Impacts of land-use and land-cover changes on surface urban heat islands in Addis Ababa city and its surrounding

Land-use and land-cover (LULC) change as a result of rapid urban expansion cause land surface temperature (LST) variations. The study aims to analyze urban LULC change and its impact on the seasonal, spatial and temporal Surface Urban Heat Islands (SUHI) of Addis Ababa city and its surrounding from 1987 to 2019 using Landsat images. The result indicates that the Impervious Surface (IS) of Addis Ababa city and the surroundings have expanded from 81.49 km(2) in 1987 to 591.85 km(2) in 2019 with a 6.2% rate of change. On the other hand, vegetation cover which has a high thermal cooling effect has been degraded from 217.66 km(2) in 1987 to 157.8 km(2) in 2019. The spatial pattern of LST increased from northern highlands towards southern lowlands. The mean temperature for January and February 1987 was 26.22 degrees C and 27.76 degrees C, respectively. On 25 January 2002, the study area exhibited the mean LST of 28.25 degrees C, whereas on 26 February 2002, the mean temperature was increased to 31.26 degrees C. On 16 January 2019, the mean LST was 31.14 degrees C, whereas on 1 February declined to 30.62 degrees C. The study area exhibited a high mean LST on 21 March 2019 which was 36.1 degrees C, whereas on 15 October 2019 the mean LST was 25.41 degrees C. The study shows that very high LST exhibited on fallow land (27.78, 30.39 and 33.38 degrees C), crop (26.5, 28.66 and 30.83 degrees C), grassland (26.52, 28.53 and 31.15 degrees C) and IS (26.58, 28.38 and 31.39 degrees C) while low LST found on vegetation cover (22.76, 21.64 and 24.44 degrees C in 1987, 2002 and 2019, respectively). The mean LST has a positive correlation with fraction of IS (R-2 = 0.5152, 0.5855, 0.7184), CL (R-2 = 0.716, 0.6294, 0.7089), FL (R-2 = 0.6373, 0.6138, 0.8667) and GL (R-2 = 0.6513, 0.6073, 0.6442) while negative with VC (R-2 = 0.6295, 0.5601, 0.6357 in 1987, 2002 and 2019, respectively). The mean LST that exhibited in Z1 and Z2 was 25.47 and 26.91 degrees C, 26.06 and 28.88 degrees C, and 29.43 and 32.33 degrees C in 1987, 2002, and 2019, respectively. The IS declined when moving from the center to the peripheral area and the mean LST increased towards the rural area along urban-rural zones (URZs). In the north-south direction along URZs, the minimum and maximum mean LST in January increased from 15.83 degrees C in 1987 to 18.57 degrees C in 2019 and 33.64 degrees C in 1987 to 35.58 degrees C in 2019, respectively. Besides, the minimum and maximum mean LST for February in 1987 and 2019 was 17.57 degrees C and 18.97 degrees C and 32.82 degrees C and 35.21 degrees C, respectively. The minimum mean temperature also increased from 18.24 degrees C to 23.4 degrees C, and maximum mean LST from 32.84 degrees C to 41.9 degrees C from October to March 2019. Along east-west direction, the minimum and maximum mean LST in January was found 18.48 degrees C and 30.8 degrees C in 1987 while it was increased to 23.66 degrees C and 35.69 degrees C in 2019, respectively. The minimum and maximum mean LST in February was also increased from 20.17 to 23.57 degrees C and 30.81 to 34.91 degrees C from 1987 to 2019, respectively. Along northeast-southwest direction, the minimum mean LST in January 1987 was 16.17 degrees C and increased to 18.16 degrees C in 2019, whereas the maximum mean temperature increased from 32.1 degrees C in 1987 to 36.06 degrees C in 2019. The minimum mean LST in February 1987 was 19.43 degrees C and decreased to 18.5 celcius in 2019, whereas the maximum value raised from 32.07 degrees C in 1987 to 36.85 degrees C in 2019. The pattern of LST decreased when moving to URZ60 to URZ90 in the northwest direction of the city center and increased city center to URZ47 and city center to URZ165 in the southeast direction. The study revealed that SUHII was highly concentrated in the urban area than the peripheral area. Therefore, to reduce SUHI and to have a sound environment, vegetation cover with dense trees canopy and greenery areas covered with grasses and trees are very important for the city and the surrounding.