Description: Groundwater is one of the most important freshwater resources for mankind and for ecosystems. Assessing groundwater resources and developing sustainable water management plans based on this resource is a major field of activity for science, water authorities and consultancies worldwide. Due to its fundamental role in the Earth’s water and energy cycles, groundwater has been declared as an Essential Climate Variable (ECV) by GCOS, the Global Climate Observing System. However, within Copernicus - the European Earth Observation Programme – there is no service available yet to deliver data on this fundamental resource, nor is there any other data source worldwide that operationally provides information on changing groundwater resources in a consistent way, observation-based, and with global coverage. Filling this gap is the goal of the G3P (Global Gravity-based Groundwater Product) project, funded since the beginning of 2020 by the European Union. G3P aims at (1) capitalizing from the unique capability of GRACE and GRACE-FO satellite gravimetry as the only remote sensing technology to monitor subsurface mass variations and thus groundwater storage change for large areas with global coverage, and (2) incorporating and advancing a wealth of products on storage compartments of the water cycle that are already part of the Copernicus portfolio and will be used for separating out the groundwater storage variations from the gravity-based total terrestrial water storage, to finally (3) developing an operational global groundwater service prototype as a cross-cutting extension of the existing Copernicus portfolio. In this contribution, we present the concept of G3P and first results.

Description: Groundwater (GW) is the world’s largest distributed freshwater storage for mankind, ecosystems, and is a key resource for industrial and agricultural demands. Due to its fundamental role in the Earth's water and energy cycles, groundwater has been declared as an Essential Climate Variable (ECV) by GCOS, the Global Climate Observing System. However, within Copernicus - the European Earth Observation Programme - there is no service available yet to deliver data on this fundamental resource, nor is there any other data source worldwide that operationally provides information on changing groundwater resources in a consistent way, observation-based, and with global coverage. Therefore, the Global Gravity-based Groundwater Product (G3P) project aims at developing an operational global groundwater service as a cross-cutting extension of the existing Copernicus - the European Earth Observation Programme - portfolio. G3P capitalizes from the unique capability of GRACE and GRACE-FO satellite gravimetry as the only remote sensing technology to monitor subsurface mass variations, and from other satellite-based water storage products that are already part of the Copernicus portfolio, to provide a data set of groundwater storage change for large areas with global coverage. G3P is obtained by using a mass balance approach, i.e., by subtracting satellite-based water storage compartments (WSCs) such as snow water equivalent, root-zone soil moisture, glacier mass, and surface water storage from GRACE/GRACE-FO monthly terrestrial water storage anomalies (TWSA). Compatibility of the observation-based WSCs with TWSA is achieved by a filtering process, where optimal filter types were derived by analyses of spatial correlation patterns. G3P groundwater variations are provided for almost two decades (from 2002 to the present), with the monthly resolution, and at a 0.5-degree spatial resolution globally. In this contribution, we also illustrate preliminary results of the G3P data set and of its uncertainties, as well as its evaluation by independent groundwater data. This study has been run in the context of the European Union’s Horizon 2020 research project G3P (Global Gravity-based Groundwater Product, grant agreement nº 870353).

Description: Operationally combined monthly gravity fields of the GRACE-FO satellite mission in spherical harmonic representation (Level-2 product) generated by the Combination Service for Time-variable Gravity Fields (COST-G; Jäggi et al. (2020):http://dx.doi.org/10.1007/1345_2020_109), a product center for time-variable gravity fields of IAG's International Gravity Field Service (IGFS). COST-G_GRACE-FO_RL01_OP is a combination of AIUB-GRACE-FO_op, GFZ-RL06 (GFO), GRGS-RL05 (unconstrained solution), ITSG-Grace_op, LUH-GRACE-FO, CSR-RL06 (GFO) and JPL-RL06 (GFO). The original time-series were provided by the analysis centers (ACs) and partner analysis centers (PCs) of COST-G.

Description: The Global Gravity-based Groundwater Product (G3P) aims at developing a satellite-based groundwater storage (GW) data set as a new product for the EU Copernicus Climate Change Service. As the world’s largest distributed freshwater storage, GW is a key resource for mankind, industrial, and agricultural demands. In Copernicus, there is no service available yet to deliver data on this fundamental resource, nor is there any other data source worldwide that operationally provides information on changing groundwater resources in a consistent way, observation-based, and with global coverage. Therefore, G3P develops an operational global groundwater service as a cross-cutting extension of the existing Copernicus portfolio. G3P capitalizes from the unique capability of GRACE and GRACE-FO satellite gravimetry as the only remote sensing technology to monitor subsurface mass variations, and from other satellite-based water storage products to provide a data set of groundwater storage change for large areas with global coverage. G3P is obtained by using a mass balance approach, i.e., by subtracting satellite-based water storage compartments (WSCs) such as snow water equivalent, root-zone soil moisture, glacier mass, and surface water storage from GRACE/GRACE-FO monthly terrestrial water storage anomalies (TWSA). For a consistent subtraction of all individual WSCs from GRACE-TWSA, the individual WSCs are filtered in a similar way as GRACE-TWSA, where optimal filter types were derived by analyses of spatial correlation patterns. G3P groundwater variations are provided for almost two decades (from 2002 to the present), with the monthly resolution, and at a 0.5-degree spatial resolution globally. In this contribution, we also illustrate preliminary results of the G3P data set and of its uncertainties, as well as its evaluation by independent groundwater data. This study has received funding from the European Union’s Horizon 2020 research and innovation programme for G3P (Global Gravity-based Groundwater Product) under grant agreement nº 870353.

