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The relationship between land surface temperature and water index in the urban area of a tropical city

This study investigated the pattern of urban heat island changes based on the relationship between land surface temperature and the water index in Banda Aceh City for 1998 and 2018. Landsat TM 1998 and Landsat OLI-TIRS 2018 were used in this study, which represents the conditions before and after the tsunami. Analysis of data used Geographic Information System (GIS) and Remote Sensing (RS). The Land Surface Temperature (LST) was negatively related to the normalized difference water index (NDWI).


Introduction
In general, urbanization emerged as a driver of economic, social, demographic, and environmental change. Urbanization is characterized by the rapid conversion of agricultural land, vegetation, and water bodies, into a built-up area [1]. This land use/cover (LUC) changes can lead to the emergence of an urban heat island (UHI), the main themes in the study of urban planning [2] [3]. Uncontrolled changes in LUC causes serious environmental problems, where the reduction of vegetation will help form UHI [4].
According to several studies, there are many factors that led to the formation of a UHI, which included the normalized difference vegetation index (NDVI), normalized difference built-up index (NDBI), normalized difference water index (NDWI), and population [3] [5]. The land surface temperature (LST) results from the effects of the UHI [6]. NDWI shows the prevalence of regions that have water content in the region recorded using reflectance ratios from NIR and green channels to penetrate light on soil surfaces that have water content and calculate absorptions [7]. NDWI is an indicator of moisture content in vegetation [2]. The higher the vegetation cover, the lower surface temperature [5]. Analysis techniques using Geographic Information System (GIS) and Remote Sensing (RS) are needed in identifying LST and NDWI [3] [5].
The relations between LST with UHI variables have been carried out by many previous researchers, who generally took locations in major urban areas such as as Harare [8], Jakarta, Manila, Bangkok [9], and Tehran [10]. For small cities with tropical climates, it has been studied for Banda Aceh City [11], but it is still limited to the relationship of LST with NDBI and NDVI. The results showed that LST had a positive relationship to NDBI, and had a negative relationship to NDVI [9] [2] [5] [11].
Banda Aceh City is in the tropical area, which lies between the isotherm line in the northern and southern parts of the earth, the area found in 23.5° north latitude and 23.5° south latitude. In the tropical area, the average air temperature is high because the sun is always vertical, generally, the air temperatures lie between 20-30°C and can even reach 30°C in areas with wet tropical climates, high rainfall, and which get rays of the sun all year long. With these conditions, it is necessary to investigate further the relationship between LST and NDWI in Banda Aceh City. The relationship between LST and NDBI and NDVI has been done before [11].

Satellite Imagery
This study used Landsat 5 TM satellite images captured on March 8, 1998, and Landsat 8 OLI/TIRS satellite images dated June 3, 2018 (http://earthexplorer.usgs.gov/) [11]. The satellite images were selected based on a cloud cover, with cloud cover of less than 10%. where MIR = Band 6 (Landsat 8) and Band 5 (Landsat TM); NIR = Band 5 (Landsat 8) and Band 4 (Landsat TM) [5]. There were 5,000 random points were determined in the study area, used for finding the relationship between LST and NDWI for 1998-2018. Scatter plots and linear regression were created for this relationship.

Results and Discussion
The built-up area increased to 62.87% (1998-2018) [11]. This increase reached 100.32% in 20 years. This indicates that during this period there was an increase in urbanization and land conversion for development. A high increase in the built-up area occurred after the tsunami struck Banda Aceh City in late 2004. The average LST also experienced an increase in this period, which reached 5 0 C [11]. Banda Aceh City has experienced an increase in population in twenty years . The growth of the city is clearly visible, marked by a built-up area that increases in width by two times. Land for vegetation and bare land is used as the development of built-up areas, such as for buildings, roads, and other pavements. Surfaces in the structure of roads, buildings, and different pavements soak up more heat from daylight radiation than vegetation [9] [5].
In 1998, wet land dominated the city coastal areas, but in 2018 many wetlands turned into water body (sea) categories. This was caused by a tsunami that was very effective in changing the condition of the coastal area. In addition, there are community settlement areas that fall into the category of built-up areas, which become water bodies [11].
The urban development took place in almost all parts of Banda Aceh City, except in the northwest. The north-west part is developing with low intensity, related to government regulations  which makes this section with low intensity. The growth of settlements and built-up area in coastal areas is not as dense as before the tsunami [14]. This is regulated by the government in order to minimize and avoid casualties if the tsunami hits the city again. Land use in the northern coastal area of the city is only for important public facilities, such as ports, schools, and escape building. For school buildings, a design that can also function as an evacuation site is sought. The construction of settlements is currently directed to locations away from the coast, so that the eastern and southern parts of the city become new growth points [15].
The density of the city centre, with the lack of green open space, will cause high heat and pollution, making the population less comfortable [16] [17]. For this reason, green spaces need to be planned in urban areas such as the east, southeast, and south of the city. This aims to encourage growth into those regions so that it can reduce the trend of growth into the city centre. The development of green open spaces in the northern part of the city, especially in the area around the coast is not to encourage growth, but to minimize the impact of tsunamis that may be recurred [18] [19].

Conclusion
The development of the built-up area of Banda Aceh City in 1998 d. 2018 has doubled, indicating that urbanization is significant in this city. As a result of changes in the LUC, there is a UHI effect, which is shown by an increase in the average LST. LST has a significant negative effect on NDWI. We recommend for sufficient allocation of vegetation and water body in the city and applying green city principles. This is done by increasing city parks, ponds which can also function economically, socially and environmentally.