On the efficacy of downscaled GRACE total water storage products

Abstract. The Gravity Recovery and Climate Experiment (GRACE) satellite mission provides estimates of total water storage anomalies (TWSA) referred as “mascons” stands for mass concentration blocks. This TWSA represents integrated changes due to all forms of water including soil moisture, groundwater, surface water, canopy, and snow. However, the coarse spatial resolution of GRACE mascons (~3°) limits its utility for regional-scale hydrological analysis. Although several downscaling methods have been proposed to improve resolution, none have been comprehensively validated against in-situ observations. In this study, we are validating both mascon and downscaled GRACE TWSA products against well-based in-situ groundwater observations across India. Furthermore, we develop an improved downscaling method by modifying the approach of Vishwakarma et al. (2021), i.e., incorporating mass conservation constraints at the GRACE mascon scale instead of catchment scale to better capture spatial variability at a finer 0.5° grid scale. We assessed the efficacy of our downscaled product and two existing downscaled GRACE products (Gou and Soja, 2024; Vishwakarma et al., 2021) using performance metrics such as gain in correlation (r) and gain in root mean square error (RMSE) relative to GRACE mascon product. The entire India is spanned in 52 mascons. Out of these 52 mascons groundwater observations are available only at 22 mascons. Our modified approach shows improved performance across 17 mascons in India, with median gain in r ranging from 1.62% to 52.49% and median gain in RMSE improvements from 4.75% to 44.68%. Whereas, the Vishwakarma et al. (2021) and Gao and Soja (2024) methods perform well in 6 and 16 mascons, respectively. These results demonstrate that our enhanced downscaling approach provides more accurate and spatially resolved estimates of groundwater storage changes, offering a valuable tool for regional water resource assessments in India. Further, the downscaled GRACE product developed in this study (Mascon wise Mass Conservation product) is publicly accessible at https://doi.org/10.6084/m9.figshare.29196617.