Geospatial Assessment of the Role of Urban Green Infrastructure in Mitigating Urban Heat Island: a Case Study of Greater Monrovia District, Liberia

Abstract

Concerning the quality of life and the wellbeing of the general public, the Urban Heat Island (UHI) phenomenon has progressively become a significant matter. Several prominent cities worldwide are presently encountering the UHI phenomenon, which happens when urban regions have elevated air and surface temperatures in comparison to their surrounding rural and peri-urban areas. For instance, in Greater Monrovia District, rapid urbanization and related LULC change have led to the creation of UHI effect. However, studies demonstrating the contributions that Urban Green Infrastructure (UGI) can make in reducing LST towards mitigating urban heat island effect in Greater Monrovia District are still lacking. In this regard, the study aimed to assess the role that UGI has played in mitigating UHI in Greater Monrovia District, Liberia for the period 1991-2020, by determining the trends of LULC in Greater Monrovia District, evaluating the link between LULC and LST in Greater Monrovia District, and examining the consequences of the relationship between LST and LULC on UHI in Greater Monrovia District. To this effect, a geospatial analysis was conducted using Landsat images for study area, for 1991-2020. Using ArcGIS software, the images were cropped, pre-processed, enhanced, and colour composited followed by LULC classification, extraction of LULC classes and generation of LULC maps. Additionally, LST was retrieved, NDVI and Emissivity calculated and LST maps created. Thereafter, threshold temperatures were estimated and intensity of UHI calculated by subtracting the LST of the least urbanized reference area (vegetated) from that of the UHI area. The results of this analysis showed that; there has been significant increase in built-up areas from 14.6% in 1991 to 36.1 in 2020, while other LULC classes have reduced in size; bare land from 90% in 1991 to 0% in 2020, shrubland from 49.2% in 1991 to 8.3% in 2020, mangroves from 26.3% in 1991 to 18.5 in 2020 and waterbodies from 32.5% in 1991 to 27.5%. Notably, grassland recorded a slight increase from 17.5% in 1991 to 18.7% in 2020. This could be attributed to the decrease in shrubland. The results also show that the built-up areas have higher mean LST (420C) as compared to land areas under vegetation (shrubland 370C, grassland 350C and mangroves 330C) and waterbodies (300C). Moreover, the vegetation index negatively correlated with LST, demonstrating the role of UGI in mitigating UHI by providing the cooling effect. The built-index on the other hand, positively correlated with LST pointing to the contribution of built-up areas to UHI effect by raising LST. The study concluded that, the conversion of natural and semi-natural lands into impervious surfaces have led to the rise in LST and formation of UHI, and therefore recommends the development of UGI within the study areas to mitigate UHI effect. The study also recommends the adoption and implementation of urban planning policies promoting the development of UGI in every development project.