Time variations of land water storage from an inversion of 2 years of GRACE geoids

By delivering monthly maps of the gravity field, the GRACE project allows the determination of tiny time-variations of the Earth gravity and particularly the effects of fluid mass redistributions at the surface of the Earth. However, GRACE data represent vertically integrated gravity measurements, thus are the sum of all mass redistributions inside the Earth system (atmosphere, oceans and continental water storage, plus solid Earth). In this paper, we apply a generalized leastsquares inverse approach, previously developed by Ramillien et al., (2004), to estimate, from the monthly GRACE geoids, continental water storage variations (and their associated uncertainties) over a two years time span (April 2002 to May 2004). Tests demonstrating the robustness of the method are presented, including the separation between liquid water reservoirs (surface waters + soil moisture + groundwaters) and snow pack contributions. Individual monthly solutions of total land water storage from GRACE, with a spatial resolution of ~660 km, are presented for the 2-year time span. We also derive the seasonal cycle as well as a trend map over the period of analysis. We further estimate water volume changes over eight large river basins in the tropics and compare with model predictions. Finally, we attempt to estimate an average value of the evapotranspiration over each river basin, using the water balance equation which links temporal change in water volume to precipitation, evapotranspiration and runoff. Amplitudes of the GRACE-derived evapotranspiration are regionally consistent to the predictions of global hydrological models.