Modeling river storage from radar altimetry and remote sensing: validation using GRACE and GPS

Since its launch in 2002, the GRACE (Gravity Recovery And Climate Experiment) mission is recording the variations of the Earth’s gravity field at unprecedented temporal (classically at 10 day to monthly samples) and spatial (a few hundreds of kilometers) resolutions, mainly due to the global circulation of surface geophysical fluids.  Continental water storage variations estimated with GRACE are classically compared to global hydrology models. However most of these models do not take into account both the groundwater and the surface water (lakes and rivers) components of the hydrological cycle.

We derive surface water storage of several large rivers, characterized by various climates, using a simple routing scheme, forced by runoff outputs of hydrology models. We adjust the flow velocity, i.e. the only free parameter in our modeling by fitting the modeled equivalent water height to the observed water elevation from radar altimetry measurements.  The conversion of the observed geometric heights into the modeled equivalent water heights requires the knowledge of the variations of the river widths, which can be derived from MODIS (Moderate Resolution Imaging Spectroradiometer) observations. We validate our river models by comparing the estimated discharge to independent in-situ measurements.

The entire continental water storage, i.e. the sum of soil-moisture, snow and the modeled surface water, is then compared to GRACE and used to compute 3-D surface displacements. We show that our full model is in better agreement with GRACE, but also better explains surface displacements recorded by GPS than the soil-moisture and snow components alone