By Wahid Palash
Bangladesh is thought to be one of the most vulnerable countries of the world to Climate Change and Sea Level Rise. There are a number of environmental issues and problems that are hindering the development of Bangladesh. Salinity is such an environmental problem which is expected to exacerbate by climate change and sea level rise in the future. The coastal area of the Ganges delta in Bangladesh is characterized by tides and salinity from the Bay of Bengal. Salinity intrusion due to a reduction of fresh water flow from upstream, salinization of groundwater and fluctuation of soil salinity are the major concern of the coastal area of the country. The higher salinity levels have adverse impacts on agriculture, aquaculture, and domestic and industrial water use and so. The present temporal and spatial variation of salinity is likely to deteriorate further as a consequence of the external drivers of change (IWM, 2014).
Figure 1: South West region and the Sundarbans forest of Bangladesh
The salinity level in the western part of the South West region (Figure 1) remains higher than the eastern part. This is because the Gorai River, distributary from the Ganges, is the only significant upstream fresh water source in the western part of the region, suffers a serious decline in dry season freshwater inflows under post Farakkha condition. The eastern part of South West region remains less saline as it receives freshwater flow from the Padma and lower Meghna River through Arial Khan, Bishkhali and Buriswar River (IWM, 2014). As a result, salinity levels in the region decrease from west to east as well as from south (the Bay of Bengal) to north.
Seasonal distribution of salinity concentration in the region completely follows the seasonality of the region’s hydrology. Average salinity concentrations at the coast are higher in the dry season than in the monsoon, due to lack of freshwater flows from upstream. The salinity normally builds up from October to the late May, and it remains higher during the dry season, usually from February to May. At the end of May, salinity level drops sharply due to upstream flows and rainfall (IWM, 2014).
The salinity conditions have been deteriorated in the last few decades because of the decrease in flow of the Ganges and empoldering effect. The role of freshwater inflows through the Gorai River to push back the salinity intrusion from the Bay has been reported in many journal papers and project reports. One of such study, carried out by IWM and CGIAR (IWM, 2014), reported that,
“In 1975, India commissioned Farakka Barrage on the Ganges at about 17 km upstream of the Indo-Bangladesh border to divert about 40,000 m3/s of flow into Bhagirathi-Hoogly river system. As a consequence of such a large-reduction of the available flow, the Ganges dependent area in Bangladesh was exposed to serious fresh water shortage. The withdrawal of freshwater flow has resulted in landward movement of salinity front in the Ganges dependent coastal area of Bangladesh. In 1996, Bangladesh and India signed Ganges Water Treaty (GWT) for Ganges water sharing between the two countries. The treaty ensured minimum flow in the Ganges River in Bangladesh during dry season, which improved the salinity condition in the South West of Bangladesh. However, during dry season the Gorai intake is almost cut off from the Ganges and there is no freshwater flow through this river. As result salinity water comes through the major rivers namely Pussur, Jamuna, Malancha and Sibsa in the western part of southwest region and increases the salinity level in the dry period. In 2012, Gorai dredging restored the dry season flow temporarily into the area decreasing the salinity level slightly.”
Hence, the mitigating role of Gorai inflows to control the salinity intrusion in the western part of South West region is very much understandable. Figure 2 shows this perfectly by plotting salinity concentration at Khulna on the Rupsha River against the upstream inflows through the Gorai River.
Figure 2: Monthly Salinity variation at Khulna on the Rupsha River with upstream freshwater flow
The plot also reveals the effect of Gorai dredging where it shows pre-dredged near zero flow through the Gorai offtake (in 2011 dry season) and post-dredged improved flow condition (in 2012 and 2013 dry season).
Meanwhile, the IWM and CGIAR study warns about other external drivers, some of them have already made the salinity problem worse, and some will trigger the problem further in the future. This is why earlier mentioned report also stated that,
“The other external drivers such as population growth and increased water use, climate change are expected to worsen the situation. Land-use change will also have important impacts on the surface water resources. To improve the situation proper water management options, water governance and water infrastructure development (including Ganges Barrage and river restoration schemes) can be implemented.”
