Global Climate Change & Bangladesh....

Climate change induced enhanced soil salinity

Over thirty percent of the net available cultivable lands of Bangladesh are located in the coastal areas. But it has been observed that all the coastal cultivable lands are not being utilised for crop production, mostly due to soil salinity. Increased soil salinity, in one hand, limits growth of standing crops and affects overall crop production and in the other hand, make the affected soil unsuitable for many potential crops. Soil salinity has been considered as a major constraint to foodgrain production in coastal areas of the country.

It is believed that the impact of climate change on physical system in combination with the effect of sea level rise would cause a net increase in salinity in the already affected soils in the coastal regions. A GCM modelling approach has indicated that, under changed climate conditions the index of aridity will increase during winter (Ahmed et al., 1996). As a result, an increased rate of desiccation in topsoil leading to higher rates of capillary action would be observed. Hence the salinity problem would be accentuated by the impacts of climate change and sea level rise. The extent of increase in soil salinity in a particular area within the coastal zone would determine the extent of crop loss in the affected areas.

It was found that the soil salinity generally increases rapidly in the winter months and reaches maximum values in April. It was found that the impacts of soil salinity would be manifold under the climate change scenarios. It was also found that the estimated crop loss under the severe climate change scenario would be the maximum. Furthermore, more areas would become severely affected by soil salinity and thereby the affected lands would become unsuitable for a number of crops. As a result, the food security of the country would be threatened under climate change.

The soil salinization process : An Overview

An estimated 1.27 to 1.67 billion tonnes of sediments are carried annually by the river systems of the Ganges-Brahmaputra-Meghna basin (MPO, 1986; Milliman and Meade,1983). Before being deposited near the sea mouth, the freshly available alluvium from upstream comes in contact with seawater and becomes salty. Thereafter, it becomes more saline by interacting with the seawater that comes along the high tides and through creeks. More salt deposition occurs when capillary action takes place in the sub-surface and topsoil zones, compounded with high evapotranspiration during winter. However, the severity decreases during the onset of monsoon due to the fact that rainwater dilutes the salinity.

Karim et al. (1990) reported that, the soil salinity starts to increase from August and continue to increase until late April when the first rainfall leaches the salt and dilutes the topsoil. The general pattern of soil salinity build-up has been observed in about 0.833 million hectares of the arable lands in 64 coastal Thanas of thirteen Districts. A database was created by compiling information on soil salinity from more than 1,100 pointsources/stations. The maximum values of soil salinity in April for all the stations under each Thana were averaged. The average values of maximum soil salinity of all the Thanas were categorised in five classes. Based on the salinity classes a database was created and the latter was presented in the form of a map showing the most and the least salinity affected Thanas in the coastal areas of Bangladesh (Karim et al., 1990).

It has been approximated, based on expert judgement, that the salinity pattern under future climate scenarios would change in the following ways: i) under moderate climate change scenario (CCS1, in 2030) 10% of the present non-saline (SO) areas would transform into slightly saline (S1) areas and similarly, 10% of the areas from lower salinity class would be transformed into areas under the next higher salinity class; ii) in 2075, under the severe climate
change scenario (CCS2), 45% of the present non-saline (SO) areas would transform into slightly saline (S1) areas and similarly, 45% of the areas from lower salinity class would be transformed into areas under the next higher salinity class.

The extent of area under each salinity class for each of the 64 Thanas were determined for all the three climate change scenarios. The relative fraction of each of the salinity affected class with respect to the total salinity affected area have been computed for all the Climate Change Scenarios (CCS).

Soil salinity development

The results of computer modelling on monthly averaged soil salinity development are shown in Figure-6 (a and b) for the months of August and April under the baseline scenario, CCSO. For comparison, the severity of salinity intrusion for the three CCS are shown in figures 7a,b,c and 8a,b,c considering soil salinity of the months December and April, respectively. It may be observed that in CCS2, under the severe climate change scenario, many of the presently lowly affected Thanas would become severely affected.

It is observed that the soil salinity increases at a considerably faster rate in drier months (i.e.,december to April) compared to the rate observed in pre-winter months. Such an observation clearly suggests that the problem concerning soil salinity is related either to low flow conditions in the surface water systems or the desiccation effects of winter climate which is marked with high evapotranspiration. Since most of the coastal areas in the south-west and southcentral regions, with an exception of the Sundarbans, are embanked, low flow does not have profound effect on the salinity build-up on soils. Desiccation through high evapotranspiration along with upward movement of saline groundwater through capillary rise is mainly responsible for salinity intrusion in the soils of the coastal areas. From the above observation one may expect that effects of climate change would accentuate the salinity problem, as reported by Ahmed et al. (1996).

Soil Salinity build-up under baseline scenario

Click for a larger view

Increase in average soil salinity for December under the three climate change scenarios

Click for a larger view

Increase in average soil salinity for April under the three climate change scenarios

Click for a larger view

Source: Huq. S., karim, Z., Asaduzzaman, M., Mahtab, F., 1999, Vulnerability and Adaptation to Climate Change for Bangladesh, Kluwer Academic Publishers, The Netherlands. and

Global Climate Change : Bangladesh Episode, 1997, DOE, MoEF-GOB.