Reconstruction of Global Groundwater Storage Anomaly from GRACE Satellite Gravimetry Using a Physics-Constrained Decomposition LSTM
Groundwater is the largest accessible store of freshwater on Earth, yet its depletion has been difficult to quantify globally, because satellite gravimetry senses total water storage rather than groundwater alone, the record is coarse and interrupted, and the models used to isolate groundwater underestimate the largest trends. Here, a continuous and physically bounded record of global groundwater storage change was reconstructed for April 2002 to December 2022 at half-degree resolution. Total water storage from the GRACE and GRACE-FO missions was separated from its land-surface components, and a physics-constrained decomposition network learned the groundwater signal as the sum of a trend, a seasonal cycle, and a residual. The trend of every cell was held within the net moisture that the atmosphere can supply, and the mass balance of each river basin was enforced during training, so the learned field could not violate water availability. The reconstruction reproduced withheld observations with a root mean squared error of 6.97 centimeters and a Nash-Sutcliffe efficiency of 0.69, and against an independent network of monitoring wells it achieved a lower trend error than an independent hydrological model and a data-assimilation product at every aggregation scale tested. Gross groundwater depletion over significantly declining cells was estimated at 358 +/- 12 cubic kilometers per year, while net storage change across all land cells was -36 +/- 18 cubic kilometers per year, equivalent to 0.10 +/- 0.05 millimeters per year of sea-level rise. The associated mass redistribution implies a drift of the Earth's rotational pole of 6.9 centimeters per year toward 68.6 degrees East, within 4.5 degrees of the direction inferred in an earlier study.