Land Use and Vegetation Changes Impact on Thermal Comfort in Urban Open Spaces of Nairobi City, Kenya

Abstract

Urban heat islands (UHI) emanates from the difference in temperature between urban climatic conditions and the surrounding peri-urban and rural zones. Kenya’s capital, Nairobi city, is amongst the cities in Sub-Saharan African (SSA) that are rapidly urbanizing and is directly associated with the UHI co-occurrence. Anthropogenic activities such as rural-urban migration, land cover transformation, and industrial operations have intensified the effects of UHI within the city. Contemporary studies have demonstrated that vegetation within urban areas can regulate urban microclimate, thereby reducing the UHI effects. To support informed decision making on climate resilience planning for Nairobi, there is a need to establish UHI’s spatial distribution and quantify the degree to which mature plant species can improve human thermal comfort and urban livability. This study focused on assessment of land use change and effect of vegetation impact on thermal comfort in urban open spaces of Nairobi City. The objectives of the study were; (i) to investigate the spatiotemporal changes of vegetation cover in local climate zones (LCZs) of Nairobi, (ii) to determine the changes in land surface temperatures (LST) of Nairobi in the period between 1988 and 2018 and (iii) to determine the effects of in-situ plant species on microclimatic parameters within two distinct LCZs in Nairobi’s CBD. Changes in land use/cover (LULC) types were evaluated using three Landsat satellite images; Landsat 4 (1988), Landsat 7 (2002) and Landsat 8 (2018). In the LCZs of Nairobi, forests and rangelands declined in area by 80 Km2 and 127 Km2, about 47% and 24%, sequentially. Built-up areas, urban agriculture and water showed an increase by 195 Km2, 9 Km2 and 3 Km2, an approximation of 71%, 4% and 40%, respectively, between 1988 and 2018. The Normalized Difference Vegetation Index (NDVI) and Land Surface Temperatures (LST) were calculated from the same Landsat images. Results showed an increase in mean LST by 4.15°C and a reduction in highest NDVI values by 0.38. A statistical correlation analysis of LST and NDVI showed a significant negative coefficient (r) value of -0.94, -0.87, -0.90 in the years 1988, 2002 and 2018 respectively. A prediction of the LULC, NDVI and LST to 2033 were simulated to establish the future development possibilities for the next 15 years. The LST gradient trends showed higher surface temperatures on the central and eastern sides of Nairobi. Similarly, predictions (2033) indicate a similar trend for LULC, NDVI and LST. The differences in tree species’ effect on microclimatic variables were established in a park (Central park) and a street (Taifa rd) within Nairobi CBD. In both sites, Terminalia mantaly species provided the best cooling effect with a Physiological Equivalent Temperature (PET) reduction of 9.6°C and 9.3°C in Central Park (Site 1, LCZ B) and Taifa Road (Site 2, LCZ 4), respectively. Tipuana tipu was the second best with 9.2°C and 8.2°C, followed by Cassia spectabilis with 8.5°C and 7.6°C, respectively. Air temperatures in Site 1 (Park) were 2.3°C, 1.3°C and 1.0°C reduced compared to those in Site 2 (Street) at 1pm, 6pm and 8am, respectively. The study highlighted the need for monitoring LULC and LST changes with respect to vegetation cover, to improve thermal comfort for the urban dwellers. Further, it recommended advanced urban climate studies to be conducted involving direct human physiological measurements, and to establish a guide for urban practitioners, with planting plans suitable for green infrastructure developments towards sustainable urban cooling of the wider Nairobi and other SSA cities.