Modelling of the surface water salinity in the southwest region of Bangladesh

Abstract

Salinities in the Bangladesh coast are dependent on the annual rainfall, evaporation, freshwater flows discharging from upstream and the impact of climate change. Average salinity concentrations at the coast are higher in the dry season than in the monsoon, due to reduction in freshwater flow from the upstream. The flow of the Ganges in Bangladesh reduced significantly due to withdrawal of water in the upstream boundary. The reduction of dry season flow in the Ganges has led to various water quality, ecological, hydrological and hydraulic problems in southwestern Bangladesh. The source and quality of surface water becoming an important issue for the safe drinking water and also for crop production in the Southwest region of Bangladesh. The simulation of the calibrated salinity model has been carried out in this study to investigate the baseline condition of salinity in the Southwest region of Bangladesh. Several scenario run have been simulated with the calibrated and validated hydrodynamic and salinity model. The scenarios are mainly based on upstream discharge condition. The minimum flow in Gorai River has been considered as the worst condition for salinity intrusion from Bay of Bengal. Another scenario with increase in upstream flow through Ganges connected rivers has been simulated to identify the saline free zone at the most south end zone. The present condition and different flow scenarios have been assessed by MIKE One-Dimensional and MIKE Two-Dimensional Modelling system. The study has been simulated from November 2011 to June 2012. It was observed that, the some of the major rivers of Southwest would be saline free and all other rivers will have significant reduction of salinity due to increase in flow of fresh water through Ganges and its distributaries. Augmentation in the Gorai River flow reduces the salinity in the Pussur River and Sibsa River. The major finding of this study is that 4200 sq. km land will be saline free respectively by increasing the upstream flow. The saline free river water could be used directly for agriculture, domestic water supply (with low cost water treatment) and industrial purposes. It is also observed that, crop production will increase by 4023 sq km area with increasing upstream flow condition.