Study area The study area comprises 140 sub-districts (upazilas) in four regions of southern Bangladesh: Barisal (38 upazilas), Chittagong (30), Dhaka (13), and Khulna (59). The study area spans the southern coastal regions of Bangladesh, with extensions to permit assessment of current and future salinity further inland. The dataset includes monthly soil salinity measures from 41 stations for the period 2001–2009, provided by the Bangladesh Soil Research Development Institute; water salinity measures from 29 stations for 2000–2008, provided by Dasgupta et al. (2014a), elevation data from DIVAGIS; and monthly temperature and rainfall data from 20 BMD weather stations for 1990–2010.
The monitoring stations for soil salinity, river salinity, and weather are located in different places, at varying distances from one another. However, the analysis requires spatial juxtaposition of soil salinity, river salinity, temperature, and rainfall measures at soil monitoring locations. The analysis incorporates river salinity measures for all stations within 30 km of each soil salinity monitor. Relative diffusion impacts are captured using weights for river stations that are inversely proportional to their squared distances from the soil stations. For weather stations, the analysis uses observations for the station that is closest to each soil salinity monitor.
Figure 2 presents average monthly station soil salinity measures for 2001–2009, color-coded in five groups for visual comparison. Soil salinity is particularly high in the Khulna coastal region. The water from the Ganges River, which flows through its tributary, the Gorai River, is the only major source of fresh water for the Khulna region. The offtake of the Gorai is almost nil during the dry season (November–May). The salinity level at the Bay of Bengal during the dry season is also comparatively high; and saline water intrudes via tidal effects through the major rivers: the Baleswar, Jamuna, lower Meghna, Malancha, Pussur, Sibsa, and Tnetulia. The topography of the region is very flat; strong tidal effects sometimes travel 200 km upstream from the coast.
Visual comparison of soil salinity measures with readings from the river stations, Panel a vs. Panel b) indicates some spatial correlation, particularly in the concentration of high salinity in central Khulna. For example, the soil salinity measures in Barisal are relatively higher than their riverine counterparts. Hence, river salinity alone cannot be used as a proxy for soil salinity.
The analysis was conducted in two stages. In the first stage, a spatial econometric model incorporating determinants of soil salinity was specified and estimated. In the second stage, location-specific future soil salinity was projected from the climate-induced changes in its determinants.