Flood havoc during 2019 in the Sangu River basin caused widespread damage to residents, crops, roads, and communications in parts of hills in Bangladesh. Developing flood hazard maps can play an essential step in risks management. For this purpose, this study assessed 12 hydro‐geomorphological factors, namely, topographic wetness index, elevation, slope, extreme rainfall, land‐use and land‐cover, soil type, lithology, curvature, drainage density, aspect, height above the nearest drainage, and distance from streams. Maps prepared by individual application of the Analytical Hierarchy Process (AHP) and Analytical Network Process (ANP) exhibit validation scores ranging from 0.77 to 0.79. It is found that the ANP‐based model under 1‐day maximum rainfall denotes a reliable hazard map presenting comparable accuracy to the field results. The hazard map under 100‐year return periods shows that a total of 0.71 million population living downstream is prone to “very high” flood because of its lowland morphology, mild slope, and high drainage density. Alarmingly, 39% of roads, 43% of farming lands, and 25% of education buildings are observed to lie in the highest flood‐prone area. Details on subdistrict level exposures have the potential to serve the decision‐makers and planners in site selection for flood management strategies and setting priorities for remedial measures.

Global food security depends on the sustainability of irrigated agriculture. Rising groundwater withdrawals from seasonally humid, alluvial plains across tropical Asia have enabled dry-season rice cultivation. This groundwater pumpage increases available subsurface storage that under favorable conditions amplifies groundwater replenishment during the subsequent monsoon. We empirically quantified this nature-based solution to seasonal freshwater storage capture described as the “Bengal Water Machine,” revealing its potential and limitations. On the basis of a million piezometric observations from 465 monitoring wells, we show that the collective operation of ~16 million smallholder farmers in the Bengal Basin of Bangladesh from 1988 to 2018 has induced cumulative freshwater capture that volumetrically (75 to 90 cubic kilometers) is equivalent to twice the reservoir capacity of the Three Gorges Dam.

After more than a decade of meeting the designated objective of increasing productivity in agriculture, the South West coastal polders of Bangladesh have ended up as different man-made disasters. The failure of the polders to deliver the intended outcome is basically attributed to the lack of understanding of their hydro-morphological characteristics, inadequacy in their operation and maintenance, and failure to take into account their social relationship and culture roles. Changes in socioeconomic settings have also forced changes in the designated functions of the polders, but now the emerging context of climate change has become a major issue in rationalizing the coastal polders. In this context, this study is an attempt to review the historical and ongoing process of rationalization of the South West coastal polders, revealing that it is essential to take an integrated view of the hydrologic cycle and the interactions of human interventions. Finally, this paper recommends that an extended cost–benefit analysis with a multi-objective focus or a multi-criteria analysis, if monetizing is not possible, should be an option in rationalizing this multi-functional infrastructure. Proper macro-planning would require development of an institution capable of dealing with a task which is multi-dimensional and multi-disciplinary in nature.

Determining soil salinity within the delta is crucial as it is the dominant factor determining crop productivity. There are numerous interacting drivers that influence soil salinity, including climate variability, saline river water inundation, storm surge inundation, depth to groundwater table, groundwater salinity, and shrimp farming (Bagda). For the study area, tidal river salinity appears to influence the soil salinity most, particularly in the south-west of the delta. In northern areas, high groundwater salinity levels, combined with a high groundwater table, are a major contributor to soil salinity. In addition, an increase in salinity of dry season irrigation water is expected to increase salt accumulation in soils, with a possibility of irrigation water salinity exceeding five parts per thousand.