Comment on essd-2025-797

<strong class="journal-contentHeaderColor">Abstract.</strong> Groundwater is one of the most important freshwater resources for ecosystems and mankind. Because of its fundamental role in the Earth's water and energy cycles, groundwater has been declared an essential climate variable by GCOS, the Global Climate Observing System. Similar to other subsurface states and fluxes, groundwater is difficult to monitor at the global scale, with sufficient spatial coverage and over climate-relevant time scales. The Global Gravity-based Groundwater Product (G3P) is a global observation-based data set of large-scale groundwater storage variations. G3P capitalizes on the unique capability of GRACE and GRACE-FO satellite gravimetry as the only remote sensing technology to monitor subsurface mass variations. In a mass balance approach, satellite-based, in situ observation-based and model-based water storage variations of snow water equivalent, root-zone soil moisture, glacier mass, and surface water storage are subtracted from GRACE/-FO terrestrial water storage anomalies to result in monthly variations of groundwater storage. For this combination, the individual compartmental storage data are spatially filtered to be consistent with the spatial resolution of terrestrial water storage from satellite gravimetry. The G3P data set presented here covers the period 2002 to 2023 with monthly resolution on a 0.5&deg; global grid and includes propagated uncertainty information. We describe the details of the G3P data processing chain and of each contributing data stream, provide examples of spatial and temporal groundwater storage variations represented by the G3P data set, and present exemplary evaluation results against in situ groundwater observations for three large aquifer systems. G3P is a prototype for an operational global groundwater service, under development as a cross-cutting extension of the existing portfolio of the Copernicus Climate Change Service C3S. The G3P data set is available via GFZ Data Services at <a href="https://doi.org/10.5880/G3P.2024.001">https://doi.org/10.5880/G3P.2024.001</a> (G&uuml;ntner et al., 2024).