Global time variations of hydrological signals from GRACE satellite gravimetry

Successfully launched in 2002 mid-March, the goal of the Gravity Recovery And Climate Experiment (GRACE) satellite mission is to measure the spatio-temporal variations of the gravity field of the Earth to high accuracy (similar to1 cm in terms of geoid height) and a spatial resolution of similar to200-300 km, for a nominal period of 5 yr. The unprecedented precision of the GRACE mission will enable us to detect tiny time variations of the gravity field related to global redistributions of water and air mass inside fluid envelops at the surface of the Earth. In this paper, we present a new approach based on linear inverse methods to separate the different contributions of the main surface fluid reservoirs (oceans, atmosphere, total continental water storage including snow, soil wetness, ground water and ice caps) from monthly synthetic GRACE geoids. The synthetic geoids were computed from outputs of global models of different climatic fields. Because of the non-uniqueness of the classical inverse problems in gravimetry, independent information was added before inverting the synthetic geoids. Geoid solutions associated with each fluid contribution were then converted into water-equivalent thickness maps. Validation of the continental water storage solutions was performed by comparing total soil water estimates [soil moisture (SM) plus groundwater] with predictions of a global hydrological model in 71 different drainage basins of the world. Analysis of the a posteriori errors of the solutions suggests that the inversion method developed in this study allows recovering monthly water mass changes with a cm precision.