ELASTIC DISPLACEMENT OF THE SOLID EARTH AND GRACE/GRACE-FO RESPONSE TO VARIATIONS IN GLOBAL LAKE AND RESERVOIR STORAGE

Estimates of subsurface water storage (groundwater + soil moisture) from gravity (GRACE/GRACE-FO) data require that the other components of terrestrial water storage be known. In many basins the largest and most seasonally variable contribution to terrestrial water storage is surface water storage, which is primarily concentrated in natural lakes and artificial reservoirs. However, the large footprint of GRACE spreads surface water bodies over a broad area making it difficult to isolate and remove variations in lake and reservoir water storage. Surface displacements of Global Positioning System (GPS) stations provide an alternative method of estimating change in terrestrial water storage and by extension subsurface water. GPS inversions for subsurface water are also subject to systematic bias caused by Earth’s elastic response to surface water loading. To improve subsurface water estimates from GPS and GRACE here we use the monthly surface water storage anomalies of 287 natural lakes and 704 reservoirs, larger than 100 km², between October 1992 to October 2020 derived from satellite altimetry and Landsat imagery from the Global database of Lake Water Storage to predict the elastic deformation of the solid Earth and mass change observed by GRACE caused by these changes in surface water storage (Yao et al., 2023). We estimate the least-squares seasonal oscillation of each water body and combine it with a linear interpolation to construct a continuous monthly record across the study period. We estimate that our final product accounts for ≈76% and ≈72% global natural lake and reservoir storage respectively, with the worst coverage above 55°N where the temporal coverage of lake altimetry is poor. When removed from terrestrial water storage estimates from GRACE the affected mascons reduce in variance by up to ≈70%, with an average variance reduction of ≈8%. The implied elastic vertical displacements due to changes in lake and reservoir storage are greater than 20 mm around the largest lakes and reservoirs such as the Caspian Sea, the Great Lakes, and the Rift lakes of East Africa. Finally, we estimate a near global map of subsurface water change from October 1992 to October 2020, by removing our estimates of lake and reservoir storage change from terrestrial water change estimated from GRACE & GPS in areas with insignificant seasonal snow accumulation.