The global continental water budget using GRACE spaceborne gravimetry and high-resolution consistent geodetic-hydrometeorological data analysis

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GRACE gravity measurements provide a direct measure of water storage changes over continents. Thus, this novel technique enables¿for the first time¿to close the continental water balance on large scales. We propose to use GRACE gravimetry to directly determine large scale actual evapotranspiration from ground-based measurements of precipitation and discharge on large basins. The project will also provide a previously not available direct determination of atmospheric moisture fluxes on large basins from storage changes and discharge. As such, it enables a novel evaluation of atmospheric model data. However, the anisotropic error structure of conventional GRACE products is limiting their utility even for the largest basins available. Hydrological quasi-signals appear in areas, e.g. deserts, where no signal exists. To this end, we develop a new approach to GRACE error modelling, that makes use of known mass changes and their uncertainties, derived from hydrological constraints for selected areas, e.g. with negligible inputs (deserts) or with negligible evapotranspiration (snow/ice -, high altitude regions). This strategy allows for a correction of the gravity signal beyond the conventional de-aliasing procedures and thus an improvement of resolution in terms of space, time and mass. The close interdisciplinary collaboration will ensure the establishment of GRACE as a reliable hydrological sensor. Our investigations of the characteristics of both the large scale actual evapotranspiration and the atmospheric moisture flux enable us to predict discharge from ungauged basins and to evaluate the corresponding uncertainty by use of GRACE data. The global coverage of data from gauged and ungauged basins will hence lead to an improved determination of the global continental and the respective atmospheric water budget with a minimum of model assumptions.