IMPACT OF SOIL AND WATER RETENTION CHARACTERISTICS ON EVAPOTRANSPIRATION BEHAVIOUR OF PLANTS IN A GREEN ROOF

Abstract

Green roofs are often used to counter rising temperatures in the built environment by reducing heat gain into the building through shading and evapotranspiration (ET). However, studies on suitable soil and water retention characteristics for green roofs are limited. This study examines the impact of soil and water retention characteristics on ET rates in a green roof. Three setups of planter boxes were placed on the roof of School of Design and Environment 1 (SDE1) in the National University of Singapore (NUS) from July to September 2015. Set-ups KA and KC consists of proprietary lightweight substrate while Setup KB consists of topsoil. To vary water retention characteristics, only Setup KC incorporates the use of an additional water retention layer. Field measurement was conducted to collect data on ET, soil moisture and weather conditions. Photos of the set-ups were taken to observe plant growth. Data show that the water retention layer is effective in maintaining about 75% higher level of soil moisture when irrigation was withheld. KC also showed higher ET rates of 4 to 6 kg m-2 d-1 compared to about 2 kg m-2 d-1 for KA and KB during the same period. It is also observed that typical topsoil is better able to support plant growth than the proprietary lightweight substrate, based on measurement of biomass and recovery of plants after the period for which irrigation was withheld. These results can be used to identify suitable substrate and water retention characteristics to be used in green roofs in order to optimise ET rates.