Abstract
The southwest coastal region of Bangladesh, being under tidal influence and dependent on sweet water supplies from upstream, has a unique brackish water ecosystem. The region, having vast low-lying areas enclosed by man-made polders, is considered to be highly vulnerable to climate change induced hazards. In this study, linear trends in hydro-climatic variables, such as temperature, rainfall, sunshine, humidity, sweet water inflow and tidal water level in the region are assessed using secondary data and following both parametric and nonparametric statistical techniques. Correlation between the sweet water flow from the Gorai River, a major distributary of the Ganges River, and the salinity level in the Rupsa-Pasur River near Khulna, a southern metropolis, is also investigated. The results reveal that the temperature in the Khulna region is increasing at a significant rate, particularly in recent years. The number of extremely cold nights is decreasing and the heat index is increasing. The sunshine duration has a decreasing trend and the humidity has an increasing trend. Rainfall is increasing in terms of both magnitude and number of rainy days. However, the annual maximum rainfall and the number of days with high intensity rainfall have remained almost static. The annual maximum tidal high water level is increasing and the annual minimum low water level is decreasing at a rate of 7 - 18 mm and 4 - 8 mm per year, respectively. There is a negative correlation between the Gorai flow and the river water salinity around Khulna. Dredging of the Gorai during 1998-2001 resulted in an improvement of the salinity situation in the Khulna region. The variation in water salinity, tidal water level and sweet water flows in different time periods indicates that the human interventions through upstream diversion and coastal polders have contributed more in hydro-morphological changes in the southwest than the climate change. However, there are some evidences of climate change in the meteorological variables at Khulna.
Highlights
Bangladesh is a low-lying, deltaic country with sub-tropical monsoon climate
This study investigated the long-term trends in temperature, rainfall, sunshine, humidity and tidal water level in the Khulna region using secondary data
The analysis of tidal water levels of the Rupsa-Pasur River at Khulna for a period of 74 years (1937-2010) indicates that the annual maximum high tidal water levels are increasing at a rate of 18 mm per year and the annual minimum low tidal water levels are decreasing at a rate of 8 mm per year (Figures 5 and 6)
Summary
Bangladesh is a low-lying, deltaic country with sub-tropical monsoon climate. About 10% of the country is hardly 1 m above the mean sea level and one-third is under tidal excursions [1]. Storm surge induced flooding, riverine coastal flooding, water logging, salinity intrusion and coastal erosion are the main climate and hydrologic hazards in the area. The inundated area could increase by about 11% due to the rise of sea level by 88 cm. Though there have been a few studies on the impacts of climate change on coastal areas using scenario-based model studies [4,5,6], no study has been conducted on existing trends of hydro-climatic variables in the coastal region, in and around Khulna. This study investigated the long-term trends in temperature, rainfall, sunshine, humidity and tidal water level in the Khulna region using secondary data. The variation in tidal water level around Khulna in relation to sea level rise and human interventions is explored
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