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Estuarine reservoirs are critical for freshwater supply and security, especially for regions facing water scarcity challenges due to climate change and population growth. Conventional methods for assessing drought severity or monitoring reservoir water level and storage are often limited by data availability, accessibility and quality. We present an approach for monitoring estuarine reservoir water levels, storage and extreme drought via satellite remote sensing and waterline detection. Based on the CoastSat algorithm, Landsat-8 and Sentinel-2 images from 2013 to 2022 were adopted to extract the waterline of Qingcaosha Reservoir, the largest estuarine reservoir in the world and a key source of freshwater for Shanghai, China. This study confirmed the accuracy of the satellite-extracted results through two main methods: (1) calculating the angle of the central shoal slope in the reservoir using the extracted waterline data and measured water levels and (2) inverting the time series of water levels for comparison with measured data. The correlation coefficient of the estimated water level reached ~0.86, and the root mean square error (RMSE) of the estimated shoal slope was ~0.2°, indicating that the approach had high accuracy and reliability. We analyzed the temporal and spatial patterns of waterline changes and identified two dates (21 February 2014 and 15 October 2022) when the reservoir reached the lowest water levels, coinciding with periods of severe saltwater intrusions in the estuary. The extreme drought occurrences in the Qingcaosha Reservoir were firstly documented through the utilization of remote sensing data. The results also indicate a strong resilience of the Qingcaosha Reservoir and demonstrate that the feasibility and utility of using satellite remote sensing and waterline detection for estuarine reservoir storage can provide timely and accurate information for water resource assessment, management and planning. [ABSTRACT FROM AUTHOR]

Estuarine projects can quickly change the estuarine topography and influence the hydrodynamics and saltwater intrusion. The Changjiang Estuary is a multiple-bifurcation megaestuary, and the outstanding feature of the saltwater intrusion is the saltwater spillover from the North Branch (NB) into the South Branch (SB). In this study, the improved ECOM-si model was adopted to numerically experiment with the impact of the sluices that are planned for construction in the upper, middle, and lower reaches of the NB on the saltwater intrusion and freshwater resources. The simulation results show that, on the one hand, sluice construction can eliminate the saltwater spillover from the NB into the SB; on the other hand, sluice construction makes water enter the NB from the SB, and the runoff discharging into the sea in the SB decreases. The water intake time of the Qingcaosha Reservoir (QCSR) increases by 3.2 days for sluice construction in the upper reaches of the NB and decreases by 0.97 and 0.94 days for sluice construction in the middle and lower reaches of the NB, respectively. Considering the impact of sluice construction in the NB on the saltwater intrusion and freshwater resources, the construction of sluices in the upper reaches of the NB is recommended. [ABSTRACT FROM AUTHOR]

Suspended sediment fronts (SSFs) significantly affect water quality, ecology, and geomorphological evolution of estuarine and coastal waters. Based on 10‐year suspended sediment concentration data derived from satellite images, the multiscale spatio‐temporal variability of SSFs in the Bohai Sea was studied. Frontal probability (FP) was used to quantify the intensity of frontal activity. The 10‐year averaged FP was higher near the coast and decreased with increasing offshore distance. The arc‐shaped distributions of FP near the Yellow River Estuary revealed two tidal shear frontal zones. In the eastern Bohai Sea, 13 finger‐shaped high‐FP areas reflected the spatial pattern and fine structures of tidal sand ridges. The complex topography and strong tidal dynamics provided the essential conditions for frontogenesis. The seasonality of frontal activity was dominantly regulated by the change in wind waves associated with monsoons and stratification. The direct or indirect effects of sediment discharges from the Yellow River and Liaohe River on seasonal and annual variations of frontal activity only existed within close proximity to the estuaries. Frontal activity generally showed an interannual weakening trend, especially in the Laizhou Bay (−0.27%/yr), while an opposite trend was identified in the Liaodong Bay (0.10%/yr). This phenomenon was related to the interannual variations of wind speeds. Meanwhile, wind speed anomalies caused by El Niño‐Southern Oscillation events could explain the significant interannual fluctuations in frontal activity such that, frontal activity was significantly enhanced (suppressed) when El Niño (La Niña) events occurred, especially during 2015–2016 (2017–2018). Plain Language Summary: Suspended sediment fronts, one of the most common phenomena in estuarine and coastal waters, are defined as narrow zones with large spatial changes in suspended sediment concentration over relatively short distances. The Frontogenesis and frontal variability of the Bohai Sea, east coast of mainland China have unique patterns because of its complex hydro‐sedimentary environment, but no relevant studies have been systematically conducted yet. In this study, the intensity of frontal activity in the Bohai Sea was quantified by using satellite observation data spanning 10 years. Our analyses revealed that strong frontal activity mainly occurred in areas with dramatic changes in topography and shoreline and distinct tidal features. The seasonal and year‐to‐year variations of frontal activity were closely related to wind waves, stratification, riverine sediment input, and El Niño‐Southern Oscillation events. Nevertheless, their influence mechanisms varied in different regions. The new findings are of significance to further investigate the sedimentary patterns and the ecological effects of sub‐mesoscale processes in the Bohai Sea, and can also shed light on frontal variability of semi‐enclosed shallow seas worldwide. Key Points: Geostationary Ocean Color Imager remote sensing data were used to quantify suspended sediment front variations in the Bohai SeaSpatial patterns and seasonal‐to‐interannual variability of frontal activity were revealedEffects of dynamic factors on the spatio‐temporal variations of frontal activity were analyzed [ABSTRACT FROM AUTHOR]