Analysis of Impervious Surface Cover and Land Surface Temperature over Key Cities in Southwest, Nigeria

The land surface where humans majorly dwell is a key feature in environmental suitability. However, there are changes that continually occur over the land surface as a result of incessant increase in human population within the urban area.This study therefore assessed the Surface Urban Heat Island (SUHI) intensity over the southwestern region of Nigeria. This was done by assessing the changes in the impervious surface and vegetation cover for state capitals; investigating the Land Surface Temperature (LST) over the cities and surrounding towns. After this, the intensity of Surface Heat Island over each state within the region was then examined. This was achieved by using remotely sensed Landsat data covering a period of 1984 to 2016, obtained from Earth Explorer. The result for this study showed major changes in the impervious surface cover as the impervious surfaces were shown to increase with time over all the metropolitan cities. The areas initially covered by vegetation around the major urban regions were modified to urban areas over the years. This was clearly seen in the vegetation cover changes over the cities such as Ibadan, Akure, Lagos metropolis, Osogbo, Abeokuta and Ado-Ekiti within the study period. The extent of vegetal cover had decreased tremendously as urban areas increased. The increase in urban areas at the locations indicated by the imperious surface index ranged between 30 % to 57.2 % between 1984 and 2016. Investigating the LST over the area, it was observed that the mean temperature of land surface had increased over the years. The most notable increase is seen between 1984 and 2016 across all the sites. The mean surface temperature values ranged between 25.15^oC and 28.8^oC in 1984 but increased to a range of 30.15^oC and 32.4^oC in 2016. This showed that the areas are undergoing considerable warming of the land surface which had considerable impact on the general environment such as the thermal comfort situation of the inhabitants. In examining the Surface Urban Heat Island intensity across the states, the state capitals were used as the major urban areas. It was showed that though the SUHI is in existence in majority of the states, the reverse was the case in other states. States such as Ekiti, Lagos, Ondo and parts of Ogun and Osun experience the SUHI phenomenon while Oyo witness the reverse. Urbanization is a major contributor to the SUHI phenomenon as it causes the temperature of urban areas to increase more than that of the surrounding rural or sub-urban areas. This is as a result of the material used within the urban areas. These materials possess high thermal capacity and as such retain heat unlike that of the surrounding rural or sub-urban areas. However, this study showed that there might be other contributors to the SUHI around the region, which could be the nature of the land surface within the rural or sub-urban area. Other land surface features such as bareland and rock-outcrops also possess high surface temperature intensity and as such may result in negative SUHI intensity as shown in Oyo. Further analysis on the evaluation of the Landsat derived surface temperature was also carried out. Results showed high correlation values of 0.94, 0.95 and 0.93 for 1984, 2000 and 2016 respectively, indicating a close correlation between the Landsat derived surface temperature and air temperature data. In conclusion, changes in the land surface features and the associated impacts on the thermal environment, need to be monitored continually as human activities over land continue. This will help foster adequate policies in view of sustainable development.