2.1.1. Ecological benefits
Due to the accelerated urbanization process and dense urban buildings, the local temperature, humidity, air convection and other factors of the urban surface have been artificially changed, thus causing the change in urban microclimate (Xu et al. , 2019). Many researchers investigated the distribution pattern of the UHI effect (Peng et al. , 2016), and the relationships between air temperature (Qiu et al. , 2017), land surface temperature (Yu, 2018), and land cover (Gaoet al. , 2019), and their correlation with ecosystem service values (Anjos and Lopes, 2017) and found that urban green space and water bodies had a good UCI effect.
A blue space, as a low-temperature corridor, can effectively alleviate the UHI effect. The shape and contour of water bodies are the main factors affecting UCI (Dai and Yao, 2021). Due to the unique continuity, linear waters can effectively divide the aggregation status of heat islands, block the regional expansion of the UHI effect, and provide heat dissipation channels for heat exchange between the internal and external parts of urban heat islands (Cheng et al. , 2019). Through the physical effect of water evaporation, a surface water body can reduce the temperature of the heat island area well and the closer it is to the center of the urban heat island, the stronger the cooling effect is (Yue and Xu, 2013).
As a natural underlying surface for climate regulation, a green space provides a city with ecosystem services maintaining biodiversity and regulating the urban climate. Vegetation coverage, planting structure, and width of green space affect the UCI effect (Anjos and Lopes, 2017; Yang et al. , 2017), and can strengthen the temperature effect in the horizontal direction of the water body. Compared with densely built asphalt roads and concrete roads, a green space has a faster heat absorption rate and a smaller specific heat capacity (Lo et al. , 1997). Good physical shading can reduce the leaf surface temperature and transpiration can prevent the air temperature from rising (Kobayashi H, 2005), thus alleviating the UHI effect.
Some scholars studied the relationship between green spaces and the cooling effect of water and found that there was a nonlinear relationship between green space vegetation coverage and the cooling effect of water. In addition, the type of vegetation coverage also affects the cooling effect of water (Table 1). Green spaces and water bodies have a synergistic effect in reducing the ambient temperature and their cooling effects on the surrounding environment are mutually influenced (Robitu et al. , 2006; Wu et al. , 2018). Therefore, in a limited urban area, the UCI effect can be maximized by constructing a blue-green space in order to promote the development of a city (Montazeri et al. , 2017). A blue-green space can provide the habitat to maintain biodiversity and regulate urban climate, displaying its ecological service function as a composite ecosystem.