Quantifying Uncertainty in Terrestrial Water Storage Change Estimates due to Impacts of Budget Component Errors and GRACE-Based Adjustments

Accurately estimating terrestrial Water Storage Changes (TWSC) remains challenging due to its small magnitude and sensitivity to errors in driving variables. The water balance equation provides a direct means of estimating TWSC, but errors in its components—precipitation (P), evapotranspiration (ET), and streamflow (Q)—inevitably propagate into the TWSC estimates. Despite its widespread use, the performance of water balance-derived TWSC estimates has not been well evaluated across global climatic zones. Here, we assess the performance of TWSC estimates using water balance computations across 84 basins spanning diverse climatic conditions. We then investigate how errors in budget components contribute to discrepancies in TWSC estimates. To improve accuracy, GRACE-derived TWSC data were integrated to adjust water balance-derived TWSC, which, in turn, impacts the precision of the input variables P, ET, and Q. We further evaluate these impacts, quantifying their effects on data consistency and accuracy. Five P products, four ET datasets, one observed Q dataset, and four GRACE TWSC products were combined into 80 combinations for each basin, ensuring robust results. Our findings show that errors in water balance-derived TWSC account for approximately 35% of the GRACE-observed TWSC magnitude. The contributions of P, ET, and to the TWSC estimation error are 47.39%, 44.82%, and 7.79%, respectively. While GRACE data corrections improved TWSC accuracy by ~20%, they also introduced a ~12% error into the budget components. These results provide crucial insights into the strengths and limitations of water balance computations for TWSC estimation and underscore the significance of accurate driving datasets. This study advances the understanding of TWSC dynamics, offering a pathway to improve global water resource assessments and management strategies.