The Sundarbans mangrove ecosystem, the world’s largest contiguous mangrove forest, is under pressure from changing salinity due to climate change, reduced freshwater input from upstream rivers, and tidal dynamics in the Bay of Bengal. Despite its ecological and socio-economic significance, previous studies have not provided a comprehensive, location-specific assessment of salinity that explicitly links spatial heterogeneity with hydroclimatic drivers. This study synthesised and analysed the best available data in the Bangladesh Sundarbans over the 26 years to quantify the characteristics, trends, and interrelationships of these variables spatially and temporally. The results reveal that salinity has been consistently higher in the western part of the Sundarbans, and extreme salinity levels have increased since 2010. There has been a statistically significant decrease in the freshwater inflow from the Ganges through the Gorai River (at the Gorai railway bridge location) since the early 1980s, which has led to increased downstream salinity. Strong statistical correlations enabled robust predictions of salinity as a function of rainfall and temperature. A rising water level trend at Hiron Point is attributed to the rise in sea levels. The absence of long-term measurement data prevents the exploration of salinity dynamics in the numerous creeks that crisscross the mangrove ecosystem. It is crucial to establish continuous monitoring and assessment to better understand and manage the impacts of climate change and development on the Bangladesh Sundarbans and similar coastal ecosystems worldwide. Graphical abstract

Tidal inundation plays a critical role in maintaining the ecosystem services of the Sundarbans mangrove forest. In this study, we configured and calibrated a coupled one-dimensional (1D) river network and two-dimensional (2D) floodplain hydrodynamic model for the Sundarbans in Bangladesh. Model calibration was performed using gauged water levels, inundation maps, and Google Earth (Version 7.3.6) imagery. Using the calibrated model, we assessed potential changes in inundation extent, depth, and duration across the Sundarbans for varying freshwater inflow and tidal height scenarios. Results show variation in inundation extent, depth, and duration spatially and temporarily across the Sundarbans. Inundation is relatively less during February-March (end of the dry season) and high in July-August (mid-wet season). Approximately 3158 km² (85.1%) of the Sundarbans experiences at least one inundation in March, increasing to about 3658 km² (98.6%) in July. Although a large proportion of the Sundarbans inundate during daily tidal cycles, the mean inundation depth remains shallow (0.24 to 0.33 m) due to flat topography. The influence of freshwater inflow on inundations is small (<2%). In contrast, the impacts of tidal magnitude are substantial on both inundation extent and depth. These findings provide valuable insights on inundation dynamics for understanding the hydrological and ecological functioning of the Sundarbans.