Impact of plant evapotranspiration rate and shrub albedo on temperature reduction in the tropical outdoor environment

Implementation of urban greenery is a widely adopted mitigation strategy to reducing the Urban Heat Island effect. Many studies have shown that urban greenery is effective in reducing ambient temperature (t_a) and surface temperature (t_s). However, there are limited studies on the effect of rooftop greenery on the reduction of mean radiant temperature (t_mrt). 

This study focuses on quantifying the effects of plant evapotranspiration rate (ET) and shrub albedo (SA) on t_mrt in a rooftop garden setting. Field measurement was conducted for three plots of rooftop greenery setup at the roof of School of Design and Environment in the National University of Singapore from May 2014 to December 2014. 

Data collected shows that t_mrt, t_a, and t_s above the rooftop garden canopy are generally lower than above the concrete roof. 525 data points were used for regression modeling of ET, SA and t_mrt. An R² of 0.851 was achieved, highlighting strong correlation between variables. The t_mrt prediction model was validated with 150 data points and was able to estimate t_mrt with sufficient accuracy. Sensitivity analysis provides the range of ET and SA required to achieve the desired t_mrt reduction. A plant selection chart derived from the tmrt prediction model can be used for selecting plants for maximum cooling potential.