Hydrological deformation induced by the West African Monsoon: Comparison of GPS, GRACE and loading models

Three-dimensional ground deformation measured with permanent GPS stations in West Africa was used for investigating the hydrological loading deformation associated with Monsoon precipitation. The GPS data were processed within a global network for the 2003-2008 period. Weekly station positions were retrieved with a repeatability (including unmodeled loading effects) of 1-2 mm in the horizontal components and between 2.5 and 6 mm in the vertical component. The annual signal in the vertical component for sites located between 9.6 degrees N and 16.7 degrees N is in the range 10-15 mm. It is consistent at the 3 mm-level with the annual regional-scale loading deformations estimated from GRACE satellite products and modeled with a combination of hydrological, atmospheric, and nontidal oceanic models. An additional 6 month transient signal was detected in the vertical component of GPS estimates at most of the West African sites. It takes the form of an oscillation occurring between September and March, and reaching a maximum amplitude of 12-16 mm at Ouagadougou (12.5 degrees N). The analysis of in situ hydro-geological data revealed a strong coincidence between this transient signal and peak river discharge at three sites located along the Niger River (Timbuktu, Gao, and Niamey). At Ouagadougou, a similar coincidence was found with the seasonal variations of the water table depth. We propose a mechanism to account for this signal that involves a sequence of swelling/shrinking of clays combined with local loading effects associated with flooding of the Niger River.