The paradoxes of the Sahel

1From 1968 to 1993, the Sahel was hit by severe drought. And yet, river flows kept increasing from the 1980s, even though it did not rain more. This peculiarity did not happen by itself: in some instrumented parts of Niger, the groundwater levels even started rising. Hydrologists were able to explain this dual paradox in retrospect by studying the environmental data gathered since the 1960s on equipped basins.

Tree uprooted by floods.

••• The water cycle in the Sahel changed over a 30-year period, with prolonged droughts affecting agriculture as well as livestock farming •••

2It appears that increased run-off had resulted from the extreme drought of 1983-1984, which destroyed whatever remained of an already damaged vegetation. This absence of a vegetative cover, notably trees, increased water run-off and reduced its infiltration into the soil. So why did the groundwater level start to rise in certain locations? Because in certain Sahel regions, water does not flow into the sea. It builds up in large ponds, where it eventually seeps into the subsoil, feeding into the water table.

Annual flood, Niger.

3There were more surprises in store for researchers, as the revegetation of the Sahel which began in the 1990s was not accompanied by reduced run-off levels as expected. IRD solved this new riddle by showing that, in the regions in question, 30% to 50% of soils are degraded and incapable of absorbing water. Without a proactive remediation policy, they would remain “impermeable”. The consequence was the modification of the overall water balance of the Sahel, with further risks of flooding and the need to adapt hydraulic structures and sanitary infrastructures to this new reality; but also to investigate new irrigation opportunities.

“IRD, through research on long-term observation sites such as AMMA-CATCH, has shown that water, a scarce resource which limits socio-economic development, flows in abundance in the Sahel in spite of climate change. The dual hydrological paradox observed thus offers a development opportunity.” 
Professor Ibrahim Bouzou Moussa, Abdou Moumouni University, advisor to the Minister of Higher Education, Research and Innovation, Niamey, Niger