Present Salinity Intrusion Scenario
The water is not usable for domestic purposes if salinity is higher than 1ppt, though it is still favourable for crop and livestock agriculture unless salinity exceeds 2ppt. Some freshwater aquaculture is still possible when the salinity is below 4ppt. However, in the south and western part of the study region salinity is higher than 4ppt during the dry season which has intrigued brackish water shrimp farming in Satkhira, Khulna and Bagerhat districts (IWM, 2014).
The base or present salinity concentration map of the South West region is shown in Figure 3. The map was prepared using the results of South West salinity model, run for Oct 2011 to May 2012. The map is not a salinity concentration map of a particular day; rather it shows a maximum salinity concentration for every point of the region that would have experience during 2012 dry season. It has been mentioned earlier that the salinity usually builds up from October to the late May and it remains higher during the dry season, usually from February to May.
Impact of Climate Change and Upstream Freshwater Inflow
The overall impact of climate change with sea level rise and no upstream freshwater inflow are summarized in Table 1. On average, area of having portable water (0 – 1 ppt) at base condition will lose its 4 and 12 percent area during CC 2030 and CC 2050 condition, which is equivalent to 2 and 6 percent of total South West area, respectively. For Gorai zero flow condition, this could be as high as up to 19 percent loss from the base area and this is equivalent to 10 percent of South West region.
Table 1: Effect of climate change and the Gorai River flow on salinity intrusion in the South West region of Bangladesh
A side-by-side comparison of base, climate change and Gorai zero flow salinity maps have been shown in Figure 3. The maps clearly indicate the salinity front towards the northern direction of the region in each scenario. The Gorai dependent area will be more saline in the climate change condition, however, the situation could be worse if there is no dry season flow through the Gorai River, even in a base climatic condition (base sea level and rainfall condition).
Figure 3: Surface water salinity map of the South West region for base, climate change and the Gorai flow condition
The area of having suitable water for irrigation (0 – 2 ppt) at base condition will lose its 3 and 4 percent of the land, equivalent to 2 and 3 percent of entire South West, in CC 2030 and CC 2050 condition, respectively. For Gorai zero flow condition, this could be 13 percent loss from the base area that is equivalent to 8 percent of South West region.
The area of having suitable water for specific fish species (0 – 5 ppt) at base condition will lose its 3 percent of the land, equivalent to 2 percent of entire South West, in both CC 2030 and CC 2050 condition. For Gorai zero flow condition, this could be 7 percent loss from the base area that is equivalent to 5 percent of South West region. The results also show that the area that are suitable for shrimp farming (more than 5 ppt) will gain about 6 to 7 percent land from its base area due to climate change condition, which are equivalent to 2 and 3 percent of total South West area of Bangladesh.
The percentage of area loss due to climate change and Gorai zero flow condition in each water quality category from base condition is shown in Figure 4. The left column chart of Figure 4 shows percentage of area loss over area of each quality group in base condition. The right chart of the figure the same, but the percentage of area loss is shown against total South West area.
Figure 4: Percentage of area loss due to climate change and Gorai River’s zero flow over base condition.
It is quite clear from this analysis that both climate change and reduction in freshwater inflow from the upstream will convert the present fresh water zones into saline zones and lower saline zones into more saline zones. The climate change will enhance the salinity problem of the region further. However, the effect of the reduced Gorai River flow will be higher than that of climate change for the western part of South West region.
IWM. 2014. Salinity in the South West Region of Bangladesh. Institute of Water Modelling, Bangladesh.
Excerpted from Chapter 7 of Surface Water Assessment of Bangladesh and Impact of Climate Change. Palash, W., Quadir, M.E., Shah-Newaz, S.M., Kirby, M.D., Mainuddin, M., Khan, A.S., Hossain, M.M. 2014. Institute of Water Modelling, Bangladesh and Commonwealth Scientific and Industrial Research, Australia.