Terrestrial water storage (TWS) is an important indicator of the hydrological cycle. In this study, GNSS-derived TWS changes across the contiguous United States and southern Canada were estimated using Green’s function (GF) and Slepian basis function (SBF) methods at 1°, 0.5°, and 0.25° grid spacings based on 2839 GNSS stations between September 2010 and September 2024. The results were evaluated against GRACE/GRACE-FO over 13 major river basins. Both methods captured the dominant seasonal variability with strong agreement in snow-dominated basins. Increasing resolution to 0.25° reduced agreement in most basins; however, exceptions exist in some cases. While regions with sparse station density suffer from noise amplification due to insufficient observational constraints, densely instrumented regions can reflect localized variability. Overall, the correlations range between −0.37 and 0.92, while the RMSE vary from 3.40 cm to 15.08 cm between GNSS-derived and GRACE TWS changes. In the Columbia River basin, correlations reached 0.92 (GF) and 0.88 (SBF) at 1° resolution, and decreased to 0.86 and 0.82 at 0.25°, respectively. The Atlantic Ocean Seaboard showed near-zero correlations across all resolutions, indicating spatially heterogeneous signals. In the Pacific Ocean Seaboard, RMSE at 0.25° reached 12.86 cm (GF) versus 8.93 cm (SBF), reflecting GF’s greater sensitivity to localized variations at finer scales. This highlights that effective resolution depends on station density, hydrological signal coherence, and regularization rather than grid spacing alone.
Nasiri et al. (Sat,) studied this question.