Continental water storage trends and budgets from the GRACE/GRACE-FO Global Gravity-based Groundwater Product (G3P)

Abstract

The Global Gravity-based Groundwater Product (G3P) provides global groundwater storage (GWS) variations derived from GRACE/GRACE-FO terrestrial water storage (TWS) change by subtracting root-zone soil moisture, glacier mass change, surface water storage, and snow water equivalent. G3P thus provides the first globally consistent, publicly available groundwater dataset from satellite gravimetry for continental-scale water resources assessment. Here, we use G3P to assess global GWS trends over the extended GRACE/GRACE-FO record, jointly with TWS and the subtracted storage components. A central challenge is that apparent long-term trends in GRACE/GRACE-FO water storage records can reflect persistent storage change, natural climate variability, hydrological memory, and observational errors, complicating trend detection and continental-scale storage-change budgets. We address this using a calibrated trend-analysis framework that jointly accounts for trend, seasonal, and stochastic variations in each grid-cell time series. Statistically significant trends are then aggregated to quantify volumetric GWS and TWS gains, losses, and net changes at continental and global scales. Applying this framework to 21.5 years of data (April 2002–September 2023), we find that groundwater depletion is a major contributor to freshwater decline at continental scales, most prominently in Asia (−55 km³ yr⁻¹), while cryospheric mass loss dominates the strongest TWS declines in high-latitude and glacierized regions. Pronounced regional contrasts include increasing groundwater storage across large parts of Africa (+37 km³ yr⁻¹), negative trends in drought-affected regions, and widespread negative or near-threshold GWS tendencies across the Northern Hemisphere. Globally, significance-filtered net GWS and TWS changes are approximately −27 and −183 km³ yr⁻¹, respectively. The corresponding gross negative GWS change of approximately 103 km³ yr⁻¹ is of the same order as model-based estimates of global groundwater depletion. Sensitivity tests indicate robust results: at 95 %, detected area increases by 30 % for TWS and 36 % for GWS, while net-loss magnitudes rise by only 9 % and 18 %. Across much of the seasonal-to-interannual spectral range, GWS is the dominant individual storage-component signal within TWS, which highlights the importance of groundwater for continental freshwater availability.