The South-to-North Water Diversion Project in China is the world’s largest water transfer project, aiming to address water shortages in northern China by channeling water from the water-rich southern regions. Water resources in Tianjin have long been in severe deficit, with excessive groundwater extraction causing significant surface subsidence, negatively impacting urban infrastructure and economic development. As a result, Tianjin has become a key beneficiary of this water diversion project. To investigate the current situation of surface subsidence, we obtained the vertical displacement time series from 21 GNSS stations across Tianjin from 2011 to 2021 and analyzed overall subsidence changes and rehabilitation status. Results indicate that no clear surface subsidence was observed in the northern regions of Tianjin due to groundwater extraction mainly in unconfined aquifers. The southwestern region experienced the most significant surface subsidence due to overexploitation of deep groundwater, with peak cumulative subsidence exceeding 600 mm during the study period. The central, eastern, and southeastern coastal regions also faced severe surface subsidence with cumulative amounts ranging from 100 mm to 400 mm. The alleviation of subsidence predominantly benefits from continuous water supply from the South to North Water Diversion Project, which resulted in most stations significantly slowing down or even stabilizing their settlement rates after 2018. Therefore, the South-to-North Water Diversion Project plays a crucial role in addressing the persistent water resource shortage and mitigating long-term surface subsidence in Tianjin by ensuring a continuous water supply and significantly reducing the need for groundwater extraction. Our findings indicate positive measures, such as water diversion projects and water management policies, can serve as valuable references for other regions around the world facing similar water scarcity and groundwater overexploitation.

We introduce a new hydrological index that enables assessment of extreme events every few days from the GRACE Follow‐On (GRACE‐FO) satellite mission. The Mass Change Index (MCI) was developed by standardizing instantaneous satellite gravity anomalies computed directly from orbit perturbations. It is based on hydrology‐related gravity change, namely, total water storage change, and thus equally sensitive to wet and dry anomalies. The key innovation of MCI is its sensitivity to instantaneous mass changes as opposed to monthly mean changes. GRACE‐FO's ground track permits MCI retrievals every 5–6 days in most low and mid latitude regions. We demonstrate the application of MCI to investigate hydrological extremes in the middle‐lower Yangtze River Basin (MLYRB). MCI detects extreme wet conditions (standardized index of 2.0–3.0) along the Yangtze River mainstream related to the catastrophic flood in 2020, consistent with daily streamflow observations. In contrast, a typical GRACE‐FO based monthly drought index significantly underestimates the severity of the event and misidentifies timing of the onset. MCI also detects extreme dry conditions (−2.0 to −2.5) prevailing within MLYRB, related to the unprecedented heatwave and drought event during the summer of 2022. A streamflow index and the monthly drought index both underestimate the severity of the event. MCI retains information in intersatellite range measurements that may be lost when processing monthly gravity solutions. It can also be processed more rapidly, increasing its potential value for hydrological monitoring systems and other operational applications.

We present the globally available dataset of Line-of-sight Gravity Differences (LGD) as a new data product to fill the long-standing gap of investigating sub-monthly surface mass change from the satellite gravimetry along-track perspective. The dataset is generated directly from Gravity Recovery and Climate Experiment Follow-on (GRACE-FO) Level-1B intersatellite ranging observations, positioned conceptually between Level-1B measurements and Level-2 monthly gravity field solutions. Provided as along-track time series, LGD delivers instantaneous, in situ gravity change at satellite altitude, thus ensuring the fidelity of measurements by avoiding the complex preprocessing, regularization, and filtering applied to monthly products. The LGD dataset enables monitoring of sub-monthly terrestrial water storage variability and provides accurate information on timing and magnitude of water mass changes. GRACE-FO’s ground track allows LGD retrievals every 5–6 days in most low and mid latitude regions, while the revisit interval becomes shorter toward higher latitudes. Our LGD data products also contain climatology in the form of along-track gravity variations. We demonstrate its potential through case studies, including along-track diagnosis of flash drought evolution in the southeastern United States and the characterization of sub-monthly hydrological extremes in global hotspots. Results show that the LGD dataset resolves the critical timing (onset, peak, and termination) of hydrological extremes, which is unattainable from monthly products. The LGD dataset is available at https://doi.org/10.5281/zenodo.17766177 (Tang et al., 2025), which is expected to pave the way for advances in sub-monthly hydrological monitoring and can serve as a unique constraint for hydrological and geophysical models, opening new opportunities for Earth system sciences.