Rising ponds in uncultivated Sahel: A delayed effect of drought, involving plant dynamics and soil erosion

International audienceIt is now well accepted that runoff has increased in the Sahel during the recent multi-decadal drought period. There is still a debate on the causes of such an increase, and especially on the role of crop area expansion, which took place over the same period of time in part of the Sahel. In 2010, Gardelle et collaborators showed that ponds' surface in uncultivated Sahel exhibited a dramatic increase in the Gourma area (Mali) over the 1972-2006 period (Gardelle et al. 2010). A following study by Ramarohetra and collaborators (unpublished) established that a rapid evolution of both vegetation and soil types occurred over a small catchment already studied in 1954-56, the Tin Adjar catchment, also located in Sahelian Mali. This catchment underwent a strong increase in rocky outcrops and hardpans areas and a strong decreased in loamy soils and associated vegetation. During the same period, gullies significantly increased in length, number and order, whereas temporary pans (in 1954) were drained by gullies in 2008. Sand was removed from the highest slopes of the catchment and deposited in the lowest parts. The mechanism proposed to explain such changes involves a decrease in vegetation cover (mainly herbaceous) caused by the lack of precipitation in the driest years (70' and 80'), which favored the concentration of runoff in gullies at the expense of sheet runoff. In turn, the concentration of runoff further deprived plants from water. Such a mechanism takes place over shallow soils and is not related to cultivation. Such shallow soils occupy approximately 30 % of the Gourma and are found in other areas of the Sahel. Whether such a phenomenon also took place in other part of the Sahel has been further investigated with series of Landsat images. Ponds in the 70' and 2000' were classified in potentially affected areas of Mauretania, Mali and Niger. Rising ponds in part of uncultivated Sahel is shown to be an important, delayed, and rather unexpected effect of the Sahel drought. Gardelle, J., Hiernaux, P., Kergoat, L., and Grippa, M.: Less rain, more water in ponds: a remote sensing study of the dynamics of surface waters from 1950 to present in pastoral Sahel (Gourma region, Mali), Hydrol. Earth Syst. Sci., 14, 309-324, doi:10.5194/hess-14-309-2010, 201