Abstract
Extensive green roof is one of the sustainable urban stormwater management alternatives to manage and mitigate the urban surface runoff. In order to implement green roofs more effectively, suitable plant species and substrate components for tropical climate must be identified. The aim of this study is to investigate the evapotranspiration (ET) behaviors in extensive green roofs based on different substrate types and local native plant species. Four green roof test beds containing pro-mixing pot and burn soils were each vegetated with Axonopus Compressus (grass) and Portulaca Grandiflora (sedum). A weather station with soil moisture sensors was installed to measure the weather and soil moisture data. The results showed that the mean ET rates for grass-pot soil, sedum-pot soil, grass-burn soil and sedum-burn soil were 1.32 ± 0.41 mm/day, 2.31 ± 0.72 mm/day, 1.47 ± 0.39 mm/day and 2.31 ± 0.43 mm/day, respectively. It is noted that environmental parameters such as ambient temperature, solar radiation and wind speed showed significantly positive relationship (p value < 0.01) with ET rates of green roofs except relative humidity. The crop coefficients (Ks) for the studied green roof plant species are estimated based on actual and reference evapotranspiration rates. The sedum planted in burn soil showed the highest crop coefficient (0.64), followed by sedum in pot soil (0.62), grass in burn soil (0.39) and grass in pot soils (0.37), respectively. The findings in this study also showed that substrate with better water retention capacity generally improved the Ks values.
Highlights
As Malaysia shifts to building itself up as a high income and developed nation, its urban growth tier has risen swiftly, especially in the last ten years
Based on the weather data, it is seen that February, March and April are classified as dry periods with high temperature, low relative humidity and high solar radiation
Most of the substrate moisture content results showed that green roof planted with sedum species contains higher initial moisture content compared to the green roofs with grass species
Summary
As Malaysia shifts to building itself up as a high income and developed nation, its urban growth tier has risen swiftly, especially in the last ten years. Evapotranspiration and ground water recharge have lessened [2,3,4,5,6] These processes have caused the increases of surface runoff volume and flow rate, resulting in the rising frequency of urban flooding [7,8,9]. Rooftops and transportation systems typically make up roughly 80% of impervious surfaces [11]. Since rooftops typically remain unused and access to open areas at ground level are restricted, the implementation of green roof is a rational option for bringing more vegetation to urban cities [12,13]. According to Mentens et al [14] and Stovin et al [15], unused rooftop space makes up roughly half of impervious urban surface areas. Rooftop runoff is a larger threat to water quantity for urban catchments in comparison to rural catchments since the runoff is able to enter the receiving water bodies with ease at a faster rate, due to the direct rooftops’ connectivity to drainage systems
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