Quantifying the transpiration cooling effect of urban vegetation in a subtropical megacity

The urban thermal environment is deteriorating by the combination of rapid urbanization and global warming, raising increasing interest in the eco-hydrological solutions for urban planning and design, especially in the vegetation transpiration cooling effect. However, due to the high heterogeneity of the urban underlying surface, most of the current understanding on the characteristics of the urban thermal environment and urban evapotranspiration is qualitative, making it difficult to understand the characteristics and cooling mechanisms of urban vegetation with limited data. Therefore, we applied the mobile transect method with high spatial and temporal accuracy to quantify the characteristics of urban thermal environment and the impacts of vegetation. Meanwhile, the evapotranspiration of typical urban lawn, shrubs and tree, were respectively observed by Bowen ratio energy balance method, three-temperature model + thermal infrared remote sensing method and sap flow method. Afterwards, the cooling effect of typical urban vegetation types are studied from the perspective of energy balance. Our results showed that the natural underlying surface had a significant cooling effect, which was more significant at night. A 10% increase in the natural underlying surface could cool the urban heat island intensity by 0.15℃ during summer daytime and 0.27℃ during summer nighttime. The average daily evapotranspiration of lawn, shrub and tree in the study area was 2.29 mm day^-1, 2.90 mm day^-1 and 1.30 mm day^-1, respectively. In the summer of 2015, the lawn, shrub and tree consumed 86.15%, 66.62% and 32.52% of the net radiation in the form of latent heat flux and they could cool down a 10 m³ air column at a rate of 1.06 ℃ min ^-1 m^-2, 1.26℃ min ^-1 m^-2, and 0.44 ℃ min ^-1 m^-2, respectively. These results could be useful for urban thermal environmental management and urban planning to adapt to global change.