Urban ponds as nature-based solutions for heat mitigation and temperature regulation in highly urbanised regions

Understanding of the role of urban blue spaces in mitigating heat risks is increasingly important as the climate changes, cities densify and the urban population, and ages, all of which heighten population exposure to extreme heat. Although urban ponds and other water bodies are a common feature of many cities, changes in the quantity, areal extent and cooling ability of urban ponds and subsequent implications for heat exposure are rarely explored. We examined this issue in the East Harima region of Japan, where urbanisation and industrialisation have progressed in recent years, using remote sensing imagery from early summer in 1985 and 2022 and spatial statistical analysis. Urban expansion increased impervious surfaces, while water bodies and forested areas decreased. The number and patch size of urban ponds decreased, but changes in their cooling effect varied. Of 337 ponds identified in 1985, 306 (90.80%) provided cooling effects. Average cooling was 2.95 °C and the mean cooling effect was 0.0230 °C/m. Of 274 ponds that remained in 2022, 206 (75.18%) provided cooling effects. Average cooling was 3.70 °C and mean cooling was 0.01855 °C/m. In central areas, the thermal gradient between water bodies and impervious surfaces enhanced the cooling effect due to an increase in buildings and reduction of green spaces. However, pond fragmentation or disappearance and installation of floating solar panels undermined cooling, turning ponds into local surface heat islands. These findings demonstrate that small urban ponds which are meaningful to urban heat mitigation, particularly in densely populated areas. Where pond quantity and size remain constant, and the density of impervious surfaces increases, cooling benefits derived from these urban blue spaces can increase. However, increased impervious surfaces often accompany pond deterioration. Further research is needed to fully understand and maximise the potential of urban ponds in enhancing urban resilience.