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This paper is devoted to the first results from the GHYRAF (Gravity and Hydrology in Africa) experiment conducted since 2008 in West Africa and is aimed at investigating the changes in water storage in different regions sampling a strong rainfall gradient from the Sahara to the monsoon zone. The analysis of GPS vertical displacement in Niamey (Niger) and Djougou (Benin) shows that there is a clear annual signature of the hydrological load in agreement with global hydrology models like GLDAS. The comparison of GRACE solutions in West Africa, and more specifically in the Niger and Lake Chad basins, reveals a good agreement for the large scale annual water storage changes between global hydrology models and space gravity observations. Ground gravity observations done with an FG5 absolute gravimeter also show signals which can be well related to measured changes in soil and ground water. We present the first results for two sites in the Sahelian band (Wankama and Diffa in Niger) and one (Djougou in Benin) in the Sudanian monsoon region related to the recharge–discharge processes due to the monsoonal event in summer 2008 and the following dry season. It is confirmed that ground gravimetry is a useful tool to constrain local water storage changes when associated to hydrological and subsurface geophysical in situ measurements.

Advances in methods of observation are essential to ensure a better understanding of changes in water resources considering climate variability and human activities. The GHYRAF (Gravity and Hydrology in Africa) experiments aim to combine gravimetric measurements with dense hydrological surveys to better characterize the annual water storage variability in tropical West Africa. The first absolute gravimetric measurements were performed in Southwest Niger,near a temporary pond where rapid infiltration to an unconfined aquifer occurs. As gravityis sensitive both to local and global variations of water mass distribution, the large-scale hydrological contribution to time-variable gravity has been removed using either GRACE satellite data or global hydrology models. The effect of the local water storage changes was modelled using in situ measurements of the water table, soil moisture and pond water level. The adjustment of these simulations to residual ground gravity observations helped to constrain the specific yield to a value ranging between 1.8 and 6.2 per cent. This range of value is consistent, albeit on the low side, with the aquifer water content (6–12 per cent) estimated by magnetic resonance soundings, which are known to slightly overestimate the specific yield in this geological context. The comparison of these two independent geophysical methods shows their potential to constrain the local hydro geological parameters. Besides, this study evidences the worth of correcting the gravity signal for large-scale hydrology before recovering local water storage parameters.