Mapping Effects of Urban Blue-Green Landscapes on Land Surface Temperature Using Geo-Spatial Techniques: The Case of Addis Ababa, Ethiopia
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Date
2021-06-28
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Addis Ababa University
Abstract
Uncontrolled, unplanned, and unprecedented urbanization characterizes most African cities. Drastic changes in the urban landscape can lead to irreversible changes to the urban thermal environment, including changes in the spatiotemporal pattern of the land surface temperature (LST). Studying these variations will help us take urban climate change mitigation and adaptation measures. This study is intended to map effects of urban blue-green landscapes on LST using geo-spatial techniques in Addis Ababa, Ethiopia from 2006 to 2021. Object-based image analysis (OBIA) method was applied for land use/land cover (LULC) classification using high-resolution imagery from SPOT 5 and Sentinel 2A satellites. Moreover, LST was retrieved from the thermal imageries of Landsat 7 ETM + (band 6) and Landsat 8 TIRS (band 10) using the Mono-Window Algorithm (MWA). Furthermore, linear regression analysis was used to determine the relationship of LST with normalized difference vegetation index (NDVI), normalized difference built-up index (NDBI), and modified normalized difference water index (MNWI). Five major LULC classes were identified namely, built-up, vegetation, urban farmland, bare land, and water. The result shows that the built-up area was the most dominant LULC in the city and has shown a drastic expanding trend with an annual growth rate of 4.4% at the expense of urban farmland, vegetation, and bare land in the last 15 years. The findings demonstrated 53.7% of urban farmland, 48.1% of vegetation, and 59.4% of bare land, was transformed into a built-up class from 2006 to 2021. The mean LST showed an increasing trend, from 25.8oC in 2006 to 27.2oC and 28.2oC during 2016 and 2021 respectively. It was found that LST varied among LULC classes. The highest mean LST was observed at bare land having an average LST value of 26.9oC, 28.7oC, and 30.1oC in 2006, 2016, and 2021 respectively. While the lowest mean LST was recorded at vegetation with average LST values of 24.3oC in 2006 and 26.0oC in 2021; and at water 25.5oC in 2016. The regression analysis showed a strong negative correlation between NDVI and LST, a strong positive correlation between NDBI and LST, and a weak negative correlation between MNDWI and LST. The findings of this study have indicated that LULC alteration had contributed to the modification of LST in Addis Ababa during the period. The regression analysis results further revealed that built-up area and vegetation cover plays a decisive role in the variation of LST in the city compared to urban surface water and altitude. The findings of this study will be helpful for urban planners and decision-makers while planning and designing future urban blue-green innervations in the city.
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LULC, OBIA, LST, NDVI, NDBI, MNDWI