Urban Heat Island Mitigation Assessment for Green Infrastructure and Implications for Urban Planning in Zhengzhou City

Abstract

Urban microclimates have undergone extreme changes against the current backdrop of global warming and rapid urbanization, which led to severe urban heat islands (UHI) in cities. For the purpose of assessing GI’s mitigation of UHI and its implication in city, chose Zhengzhou, China, as the study area. A comprehensive literature review, remote sensing, field survey, and statistical analysis were utilized in this dissertation. At the regional scale, performed using the Google Earth Engine (GEE) platform and ArcGIS 10.8, analysis the long-term series Landsat images from 1990 to 2022. The spatial and temporal characteristics of the Normalized difference vegetation index (NDVI), Land Surface Temperature (LST), Land use/land cover (LULC) changes were examined over a 33-year period. Additionally, the Surface Urban Heat Island intensity (SUHI), SUHI level, and SUHI hotspots were also investigated. At the same time, at the neighborhood scale, real-time temperature and vegetation parameters were recorded during the summer of 2022. Statistical analysis assessed the cooling effects of four GI types (urban park, urban forest, urban street, and green roof. Moreover, the microcosmic mechanism of the cooling effect were explored across four vegetation types, Tree-shrub-grass (T-S-G), Tree-grass (T-G), Shrub-grass (S-G), Grass (G). Furthermore, the relationships between NDVI and LST, between green space and LST, as well as temperature variations and the outside distances of different GI types, and temperature variation and vegetation types were analyzed. 1)On a regional scale, the NDVI showed a slight overall increase in Zhengzhou from 1990 to 2022. Notably, Zhongmou country, Gongyi, Xinmi, Dengfeng, and Xingyang cities, as well as Huiji district, all experienced NDVI increase. Conversely, decrease were observed in Guancheng and Jinshui districts, along with Xinzheng city. The lowest NDVI was 2010, while Dengfeng City presented the highest NDVI in 2022. Regarding the LST, there was an overall upward trend, increased from 29.52℃ to 32.59℃. The maximum (Max) LST followed suit, with an increase from 48.15℃ to 55.95℃. Significant LST increases were detected in Zhongyuan, Shangjie, and Erqi districts, with the most substantial changes occurring in the northern and eastern regions of Zhengzhou. LULC changes revealed that both built-up areas and green space expanded, reaching 1013.63 km2 and 316.05 km2, respectively. From 1990 to 2022, the green space percentage grew slightly, with an 11.34% increase, and the built-up area expanded by 107.33%. The growth in green space percentage varied across different ranges. Specifically, for the ranges of 0.00 < LR ≤ 0.20, 0.20 < LR ≤ 0.35, 0.35 < LR ≤ 0.60, 0.60 < LR ≤ 0.80, and 0.80 < LR ≤ 1.00, the increases were -8.86%, 41.15%, 6.37%, 29.10%, and 6.10%, respectively. The SUHI intensity varied in both location and area. Notable expansions were observed in Xingyang, Dengfeng, and Xinmi cities, whereas significant reduction occurred in Jinshui and Guancheng districts and Gongyi city. The highest SUHI intensity was recorded in Xingyang city, and the lowest in Jinshui district, both in 2022. The SUHI level experienced significant changes, with consistently high levels observed in Guancheng, Zhongyuan, Erqi, and Shangjie districts. The high temperature (HT) first appeared in Zhongyuan district in 2000, followed by Guancheng district in 2022. For SUHI hotspots, the hot spot areas increased slightly, and their locations shifted noticeably every ten years. There was a trend toward scattered multi-regional hotspots throughout the city, different with the previous concentrated distribution. 2) On a neighborhood scale, all four GI types demonstrated cooling effects on the surrounding environment. As the distance increased from the GI type, temperatures tended to increase. The cooling effect was found greater in these two vegetation types, Tree-Shrub-Grass (T-S-G) and Tree-Shrub (T-S) vegetation types, then followed by Shrub-Grass (S-G) and Grass (G). Also, the Canopy Density (CD), Photosynthetically Active Radiation Leaf Area Index (Par LAI) index followed a similar pattern that in T-S-G > T-G > S-G > G. In urban park and urban forest, the vegetation types, particularly T-S-G and T-G, had a significant influence on temperature variation. Furthermore, a negative correlation was observed between the NDVI and LST, as well as green space and LST. This shows that vegetation's effect on LST regulation remains consistent across multiple spatial scales. Additionally, a negative correlation was noted between temperature variation and the outside distance of different GI types, as well as between temperature variation and T-S-G/T-S vegetation types. To conclude, this study put forward effective strategies for urban planners and policymakers, and construct a GI system with suitable vegetation types in Zhengzhou. The methodology employed in this study to assess the mitigation potential of GI in addressing UHI can be applied to other cities, thereby enhancing GI planning and implementation efforts. Overall, the research contributes to the assessment of GI in mitigating UHI, providing a new vision of GI providing a cooling effect, and promoting a better quality of life for city residents.