Water storage trends derived from the GRACE/-FO global gravity-based groundwater product (G3P)

Abstract. The Global Gravity-based Groundwater Product (G3P) provides observations of global groundwater storage (GWS) variations, calculated from GRACE/-FO-derived terrestrial water storage (TWS) by subtracting the contributions of root zone soil moisture, glaciers, surface water storage, and snow water equivalent. As such, G3P provides the first globally consistent, publicly available groundwater dataset from satellite gravimetry for continental-scale trend assessment. Such data are a crucial observational constraint for assessing global groundwater depletion, recharge, and water storage trends related to climate change and human activities. A challenge is the reliable separation and quantification of long-term trends from stochastic signals attributable to natural climate variability (“climate noise”) and observational system errors. To address this, we introduce a trend-analysis framework that uses calibrated time-series models to account for trends, seasonal, and stochastic variations. The approach requires minimal assumptions about underlying processes and enables the separation of significant long-term trends of GWS and TWS from stochastic variability.

Applying this framework to 21.5 years of data, our results show (1) that groundwater depletion dominates freshwater decline at continental scales – most prominently in Asia (-55 km³ yr^-1) – whereas ice mass loss remains the largest global contributor by component, and (2) reveal previously unobserved trends, including increasing groundwater storage in large parts of Africa (+37 km³ yr^-1) and declining trends attributed to droughts, e.g., in Southern Africa, Asia, and parts of Europe. Our global aggregation of statistically significant trends indicates net volumetric GWS changes of -27 km³ yr^-1 and TWS changes of -145 km³ yr^-1 (excluding Antarctica and Greenland). We also find that many regions in the Northern Hemisphere are prone to climate-induced drying, with parts of Europe close to persistent long-term groundwater decline.