Description: The Global Gravity-based Groundwater Product (G3P) provides groundwater storage anomalies (GWSA) from a cross-cutting combination of GRACE/GRACE-FO-based terrestrial water storage (TWS) and storage compartments of the water cycle (WSCs) that are part of the Copernicus portfolio. The data set comprises gridded anomalies of groundwater, TWS, and the WSCs glacier, snow, soil moisture and surface water bodies plus layers containing uncertainty information for the individual data products. All WSCs are spatially filtered with a Gaussian filter to be compatible with TWS. Spatial coverage is global, except Greenland and Antarctica, with 0.5-degree resolution. Temporal coverage is from April 2002 to December 2020 with monthly temporal resolution. Gridded data sets are available as NetCDF files containing variables for the parameter value as anomaly in mm equivalent water height and the parameter’s uncertainty as mm equivalent water height. The latest version of the data is visualized at the GravIS portal: http://gravis.gfz-potsdam.de/gws. From GravIS, the data is also available as area averages for several large river basins and aquifers, as well as for climatically similar regions. G3P was funded by the EU Horizon 2020 programme in response to the call LC-SPACE-04-EO-2019-2020 “Copernicus evolution – Research activities in support of cross-cutting applications between Copernicus services” under grant agreement No. 870353.

Description: In 2019 the Combination Service for Time-variable Gravity fields (COST-G) started operation with the release of the complete time-series of combined GRACE monthly gravity fields COST-G GRACE RL01. Meanwhile additional time-series of monthly GRACE gravity fields have become available, mainly from Chinese analysis centers. The COST-G quality control confirms the high quality of these solutions that are derived with independent analysis tools. In the frame of the Horizon 2020 project Global Gravity-based Groundwater Product (G3P) the weighting scheme of the monthly GRACE-FO gravity fields was adapted to take into account the limitations on the solution space imposed by the specific observation geometry of the GRACE/GRACE-FO missions. We present the new COST-G GRACE RL02 combination, based on an extended set of time-series including COST-G's new Chinese partners, and combined applying the G3P weighting scheme.

Description: The Combination Service for Time-variable Gravity fields (COST-G) operationally provides combinations of monthly Earth gravity field models derived from observations of the microwave ranging instrument of the GRACE Follow-on (GRACE-FO) satellite mission, applying the quality control and combination methodology originally developed by the Horizon 2020 project European Gravity Service for Improved Emergency Management for the data of the GRACE satellites. In the frame of the follow-up Horizon 2020 project Global Gravity-based Groundwater Product (G3P), the GRACE-FO combination is used to derive global grids of groundwater storage anomalies. To meet the user requirements and achieve optimal signal-to-noise ratio, the combination has been further developed and extended to incorporate: • new time-series based on the alternative accelerometer transplant product generated in the frame of the project by the Institute of Geodesy at the Graz University of Technology, which specifically improves the estimation of the C30 coefficient and also reduces the noise at medium to short wavelengths, and • the new time-series AIUB–GRACE-FO–RL02 of monthly GRACE-FO gravity fields, which is derived at the Astronomical Institute of the University of Bern by applying empirical noise modelling techniques. The COST-G quality control confirms the consistency of the contributing GRACE-FO time-series concerning the signal amplitude of seasonal hydrology in large river basins and the secular mass change in polar regions, but it also indicates rather diverse noise characteristics. The difference in the noise levels is taken into account in the combination process by relative weights derived by variance component estimation on the solution level. The weights are expected to be inverse proportional to the noise levels of the individual gravity field solutions. However, this expectation is violated when applying the weighting scheme as developed for the GRACE combination. The reason is found in the high-order coefficients of the gravity field, which are poorly determined from the low–low range-rate observations due to the observation geometry and suffer from aliasing due to the malfunctioning accelerometer onboard one of the GRACE-FO satellites. Hence, for the final G3P-combination a revised weighting scheme is applied where the gravity field coefficients beyond order 60 are excluded from the determination of the weights. The quality of the combined gravity fields is assessed by comparison of the noise content and the signal-to-noise ratio with the individual time-series. Independent validation is provided by the COST-G validation centre at the GFZ German Research Centre for Geosciences, where orbit fits of the low-flying Gravity and steady-state Ocean Circulation Explorer satellite are performed that confirm the high quality of the combined GRACE-FO gravity fields. By the end of the G3P project, the new combination scheme is implemented by COST-G as the new COST-G–GRACE-FO–RL02 and continued to be used for the operational GRACE-FO combination.