Résumé
The water cycle in the Sahel region is known to be subject to great variability, from infra-rainfall time scale during the West African monsoon (e.g., surface runoff surveys) to century or even millennium time periods (i.e., for large sedimentary aquifers). Detailed hydrological and heat flow surveys, using a large set of instrumental methods or approaches have increased our understanding of eco-hydrological processes leading to the observed changes in green (soil moisture) and "blue" (surface and ground water) resources. In semiarid SW Niger, change from fallow to rainfed millet crop had a important impact on the water cycle, increasing runoff on sandy slopes and focused recharge through endoreic ponds and gullies, but also deep drainage and direct recharge below crop fields. As a consequence, evapotranspiration fluxes decreased, both during the dry and rainy seasons. Conversion from semi-natural dryland savannah to rainfed crop production reversed fluxes from upward (small discharge from deep rooted ligneous species) to downward (net recharge) in the Lake Chad basin aquifer. Moreover, the use of irrigation using river water increased drainage in the river plains and hence, increased aquifer recharge, decreasing evapotranspiration fluxes and river discharge. In endoreic, large sedimentary basins of the Sahel region, ground water represents the terrestrial outlet of the system and changes may be recorded in long-term water table fluctuations. A detailed analysis of trends in aquifer reserves in the Sahel indicates that it matches diachronic changes in land use, both in time and space. This further suggests, in turn, diachronic retroaction and feedback of the terrestrial water cycle to the atmosphere.