Effects of Water Bodies with Different Coverage Ratios on Urban Microclimate in Street Canyons: Scaled outdoor experiments

Water bodies have been recognized as an effective strategy for mitigating the urban heat island effect. However, the underlying cooling mechanisms remain insufficiently explored, particularly regarding the magnitude of evaporative cooling and cooling potential at different heights within street canyons under different water coverage ratios. Therefore, this study conducted scaled outdoor experiments from July to September 2025 in a temperate region (suburban Xingtai, China), aiming to examine the effects of different water coverage ratios (0%, 50%, and 100%) on microclimate parameters of wind speed, radiation flux, and temperature within street canyons.

The results indicate that water bodies absorb more shortwave radiation and emit less longwave radiation, resulting in increased net radiation capture and lower albedo. Additionally, water bodies attenuate the wind speed ratio (U_0.25H/U_2H) within street canyons by 16% as water coverage increases from 0% to 100%. With higher water coverage, temperatures of the south-facing wall (T_S-w), canyon air (T_a), ground (T_g), mean radiant temperature (T_mrt), and Physiological Equivalent Temperature (PET) all decrease significantly, with the cooling effect intensifying closer to the water surface. Specifically, the maximum temperature reduction on the south-facing walls is observed at a height of 0.1 m, reaching 4.9°C for 50% water coverage and 5.5°C for 100% coverage in street canyons. In contrast, the cooling effect on the north-facing walls is relatively weaker and shows little difference between the two coverage scenarios. The maximum reductions in T_a at 0.1m height are 0.9°C and 1.3°C in street canyons with 50% and 100% water coverage. Water bodies significantly improve daytime pedestrian-level thermal comfort, with maximum PET reductions of 4.6°C (50% coverage) and 10.0°C (100% coverage), respectively, while their influence on nighttime thermal comfort is negligible. Moreover, the evaporation fluxes of the water bodies in street canyons with 50% and 100% water coverage were quantified, with maximum values of up to 92 Wm^-2 and 155Wm^-2 at 14:00, respectively.

 

Figure 1. (a) Schematic illustration of the thermal effects of water bodies; (b) Schematic illustration of experimental design; (c) Photographs of the street canyons for each experimental cases.