ALBERT

Description: A major uncertainty in determining the mass balance of the Antarctic ice sheet from measurements of satellite gravimetry, and to a lesser extent satellite altimetry, is the poorly known correction for the ongoing deformation of the solid Earth caused by glacial isostatic adjustment (GIA). In the past decade, much progress has been made in consistently modelling the ice sheet and solid Earth interactions; however, forward-modelling solutions of GIA in Antarctica remain uncertain due to the sparsity of constraints on the ice sheet evolution, as well as the Earth's rheological properties. An alternative approach towards estimating GIA is the joint inversion of multiple satellite data - namely, satellite gravimetry, satellite altimetry and GPS, which reflect, with different sensitivities, trends of recent glacial changes and GIA. Crucial to the success of this approach is the accuracy of the space-geodetic data sets. Here, we present reprocessed rates of surface-ice elevation change (Envisat/ICESat; 2003-2009), gravity field change (GRACE; 2003-2009) and bedrock uplift (GPS; 1995-2013). The data analysis is complemented by the forward-modelling of viscoelastic response functions to disc load forcing, allowing us to relate GIA-induced surface displacements with gravity changes for different rheological parameters of the solid Earth. The data and modelling results presented here form the basis for the joint inversion estimate of present-day ice-mass change and GIA in Antarctica. This paper presents the first of two contributions summarizing the work carried out within a European Space Agency funded study, REGINA.

Description: Groundwater is a crucial resource for drinking water, agricultural irrigation, and industry, and its sustainable management is essential for maintaining economic development and healthy ecosystems. Climate change (e.g., droughts) and human interventions (e.g., land use change, and water withdrawals) increase global reliance on groundwater, leading to more pressure on already depleted aquifers. A lack of direct groundwater observations presents many challenges to assess groundwater stores, especially when under stress caused by groundwater drought. Thomas et al., (2017) developed a framework to evaluate groundwater drought occurrence across California Central Valley (CCV), based on observations from NASA's Gravity Recovery and Climate Experiment (GRACE) satellite mission. The GRACE Groundwater Drought Index (GGDI), a normalized GRACE-groundwater time series, was shown to quantify groundwater storage deficits attributed to groundwater drought. The GGDI demonstrated a good agreement with in-situ groundwater drought timeseries, capturing the characteristics of groundwater drought in the CCV. As applied for the CCV, in-situ records of surface water storage changes were used to derive GRACE-groundwater. However, many follow-on studies have applied GGDI without strict accounting for surface water storage change. Accurate extraction of GRACE-groundwater requires robust estimation of water budget components; thus, it stands to reason that accounting for surface water stores within GGDI would result in a more robust estimate of groundwater drought. The aim of this project is to investigate the influence of accounting for surface water changes within GGDI on capturing groundwater storage deficits. Our findings indicated that a strict accounting of changes in surface water stores improved GGDI’s ability to identify groundwater drought characteristics across large-scale basins.

Description: Changing ice masses cause deformation of the solid Earth on different time scales. The classic view discriminates between elastic deformation and glacial-isostatic adjustment (GIA) due to present-day and past changes, respectively. The increasing availability of observational data and modelling advances allows our understanding of the complex pattern of solid Earth response to improve, including observation of rapid GIA.Geodetic GNSS provides a technique to directly observe bedrock motion. In Antarctica, several studies already utilized such GNSS data but were limited in time or to a specific region, or could use recordings of only a limited number of stations. Within the SCAR-endorsed project GIANT-REGAIN a reprocessing of all available Antarctic GNSS data was realized, comprising data acquired by episodic and permanent recordings at about 280 bedrock sites between 1995 and 2021. Special attention was given to metadata which are indispensable for a correct assignment of the hardware set-up. The four processing centres applied precise point positioning or differential GNSS using different scientific software. Time series of consistent point coordinates were generated as the major product.We will report on the comparison of the different solutions which allows to quantify time series analysis uncertainty. From the time series, coordinate velocities will be estimated. Here, we will discuss different aspects such as useful noise models, spatial correlations and suitable trajectory models. The treatment of the solid Earth response to ice-mass changes occurring over the last decades up to present day is currently under strong discussion and will be touched briefly.

Description: Traditional methods for evaluating groundwater resources rely on in-situ observations, which are often limited due to sparsely distributed monitoring wells and temporal inconsistencies in measurements. However, InSAR techniques have the potential to measure groundwater storage change indirectly through measuring ground deformation. The ground deformation associated with groundwater withdrawal is mainly recoverable or as a result of elastic compression. However, when effective stress exceeds the maximum past stress on the aquifer, inelastic subsidence occurs, which can permanently lower the storage capacity of the aquifer. Thus, in addition to monitoring changes in the volume of groundwater storage, analysing elasticity properties is necessary to fully understand the influence of overextraction on long-term aquifer sustainability. This research aims to investigate the capability of InSAR to contribute to a detailed understanding of groundwater storage change and the sustainability of groundwater use across Delhi, India. Specifically, we apply the ISBAS-InSAR technique to Sentinel-1 SAR data to produce a time-series of deformation across the region and investigate the relationship between in-situ groundwater storage change and ground deformation. Finally, the spatio-temporal variability and trends of elasticity in the underlying aquifer system, quantified using the elastic skeletal storage coefficient, will be analysed by examining the ratio of seasonal deformation signal provided by InSAR and groundwater level change from well measurements across the time-series of observations. These results shall inform both groundwater management in the Delhi region and provide insights into the applicability of InSAR for inferring large-scale aquifer dynamics.

Description: GNSS observations of M〈sub〉2〈/sub〉 ocean tide loading horizontal displacements (OTLHD) at 256 stations in western Europe generally show amplitudes of less than 10 mm. When compared to predictions using the standard Earth model PREM and the high-resolution ocean tide model TPXO9v5, they show discrepancies (or residuals) greater than 0.4 mm at most of these GNSS stations. This can be due to uncertainties in ocean tide models or deviations in the Earth’s internal structure. To assess the former, we computed the predicted OTLHD across western Europe using the recent global ocean tide models DTU10, FES2014b, GOT4.10 and TPXO9v5. It is found that the inter-model standard deviation is under 0.2 mm at all but 18 and 30 stations (less than 10% of all stations), respectively, for the West and South displacements, and the mean size of the standard deviations is less than 0.16 mm. Therefore, it is necessary to look into the structural deviations of the Earth from those revealed by PREM. Here, using both one-dimensional (1-D) and recently developed three-dimensional (3-D) Earth models, we investigate the effect of structure variations of the Earth’s lithosphere and mantle on OTLHD. In particular, given that lateral heterogeneity induces an additional toroidal displacement, we explore if the presence of lateral inhomogeneity in the mechanical structure, characterized by elastic moduli, can reduce the discrepancies in horizontal displacement.