Abstract
Abstract. Observations of changes in terrestrial water storage (TWS) obtained from the satellite mission GRACE (Gravity Recovery and Climate Experiment) have frequently been used for water cycle studies and for the improvement of hydrological models by means of calibration and data assimilation. However, due to a low spatial resolution of the gravity field models, spatially localized water storage changes, such as those occurring in lakes and reservoirs, cannot properly be represented in the GRACE estimates. As surface storage changes can represent a large part of total water storage, this leads to leakage effects and results in surface water signals becoming erroneously assimilated into other water storage compartments of neighbouring model grid cells. As a consequence, a simple mass balance at grid/regional scale is not sufficient to deconvolve the impact of surface water on TWS. Furthermore, non-hydrology-related phenomena contained in the GRACE time series, such as the mass redistribution caused by major earthquakes, hamper the use of GRACE for hydrological studies in affected regions. In this paper, we present the first release (RL01) of the global correction product RECOG (REgional COrrections for GRACE), which accounts for both the surface water (lakes and reservoirs, RECOG-LR) and earthquake effects (RECOG-EQ). RECOG-LR is computed from forward-modelling surface water volume estimates derived from satellite altimetry and (optical) remote sensing and allows both a removal of these signals from GRACE and a relocation of the mass change to its origin within the outline of the lakes/reservoirs. The earthquake correction, RECOG-EQ, includes both the co-seismic and post-seismic signals of two major earthquakes with magnitudes above Mw9. We discuss that applying the correction dataset (1) reduces the GRACE signal variability by up to 75 % around major lakes and explains a large part of GRACE seasonal variations and trends, (2) avoids the introduction of spurious trends caused by leakage signals of nearby lakes when calibrating/assimilating hydrological models with GRACE, and (3) enables a clearer detection of hydrological droughts in areas affected by earthquakes. A first validation of the corrected GRACE time series using GPS-derived vertical station displacements shows a consistent improvement of the fit between GRACE and GNSS after applying the correction. Data are made available on an open-access basis via the Pangaea database (RECOG-LR: Deggim et al., 2020a, https://doi.org/10.1594/PANGAEA.921851; RECOG-EQ: Gerdener et al., 2020b, https://doi.org/10.1594/PANGAEA.921923).
Highlights
The dynamic global water cycle influences our everyday lives by affecting freshwater availability, weather/climate fluctuations and trends, seasonal variations, anthropogenic water use, and single extreme events such as floods and droughts
The German GeoForschungsZentrum in Potsdam (GFZ) is the only processing centre that provides a total water storage dataset corrected for earthquakes (Boergens et al, 2019); a data-based global earthquake correction for different GRACE solutions is not available yet. To account for both the localized surface water storage in lakes/reservoirs and the earthquake signal, we present the first release of a new global correction dataset, RECOG (REgional COrrections for GRACE) RL01, which can be used for disaggregation of the integral GRACE water storage estimates in addition to applying standard corrections such as GIA models and the atmosphere/ocean de-aliasing products
The dominant negative part can be found to the west of the Tohoku region, while the positive parts are apparent in the Pacific Ocean, southeast of Tohoku. These results suggest that uncorrected terrestrial water storage (TWS) changes might hinder the correct analysis of the data for hydrological studies, because the post-seismic part of the earthquake might falsely be interpreted as a linear trend in the uncorrected TWS changes
Summary
The dynamic global water cycle influences our everyday lives by affecting freshwater availability, weather/climate fluctuations and trends, seasonal variations, anthropogenic water use, and single extreme events such as floods and droughts. Large model uncertainties caused by errors in climate forcings and an incomplete realism of process representations limit the models’ ability to accurately simulate water storages and fluxes, making independent observations indispensable for model validation/calibration and data assimilation. Several challenges are involved with using GRACE for improving hydrological models, among them (1) the low spatial resolution of GRACE, integrating spatially over regions as large as ∼ 200 000 km and hampering the representation of concentrated and subscale water storage changes, and (2) the fact that gravity observations contain non-hydrology-related mass variations. The first problem is caused by the GRACE orbit configuration in combination with unmodelled short-periodic mass changes, resulting in the gravity field models being strongly corrupted by spatially correlated noise. The necessary spatial filtering approach (e.g. Kusche, 2007) inevitably leads to signal loss and to leakage effects resulting in a rather coarse spatial resolution of the gravity field models of a few hundred kilometres
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