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A large share of surface water resources in Sahelian countries originates from Guinea’s Fouta Djallon highlands, earning the area the name of “the water tower of West Africa”. This paper aims to investigate the recent dynamics of the Fouta Djallon’s hydrological functioning. The evolution of the runoff and depletion coefficients are analyzed as well as their correlations with the rainfall and vegetation cover. The latter is described at three different space scales and with different methods. Twenty-five years after the end of the 1968–1993 major drought, annual discharges continue to slowly increase, nearly reaching a long-term average, as natural reservoirs which emptied to sustain streamflows during the drought have been replenishing since the 1990s, explaining the slow increase in discharges. However, another important trend has been detected since the beginning of the drought, i.e., the increase in the depletion coefficient of most of the Fouta Djallon upper basins, as a consequence of the reduction in the soil water-holding capacity. After confirming the pertinence and significance of this increase and subsequent decrease in the depletion coefficient, this paper identifies the factors possibly linked with the basins’ storage capacity trends. The densely populated areas of the summit plateau are also shown to be the ones where vegetation cover is not threatened and where ecological intensification of rural activities is ancient.

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International audienceThe evolution of the biophysical characteristics of the African Western Sahelian environment and the Senegalese Ferlo in particular is marked by strong fluctuations in the rainfall intervened during these last decades and which had important consequences on the biological potential of the region. The modifications that they have induced on the ecosystems are read through the floristic composition, the chorology of species found in the region and the contours of the landscapes; flora is a good indicator of the biophysical environment evolution during the last decades. So the phytoclimatic organization at the micro-regional scale deserves to be reviewed and updated in light of major changes. The floristic study of plant resource, seen through the prism of dual anthropogenic practices and changing environmental conditions, allows to understand better the complexity of the internal physical and societal dynamics.L'évolution des caractéristiques biophysiques de l'environnement sahélien Ouest africain et du Ferlo sénégalais en particulier est marquée par les fortes fluctuations de la pluviosité intervenues au cours de ces dernières décennies et qui ont eu des conséquences importantes sur le potentiel biologique de toute la région. Les modifications qu'elles ont induites sur les écosystèmes se lisent aujourd'hui à travers la composition floristique, la chorologie des espèces rencontrées dans la région et les contours des paysages ; la flore est une bonne indicatrice de l'évolution des milieux biophysiques au cours des dernières décennies. L'organisation phytoclimatique à l'échelle micro-régionale mérite donc d'être réexaminée et réactualisée en tenant compte des principaux changements intervenus. L'étude floristique de la ressource végétale, vue au travers du double prisme des pratiques anthropiques et de l'évolution des conditions du milieu, permet de mieux comprendre la complexité des dynamiques physiques et sociétales internes