An in-situ measurement and assessment of evaporative cooling effects of low impact development facilities in a subtropical city

The urban green infrastructure such as the low impact development (LID) facility and traditional garden that are relatively small and characterized by decentralized distributions has been proposed as the most effective way to mitigate urban heat through its evaporative cooling effect. Recently, there have been increasing studies on its temperature reduction and evapotranspiration (ET) rate, but few of them correlate ET with external surface temperature reductions. Therefore, this study investigated the evaporative cooling effects, ET rates, and their relationships by the three-temperature (3T) model and ground-based thermal infrared remote sensing. Results show that the cooling effect of both vegetated LID facilities and traditional gardens is significantly stronger than that of non-vegetated LID facilities. Due to a thinner soil layer and lower water connectivity of LID facilities, their ET rates are significantly reduced in the dry period while the evaporative cooling effect of traditional gardens covered by the same vegetation can maintain high. The dependency of their cooling effect can be largely explained by the ET rates. When ET < 0.6 mm h-1, an increase in ET of 0.1 mm h-1 can enhance the cooling effect by 3.66 C. When ET exceeds 0.6 mm h-1, the evaporative cooling effect saturates. Vegetation types and soil water conditions are two main factors that govern evaporative cooling effect. Specifically, shrubs with higher ET rates are more efficient in urban heat mitigation than herbs. The responses of the evaporative cooling effect to soil water availability vary among species, which may require species-specific irrigation regime. These results may have implications on the best management practices for urban heat mitigation by the small widely- distributed green spaces.