Assessment of spatiotemporal distribution pattern of land surface temperature with incessant urban sprawl over Khulna and Rajshahi City Corporations

Abstract

Urban sprawl and urban forest fragmentation are the dominating driving factors of land use land cover changes in city areas and changes in land surface temperature subsequently. Khulna and Rajshahi City Corporations, the major divisional city corporations of Bangladesh, were purposively selected in this study because of their difference in geography and demographic pattern. Khulna is the southwestern city of the country and is near to the Sundarbans, whereas Rajshahi is situated at the northwestern side. The population trend of Khulna has been decreasing day by day after 2001, but an increasing trend is observed in Rajshahi. The present study investigates the urban sprawl, change in urban greenspace, and spatiotemporal pattern of land surface temperature over these two cities from 1989 to 2021 at an interval of 16 years. Based on season, having the least cloud cover and shade-free images of Landsat 4–5 Thematic Mapper (TM), and 8 Operational Land Imager (OLI) and Thermal Infrared Sensor (TIRS) were collected from USGS Earth Explorer. Images of November and April, which had closer dates along with the required criteria, were downloaded for analysis. Land use land cover change was detected by using images from November, and land surface temperatures were retrieved from images of November and April to compare the temperature variation in winter and summer. The result of the study revealed that in 1989, urban green coverage was 21.07% in Khulna City Corporation (KCC) and 21.29% in Rajshahi City Corporation (RCC). The percentage was more than the threshold level of green coverage (20% of total land) required by plain lands. But these proportions reduced severely by 2021, to 11.86% and 8.56%, respectively. Urban sprawl was found as the major driver for destroying greenspace, which increased land surface temperature in both cities, but in Rajshahi City Corporation, it is comparatively more rapid than in Khulna City Corporation. The urban land cover increased by 32.40% in Khulna and by 71.92% in Rajshahi City Corporations during 1989–2021. The Kappa coefficients were found satisfactory for the years 1989, 2005, and 2021 in both cities – 93.03%, 86.3%, 93.15% for KCC, and 92.48%, 78.87%, 92.83% for RCC, respectively. In the time between 1989 and 2021, the overall land surface temperature increased by 2.72 °C in Khulna and 4.76 °C in Rajshahi City Corporations. In KCC and RCC, the total temperature decreased in winter season by 2.14 °C and 0.90 °C, correspondingly. The year 2005 was the warmest year in both cities among all the studied years. The summer season is getting warmer, and the winter is getting colder with the passing of decades. This significant change in temperature must be a sign of global warming. The results of the study will be scientific evidence to the policymakers, planners, city developers, city dwellers, as well as researchers.