Résumé
The humid sudanian zone of West-Africa undergoes a monsoon climate, implying a strong seasonality in water storage changes (WSC). The GHYRAF (Gravity and Hydrology in Africa) project aims at monitoring both these local and non-local hydrological contributions with the main gravity sensors available today (FG5 absolute gravimeter, superconducting gravimeter--G--and CG5 micro-gravimeter). The study area is located in hard-rock basement context in Djougou, northern Benin, and is also part of the long-term observing system AMMA-Catch, and thus under intense hydro-meteorological monitoring (rainfall, soil moisture, water table, evapotranspiration, ...). Gravity-derived WSC are compared to hydrological data and to physically-based or conceptual hydrological models calibrated on these data. This presentation shows the results and limitations of each gravimeter in the context of WSC retrieval. This site was first measured with a FG5 absolute gravimeter four times a year from 2008 to 2013. This can be considered as a high sampling rate, given the remote location and the complexity of FG5 carriage and installation. It allowed to derive an average specific yield for the local aquifer, and preliminary estimates of seasonal WSC (up to 120 nm/s2-270 mm). Yet the lack of continuity in the data avoids further investigations. The SG-060 superconducting gravimeter has been installed in 2010 in order to monitor gravity response to WSC in a continuous way. A strong drift is present (230 nm/s2/yr), and FG5 data together with a-priori information on WSC are needed for estimating its effect. Also, frequent power-failures lead to some significant gaps and offsets during which fast WSC may occur (e.g. rain), yielding to a challenging correction for these events. The retrieval of inter-annual WSC suffers from these strong and limiting instrumental effects. At higher frequencies, up to a few days, continuous gravity monitoring may help to quantify evapotranspiration (ET), a poorly-known variable of the hydrological cycle. In Djougou, favorable--flat--topographic conditions and significant ET (up to 5 mm/day) are present. However, the shelter size together with the low altitude of the SG sensor with respect to the ground yield to diminish the expected effect of ET. Also, atmospheric contribution at such frequencies in the equatorial band is governed by S1 and S2 pressure waves of planetary extension, with rather complicated behavior. Therefore, the retrieval of ET is limited by the SG environment (shelter and instrument height) and our ability to fully correct for atmospheric effects. The spatial variations of gravity changes on the local catchment are also investigated by CG5 micro-gravity surveys since July 2011 with weekly measurements in the wet season and monthly in the dry season, resulting in more than two years and 3 wet seasons coverage. This survey helped to identify preferential recharge areas and some specific water-redistribution processes at the catchment scale, driven by subsurface heterogeneities.