Extending terrestrial water storage anomalies beyond the GRACE era using tree rings

Abstract. Satellite observations from NASA’s Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On have transformed monitoring of terrestrial water storage (TWS) anomalies, but their short record (2002–present) limits characterization of long-term variability and extremes. Here, we evaluate whether tree-ring width indices (RWI) can be used to extend TWS variability across western North America. Using 144 chronologies, we find predominantly positive correlations between RWI and GRACE-observed TWS anomalies, with mean annual correlations of r = 0.25 (IQR: 0.07–0.50) and the strongest relationships during summer (mean r = 0.27; 37% of sites significant at p < 0.1). Correlation strength is highest in moisture-limited environments and among Pinus species. In the Upper Colorado River Basin, a two-site composite model explains 69% of the variance in GRACE-observed summer TWS anomalies (r = 0.83) and enables reconstruction back to 1567 CE. The reconstruction exhibits a standard deviation of 29.9 mm and a peak-to-trough range of 167.6 mm, and agrees well with other hydroclimatic records, including reconstructed Palmer Drought Severity Index over 1567-2000 (r = 0.79) and climate-data-driven TWS estimates over 1901-2000 (r = 0.73). Using climate-data-driven TWS estimates as a longer reference record, moving-window analyses show that calibration stability improves rapidly from 10-year to approximately 25–30-year windows, after which additional years yield more gradual gains. Together, these results suggest that the now nearly 25-year GRACE and GRACE Follow-On record is approaching the duration needed for more stable tree-ring–TWS calibration, and that targeted extension of moisture-sensitive chronology networks through the satellite era may provide a robust pathway for reconstructing multi-century TWS variability in western North America and other similar regions.