas, 20% to 30% of the soil is completely damaged: it is no longer home to any vegetation and is conducive to run-off. Half of all arable areas have yet to regain their infiltration capacity for the same reasons.” Is this the only explanation? No: hydroclimatic conditions have also changed. “While annual rainfall has significantly picked up since the 1990s, it is now characterised by scarce and very heavy rains”, indicates hydro-climatologist Gérémy Panthou. “Between these intense rainfall events, dry spells can sometimes last longer than during the great drought of the 1970s.” This rainfall intensification contributes to run-off, floods and soil erosion.
The rural populations of the Sahel have now begun to adopt simple and effective agro-ecological practices to respond to new environmental conditions. By developing bench terracing, spreading the local zai technique3 or planting leguminous trees4 in crop fields, they manage to improve infiltration as well as yields. “Fortunately, these virtuous processes develop most successfully in densely populated regions, where environmental pressure is highest”, believes Luc Descroix.
1. Resulting from in-situ observations by the teams of UMR IGE 252 in Grenoble on rainfall intensification, UMR GET 234 in Toulouse on the performance of non-crop areas, and data from AMMA programmes (African Monsoon Multidisciplinary Analysis)
2. Luc Descroix, Françoise Guichard, Manuela Grippa, Laurent A. Lambert, Gérémy Panthou, Gil Mahé, Laetitia Gal, Cécile Dardel, Guillaume Quantin, Laurent Kergoat, Yasmin Bouaita, Pierre Hiernaux, Théo Vischel, Thierry Pellarin, Bakary Faty, Catherine Wilcox, Moussa Malam Abdou, Ibrahim Mamadou, Jean-Pierre Vandervaere, Aïda Diongue-Niang, Ousmane Ndiaye, Youssouph Sané, Honoré Dacosta, Marielle Gosset, Claire Cassé, Benjamin Sultan, Aliou Barry, Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review, Water 2018, 10(6), 748.
3. Cultivation technique aiming at concentrating the water and manure around the plant, in a mini-pit approximately thirty centimetres in diameter.
4. They enrich the soil and associated plants by capturing atmospheric nitrogen.