A data service for global groundwater and terrestrial water storage variations based on satellite gravimetry

Abstract

Among the Essential Climate Variables (ECVs) defined by the Global Climate Observing System (GCOS), groundwater is one of the terrestrial ECVs in the field of hydrology. As the world’s largest distributed freshwater storage, groundwater is a key resource for mankind, industrial, and agricultural demands, and for ecosystems. Very recently, in its Implementation Plan of 2022, GCOS defined terrestrial water storage (TWS) as a new hydrological ECV. The state variable TWS quantifies the net effect of climatic, hydrological and anthropogenic change on the continental water cycle and is essential for closing the terrestrial water balance. In spite of their importance, there is no data service or product yet on the ECVs groundwater and TWS in Copernicus, the European Union’s Earth observation program. The EU-funded project G3P (Global Gravity-based Groundwater Product) recently developed a satellite-based global-scale data set of groundwater storage anomalies (GWSA) for the period 2002-2020, with monthly resolution and on a 0.5-degree global grid. We present this data service developed as a prototype for later implementation into the EU Copernicus Climate Change Service. G3P is a global data set of groundwater storage variations as a cross-cutting extension of the existing Copernicus portfolio. G3P capitalizes from the unique capability of the satellite gravimetry mission GRACE (Gravity Recovery and Climate Experiment, 2002-2017) and its successor mission GRACE-FO (GRACE-Follow-On, since 2018) being the only remote sensing techniques 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 such as snow water equivalent, root-zone soil moisture, glacier mass, and surface water storage from GRACE/GRACE-FO monthly TWS anomalies. The resulting TWS and groundwater data sets are currently made available via the GravIS portal and within GGMN, the Global Groundwater Monitoring Network of IGRAC, the International Groundwater Resources Assessment Centre. The GravIS (‘Gravity Information Service’, gravis.gfz-potsdam.de) portal is operated by the German Research Centre for Geosciences (GFZ), together with the Technische Universität Dresden and the Alfred-Wegener-Institute (AWI). It facilitates the dissemination of user-friendly products of mass variations in the Earth system, based on GRACE/GRACE-FO. In addition to TWS and GWSA data, GravIS provides ocean bottom pressure (OBP) variations from which global mean barystatic sea-level rise can be estimated, as well as mass changes of the Greenland and Antarctic ice sheets. All these data sets can be interactively displayed at the portal and are freely available for download, either provided as gridded products or as regional averages. 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.