Assessing the Potential of Next-Generation Gravity Missions for Estimating Total Drainable Water Storage

Total Drainable Water Storage (TDWS) represents the fraction of terrestrial water storage that can drain naturally from a basin. It is a key indicator of basin-scale hydrological responses, acting as a proxy for a basin’s water-retention capacity  and  availability for ecosystems and society. Satellite gravimetry provides a unique observational constraint on terrestrial water storage changes by sensing gravity variations caused by the redistribution of water mass on and beneath the land surface. While current missions such as GRACE and GRACE-FO successfully observe total water storage anomalies, they do not measure absolute water storage or any proxy of it. TDWS must therefore be inferred by interpreting gravity-based storage changes through the storage–runoff relationship, which governs how storage variations translate into drainage and river discharge. However, the limited effective spatial resolution of current gravity missions restricts robust analyses to large river basins and prevents investigations of smaller basins and sub-basin-scale hydrological processes. These limitations lead to the question of what improvements in TDWS estimation can be expected from next-generation gravity missions with enhanced spatial resolution and sampling.

In this study, we assess the potential impact of next-generation gravity missions, specifically NGGM and MAGIC, on the global-scale estimation of TDWS. We use simulated gravity observations, with two generations of the ESA Earth System Model (ESM2.0 and ESM3.0) providing the Total Water Storage Anomaly (TWSA) as the reference signal. TDWS is then estimated using a storage–runoff relationship, with TWSA representing storage and runoff taken from in situ observations. All mission scenarios, including GRACE-C, NGGM, and MAGIC, are processed using an identical TDWS estimation framework, ensuring that differences in the resulting TDWS parameters arise solely from mission design characteristics such as spatial resolution, temporal sampling, and noise levels.

Mission performance is evaluated at the basin scale by comparing basin-averaged total water storage anomalies and TDWS-related parameters against ESM reference values. The impact of each mission is quantified in terms of (i) accuracy, defined as the closeness of mission-based parameters to the model reference, and (ii) parameter uncertainty, assessed through confidence intervals derived from the storage–runoff fitting. The analysis is further stratified by basin size, storage–discharge coupling, and hydrological complexity.

The results show that NGGM and MAGIC reproduce basin-scale TDWS parameters more accurately than a GRACE-C–like scenario, particularly for smaller basins. Comparison with the ESM reference demonstrates that future missions reduce parameter errors, tighten confidence intervals, and better capture differences in hydrological behavior across basins. At the same time, the study demonstrates that improved gravity observations must be complemented by physically meaningful storage–runoff relationships to fully exploit the potential of future missions. A comparison between results obtained from ESM2.0 and ESM3.0 is therefore required to assess how advances in the representation of basin-scale hydrological processes affect the evaluation of future mission impacts on complex hydrological behavior.

This work was carried out within the SING project, funded by the European Space Agency under the ‘NGGM and MAGIC Science and Applications Impact Study’ ESA Contract No. 4000145265/24/NL/SC.