Understanding the relationship between urban blue infrastructure and land surface temperature.

Urban heat island (UHI) effect has serious negative impacts on urban ecosystems and human well-being. Mitigation of UHI using nature-based solutions is highly desirable. It was well known that urban green infrastructure (UGI), i.e., urban vegetation, can effectively mitigate UHI effect. However, the potential of urban blue infrastructure (UBI), i.e., urban surface water, on UHI mitigation is not well understood, although its potential to lower UHI effect via evaporation is similar to the biophysical mechanism of evapotranspiration through vegetation. In this paper, we study the relationship between UBI and land surface temperature (LST) in Wuhan city in central China, using a normalized difference water index (NDWI), maximum local cool island intensity and the maximum cooling distance as indicators for the cooling effects of UBI, respectively. We found a significant negative linear relationship between mean LST and NDWI after NDWI passes a critical threshold value. NDWI is an effective biophysical parameter to delineate the spatial distribution of UBI. The cooling effects of UBI are influenced both by its size and shape. Water surface temperature decreased logarithmically with increasing UBI size, critical threshold values of UBI size corresponding to maximum cooling efficiency do exists. Maximum cooling distance and maximum local cool island intensity are also affected by the shape and size of UBI, and exhibit seasonal and spatial variations. These results provide insights for urban landscape planning regarding how to use UBI as a nature-based solution to improve urban thermal environment.