Downscaling GRACE-FO Data with CYGNSS Soil Moisture for Improved Representation of Floodplain Inundation: Application to the Murray-Darling Basin, Australia

In the era of global warming, accurate monitoring of terrestrial water storage across scales is essential for effective water resource management and mitigating the impacts of a changing climate. Since 2002, the Gravity Recovery and Climate Experiment (GRACE) satellite mission and its successor, GRACE Follow-On (GRACE-FO), have provided valuable insights into terrestrial water redistribution processes by monitoring Earth’s time-varying gravity. However, their coarse spatial resolution limits their utility for estimating water mass changes at local scales. While various downscaling methods integrating GRACE data with high-resolution land surface models have been proposed, the accuracy of these approaches often depends on the fidelity of the models used. This presents challenges in the regions where water redistribution is driven primarily via streamflow, flood inundation or agricultural irrigation—processes often poorly represented in many land surface models. In this study, we employ soil moisture contents retrieved from the Cyclone Global Navigation Satellite System (CYGNSS) as a priori soil water mass to downscale GRACE-FO data. The downscaling is applied to the Murray-Darling Basin, Australia, with particular focus on interior arid regions where water redistribution is dominated by streamflow and associated inundation of floodplains. The downscaled GRACE-FO data demonstrate superior performance in capturing local hydrological processes, including a major flood inundation event in late 2022, mapped using the Water Observation product (Digital Earth Australia), and regions of intensive agricultural water use identified through the CSIRO MODIS Reflectance-based Scaling EvapoTranspiration (CMRSET) product.