RECOG RL01: Correcting GRACE total water storage estimates for global lakes/reservoirs and earthquakes

Abstract

Abstract. Observations of changes in terrestrial water storage 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 neighboring 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 & 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 re-location 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 9 Mw. We can show 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, even in neighboring river basins, 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 as open access via the Pangea database (RECOG-LR: Deggim et al. (2020a) https://doi.org/10.1594/PANGAEA.921851; RECOG-EQ: Gerdener et al. (2020b, under revision), https://doi.pangaea.de/10.1594/PANGAEA.921923).

Seismic Contributions to Secular Changes in Global Geodynamic Parameters

“…The secular effect of coseismic deformation has been gradually emphasized in the realization of the terrestrial reference frame (Tregoning et al., 2013), global mean ocean mass change (Tang et al., 2020), and global terrestrial water storage estimate (Deggim et al., 2021). Moreover, all these estimations concentrate only on global coseismic deformation and neglect postseismic deformation.…”

Uncertainty of low-degree space gravimetry observations: surface processes versus internal signal

“…We refer to certain surface processes with the term "loading" defined here as the Newtonian attraction and mass redistribution associated with elastic deformation. By approximate order of magnitude, the processes include in GRACE records are tidal effects from extraterrestrial bodies, post-glacial rebound (Purcell et al, 2011), hydrological (Rodell et al, 2018), atmospheric (Kusche & Schrama, 2005) and oceanic (Dobslaw et al, 2017) loading, water mass displacement across ocean, hydrosphere and cryosphere (Pfeffer et al, 2021), pre-seismic (Bouih et al, 2022), co-seismic and post seismic (Deggim et al, 2021) mass re-distributions, sea level changes (Adhikari et al, 2019;Horwath et al, 2022) and finally core processes.…”

“…The main objective of this work is to estimate the uncertainty associated with each category of models at large spatial scales over 1200 km and inter-annual time scales to compare with the expected gravitational signature of some core processes. This estimation can not be done for the earthquakes and for the cryosphere because the existing models are not independent from GRACE observations (Deggim et al, 2021;Adhikari et al, 2016).…”