Drought-induced regime shift and resilience of a Sahelian ecohydrosystem

The Sahel (semi-arid fringe south of the Sahara) experienced a long and prolonged drought from the 1970s to the mid-1990s, with a few extremely severe episodes that strongly affected ecosystems and societies. Long-term observations showed that surface runoff has increased during this period, despite the rainfall deficit. This paradox stems from the soil degradation that was induced by various factors, either directly linked to the drought (impact on vegetation cover), or, in places, to human practices (land clearing and cropping). Surface runoff is still increasing throughout the region, suggesting that Sahelian eco-hydrosystems may have shifted to a new hydrological regime. In order to explore this issue, we have developed a simple system dynamics model incorporating vegetation-hydrology interactions and representing in a lumped way the first order processes occurring at the hillslope scale and the annual time-step. Long term observations on a pilot site in northern Mali were used to constrain the model and define an ensemble of plausible simulations. The model successfully reproduced the vegetation collapse and the runoff increase observed over the last 60
years. Our results confirmed that the system presents two alternative states and that during the drought it shifted from a high-vegetation/low-runoff regime to the alternative low-vegetation/high-runoff one where it has remained trapped until now. We showed that the mean annual rainfall deficit was sufficient to explain the shift. According to the model, vegetation recovery and runoff reduction are possible in this system, but the conditions in which they could occur remain uncertain as the model was only constrained by observations over the collapse trajectory. The study shows that the system is also sensitive to the time variability of rainfall, and that larger variability leads to higher runoff. Both mean rainfall and rainfall variability may increase in central Sahel under climate change, leading to antagonist effects on the system, which makes its resilience uncertain.