Quantifying landscape-metrics impacts on urban green-spaces and water-bodies cooling effect: The study of Nanjing, China

Urban green spaces and water bodies play major roles in effectively alleviating the urban heat island (UHI) effect by decreasing temperature. Here, statistical correlation and regression analyses were carried out to examine the relationships between metrics (vegetation coverage, water coverage, area, leaf area (LA), and patch shape index (PSI)) and land surface temperature (LST) or cooling effect intensity. Besides, the cooling ranges of green spaces and water bodies were explored by sampling and thresholds detection. Landsat satellite images were used to derive land cover map and LST map. Results show that: (1) average LST within given residential area decreases with the vegetation coverage and water coverage increasing; (2) area and LA are both significantly positively correlated with the cooling effect beneficial to people entering green spaces; (3) increasing the vegetation coverage in the range from approximately 0-28 % can effectively and economically alleviate the thermal environment of residential areas, and the economical optimal green space area and LA are both 0.26 ha; and (4) from 2010 to 2017, the cooling range of green spaces in the central districts of Nanjing is around between 180 and 810 m with the average value of 540 m, while the cooling range of water bodies ranges approximately from 510 to 1260 m with the average value of 840 m.