Multi-Lagrange multiplier method to improve the region-specific GRACE estimation of water storage change in eleven sub-basins of the Yangtze River

Study Region: Yangtze River basin Study Focus: To improve the potential of the Gravity Recovery and Climate Experiment (GRACE) mission for estimating terrestrial water storage variations (ATWS) in smaller basins than its typical footprints (150,000 - 200,000 km2), an improved method named the Multi-Lagrange multiplier method (MLMM) is proposed to decrease leakage errors and restore signals from an independent hydrologic model. The MLMM is applied to estimate ATWS in the Yangtze River basin (YRB) and eleven of its sub-basins, and is assessed from different GRACE solutions and ground-based precipitation (P), evapotranspiration (ET), and runoff (R). Results indicate that MLMM estimates the leakage correction satisfactorily for small sub-basins. In the Tai Lake basin, the leakage ratio is improved by 16%. Excluding the Han River basin, ATWS from MLMM over the YRB are the best among different tested methods with a Nash-Sutcliffe Efficiency larger than 0.80, and the best root means square error (1.34 cm). New Hydrological Insights for the Region: The main contributor to ATWS over the YRB is P with 60%. The contribution of ET to ATWS is more extensive than that of R over the middle of YRB. The ATWS and the total groundwater and surface water were all increasing over the YRB and eleven of its sub-basins. This paper concludes that the proposed MLMM is helpful for ATWS estimation in the small ungauged basins.