Soil and river water salinity dynamics in coastal Bangladesh

Changes in soil and water salinity pose critical challenges to agriculture, water management, and livelihoods in deltaic environments globally, and particularly in the densely populated Asian mega-deltas. Using observations from 24 stations over nearly two decades (2004-2022) in the Ganges-Brahmaputra-Meghna delta of Bangladesh, our scientific study examined the influences of seasonal weather and climate conditions, tropical cyclones, and hydrology on the seasonal variability in soil and river water salinity. We applied statistical analyses including cross-correlation, seasonal-trend decomposition, and wavelet analysis to explore the spatiotemporal dynamics of soil and river water salinity. We developed statistical models to assess how hydrological, meteorological, and climatic factors explain the variability of salinity. Pronounced seasonal fluctuations in soil and river water salinity are observed, with levels rising during the dry season and declining sharply during the monsoon season (June‒October). We also observed how tropical cyclones contribute to short-term spikes in both soil and river water salinity, with stronger impacts observed for those making a landfall during early monsoon period (April‒May). Storm-surge-driven polder breaches further exacerbate salinisation, compounded by land subsidence and rising sea levels. Statistical models reveal a significant positive association between soil and river water salinity and sea-surface salinity during the pre-to-early monsoon season. In contrast, the seasonal rise in sea levels during the monsoon coincides with reduced soil and river water salinity due to monsoon rainfall and freshwater discharges to the sea.