Water as an urban heat sink: Blue infrastructure alleviates urban heat island effect in mega-city agglomeration

Surface urban heat island (SUHI) shows higher surface thermal intensification in urban areas than suburban areas and countryside. Green spaces are widely recognized as the dominant nature-based strategy to mitigate thermal stress. In contrast, natural cooling processes of urban blue spaces are seldom addressed. In this study, we applied the Google Earth Engine (GEE) to map SUHI intensity and blue spaces in order to explore the cooling effect and efficiency of blue spaces in the Pearl River Delta Metropolitan Region (PRD). Significant in-situ cooling is observed, with a 10% increase in water-body coverage leading to a depression of SUHI intensity by 11.33%. The irregular shape of lakes and reservoirs may weaken the cooling effect, but the reshaping of rivers may not. Our results indicate ex-situ cooling spillover of water bodies that will benefit the surrounding areas within an envelope of 100-m effective cooling distance. Such findings underline the importance of urban blue infrastructure in alleviating the thermal discomfort and advocate blue spaces as an essential component of urban infrastructure to minimize the SUHI effect.