The impacts of land-use/land-cover change and climate variability on the hydrology of the Sahel

The joint effect of climate change and of human activities on land cover is responsible for an increase of the runoff coefficients of the West African Sahelian rivers since the 1970s, as revealed by the analysis of runoff time series of rivers from Mauritania, Burkina-Faso and Niger. The runoff coefficients have increased in regions with less than 750 mm of annual rainfall, under Sahelian and sub-desertic climates, leading to increased flood peaks, occurring earlier in the season. Studies have shown a rapid change in landuse/land-cover (LUCC) since the 1970s over Sahelian river basins in Burkina-Faso and Niger. It is likely that the aridification of the environment-triggered by climatic change, especially rainfall shortage-is, to an important extent, enhanced by increasing agricultural activities. The relationships between hydrology and LUCC are studied using hydrological modelling. Rainfall/runoff modelling is improved when taking into account the spatio-temporal evolution of the LUCC through a time varying water holding capacity (WHC). The WHC, considered as the soil water reservoir in hydrological models, decreases as the natural vegetation is replaced by cultures or bare soil, leading to increased surface runoff. Long-term observations on "reference river basins" should be set from now on, to collect comparative information for the future.