Combined effects of urban blue–green spaces on the thermal environment: a case study of Kobe, Japan

Global climate change has led to the deterioration of urban thermal environments, resulting in serious negative impacts on human well-being. As a countermeasure, the public sector has established ad hoc policies, but there are concerns about the financial sustainability of these policies. This study focuses on economic land-use policies for blue–green spaces, using Kobe City as a case study, and evaluates and discusses economic policies such as the Production Green Space Law by quantitatively assessing the combined effects of blue–green spaces. Using Landsat-8 remote-sensing images from the summers of 2014 and 2019, this study retrieved land surface temperatures (LST) by applying the Radiative Transfer Equation method. The results indicated that (1) the area with LST above 32 ℃ in 2019 showed a significant trend of expansion compared with that in 2014. (2) The LST in 2014 was ranked as follows: built-ups, bare land, farmland, water bodies, grassland, and forests. (3) The impact of landscape pattern metrics on LST varied by grid scale, and the correlation was validated at a grid scale of 1200 m. However, the correlation was not significant at a grid scale of 1500 m. (4) The higher the concentration and area of the forests, the better the cooling effect. Regression analysis revealed that water bodies had a mitigating effect on LST. Water bodies and forests exhibited a weak combined warming effect; however, the diminutive regression coefficients suggested that the overall combined effect was not notable. Moreover, the Law on Productive Green Areas is conducive to improving the urban thermal environmental effect, providing the necessary agricultural production support for the city, and improving the well-being of residents.