EFFECTS OF PLANT EVAPOTRANSPIRATION RATE ON OUTDOOR MEAN RADIANT TEMPERATURE IN THE TROPICAL URBAN ENVIRONMENT

Abstract

Urbanisation has brought about adverse environmental impacts such as the Urban Heat Island (UHI) effect, which has resulted in urban regions becoming warmer than rural surroundings, forming an ‘island’ of higher temperatures in the landscape. Various studies have established several ways to mitigate UHI, one of which is to incorporate greenery into the built environment in the form of rooftop gardens, balcony gardens, vertical landscaping, as well as neighbourhood parks. Although studies have shown that the cooling effect of plants has an influence to the surrounding environment, there is no objective criteria for implementing the appropriate type of greenery on the rooftop. In this paper, various factors that contribute to the cooling effect of plants, such as rate of evapotranspiration, are examined to see how it is associated with the reduction of mean radiant temperature. In this study, measurements are conducted for the evapotranspiration rate for 4 different types of vegetation (ie. Phyllanthus Cochinchinensis, Heliconia American Dwarf, Sphagneticola Trilobata, and Cow Grass) and the corresponding effect on mean radiant temperature. The measurement location is at the rooftop of a university campus building. Results obtained are compared against a normal concrete roof which serves as the experimental control. Quantitative analysis is conducted to determine the effect of plant evapotranspiration rate on outdoor mean radiant temperature. Results show that evapotranspiration from green roof has a cooling effect to the surrounding, thus the ability to influence the surface temperature of the roof. Furthermore, evapotranspiration characteristics differ from each individual plant, with a higher Leaf Area Index does not potentially assure to higher evapotranspiration rate or higher reduction of mean radiant temperature. The study shows that it is possible to reduce outdoor temperature by implementing greenery in the built environment. More importantly, plant selection based on temperature reducing attributes is a significant criteria that should be considered by architects and planners in the early stages of design development.