Abstract
Three-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.