Conjugate Vegetation Model for Evaluating Evapotranspirative Cooling in Urban Environment

Vegetation in urban environment is increasingly being utilized to mitigate the Urban Heat Island (UHI) which intensity is growing due to climate change and increasing urbanization. Trees in cities can offer natural cooling as they provide shading below the crown. Moreover, they extract heat from the air due to the phase change of water at the leaves during evapotranspiration. Urban microclimate models often employ empirical parameterization of vegetation. Estimating the cooling potential of vegetation using such models is often too simplified to capture the detailed effects  of vegetation on varying environmental conditions. Further such models almost never include the response of vegetation in severe conditions such as heat wave and drought.

In this study, we investigate the evapotranspirative cooling potential of vegetation using a coupled airflow, radiation and vegetation model. The advantage of using such model to investigate the response of vegetation in an urban environment is that is provides a detailed description of heat, mass and momentum exchange between the vegetation and the environment. In addition, the water cycle driven by the evapotranspiration process can be numerically modelled. This is important as the transpiration rate through the stomata is directly linked to the water availability at the roots of the plant. Therefore, the proposed method helps us understand the response of vegetation during extreme environmental conditions such as drought and provides a more accurate prediction towards the cooling performance of vegetation.

The flow field is numerical modelled by solving the Reynolds-averaged Navier-Stokes equations (RANS) in OpenFOAM with realizable k-ε turbulence closure model. The vegetation is modelled as a porous medium providing the source/sink terms for momentum and turbulence. Furthermore, a radiation model that differentiates between the short-wave photosynthetically active radiation (PAR) component and the long-wave near-infrared radiation (NIR) component captures the photosynthetic activity of the vegetation. As a result, the heat and mass fluxes (moisture, CO₂ and O₂) inside the vegetation could be determined. At this point, the moisture aspect has been implemented and is presented here.

A study on the cooling performance of vegetation shows that the water availability has a significant effect. When the vegetation is unable to perform evapotranspiration and to extract heat from the flow, the net radiation absorbed at the leaves is transferred back to the environment as sensible heat. Furthermore, in a climate with high relative humidity, the cooling performance of vegetation through evapotranspiration is reduced substantially. Therefore, to ensure that trees provide cooling in such conditions, one solution could be in the appropriate selection of the vegetation species.

Finally, this study provides a firm step towards a more realistic parameterization of the urban vegetation. Furthermore, this study provides an insight to enhance the present urban microclimate models for more accurate prediction of the cooling effect of vegetation.