[1] A ground-based GPS network has been established over West Africa in the framework of African Monsoon Multidisciplinary Analysis (AMMA) in tight cooperation between French and African institutes. The experimental setup is described and preliminary highlights are given for different applications using these data. Precipitable water vapor (PWV) estimates from GPS are used for evaluating numerical weather prediction (NWP) models and radiosonde humidity data. Systematic tendency errors in model forecasts are evidenced. Correlated biases in NWP model analyses and radiosonde data are evidenced also, which emphasize the importance of radiosonde humidity data in this region. PWV and precipitation are tightly correlated at seasonal and intraseasonal timescales. Almost no precipitation occurs when PWV is smaller than 30 kg m À2 . This limit in PWV also coincides well with the location of the intertropical discontinuity. Five distinct phases in the monsoon season are determined from the GPS PWV, which correspond either to transition or stationary periods of the West African Monsoon system. They may serve as a basis for characterizing interannual variability. Significant oscillations in PWV are observed with 10-to 15-day and 15-to 20-day periods, which suggest a strong impact of atmospheric circulation on moisture and precipitation. The presence of a diurnal cycle oscillation in PWV with marked seasonal evolutions is found. This oscillation involves namely different phasing of moisture fluxes in different layers implying the low-level jet, the return flow, and the African Easterly Jet. The broad range of timescales observed with the GPS systems shows a high potential for investigating many atmospheric processes of the West African Monsoon.

International audienceThree-dimensional ground deformation measured with permanent GPS stations inWest Africa was used for investigating the hydrological loading deformation associatedwith Monsoon precipitation. The GPS data were processed within a global network for the2003–2008 period. Weekly station positions were retrieved with a repeatability (includingunmodeled loading effects) of 1–2 mm in the horizontal components and between 2.5and 6 mm in the vertical component. The annual signal in the vertical component forsites located between 9.6N and 16.7N is in the range 10–15 mm. It is consistent at the3 mm-level with the annual regional-scale loading deformations estimated from GRACEsatellite products and modeled with a combination of hydrological, atmospheric, andnontidal oceanic models. An additional 6 month transient signal was detected in the verticalcomponent of GPS estimates at most of the West African sites. It takes the form of anoscillation occurring between September and March, and reaching a maximum amplitude of12–16 mm at Ouagadougou (12.5N). The analysis of in situ hydro-geological data revealeda strong coincidence between this transient signal and peak river discharge at three siteslocated along the Niger River (Timbuktu, Gao, and Niamey). At Ouagadougou, a similarcoincidence was found with the seasonal variations of the water table depth. We propose amechanism to account for this signal that involves a sequence of swelling/shrinking of clayscombined with local loading effects associated with flooding of the Niger River

International audienceThis 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