Global Lakes and Reservoirs: An investigation to which extent dynamic water body shapes have an impact on the estimates of the total water storage derived from GRACE

The satellite mission Gravity Recovery and Climate Experiment (GRACE) measures variations of the gravity field of the earth which are caused by the redistribution of masses across the atmosphere, the continents and the oceans. Over the continents, those mass redistributions are directly linked to changes in the total water storage (TWS), which are expressed in terms of equivalent water heights (EWH). The derived observations have been frequently used for water cycle studies, including the improvement of hydrological models in terms of calibration, data assimilation and validation. The problem herewith is, that the respective gravity field models have a low spatial resolution, which means that spatially localized changes concerning the water storage of for instance lakes and reservoirs, cannot properly be represented in the GRACE estimates. Since those surface storage changes can represent a large part of the total water storage, the leakage effect assimilates the surface water signal into the neighbouring water storage compartments. This means, that the observations, which were derived from GRACE, cannot be used to investigate the influence of regional and grid-based water storage changes on the total water storage. Furthermore, also non-hydrology related phenomena, such as earthquakes, which also cause a redistribution of the mass, hamper the usage of observations derived from GRACE for
hydrological studies in the respective regions.

To counteract this tendency, this study presents an approach to which satellite altimetry and remote sensing data were used to forward model surface water volume estimates which can then be subtracted from the initial GRACE observations to remove their influence from the mass estimates. In order to do so, static as well as temporally variable and thus dynamic surface area extents were used. It will be discussed, that using a dynamic instead of a static surface area extent (1) will change the equivalent water height values of the corrected GRACE signal in a magnitude between 0.006 cm and 0.243 cm, (2) causes the largest deviation for the Lake Mead which does not indicate any specific features and (3) that the question whether a dynamic or a static water body shape should be considered is driven by the interaction of various parameters. A first comparison of the retrieved results to an existing work confirms the derived observations.