Wetlands are an effective green roof system
Introduction
Urban development alters the environment in many ways, including raising temperatures in cities above those of surrounding areas. This “heat island effect” results from landscape modifications that replace green surfaces with buildings and pavements. These structures are made of materials that retain heat more than natural surfaces. Further, buildings discharge anthropogenic heat and gaseous pollutants that restrict normal patterns of airflow. Green spaces that are necessary to buffer these effects simultaneously disappear. Commensurate with its rapid development into the world's third largest megacity, Seoul, Korea, has experienced one of the highest rates of temperature increase over the last few decades [1].
The development of green areas may mitigate, to some extent, the heat island effect. However, in most cases, available space for greening is limited to rooftops and the outside walls of buildings [2]. Rooftop gardening can be used to create green, living roofs which are expected to reduce absorption of excessive solar energy, thus resulting in a significant savings in the energy used for air-conditioning in summer [3]. Green roofs also serve as insulation in cool weather [4]. Also, green roofs have other social costs benefits such as carbon reduction, air quality improvements, provision of recreational space and Habitat creation [5]. Green-roof methods have advanced with studies of appropriate species and growing media [3], [6], [7], [8], system design [9], [10], thermal and energy properties [11], [12], [13], and economic and environmental impacts [14], [15]. However, the possible use of wetland plants or systems for green roofs has been largely overlooked.
Species of drought-adapted succulents (genus Sedum) currently are favorites for populating green roofs [16]. In comparison, the aqueous barrier provided by a wetland roof system should have better evaporation and insulation performance properties than the substrate used for terrestrial green roof systems. The higher water holding capacity of wetland plants, because of minimal leaching associated with them, increases their effectiveness. Therefore, using wetlands for green roofs offers distinct advantages in comparison to those provided by traditional green roof systems. Additional benefits of creating wetlands in urban areas include improvement in air quality, microclimate regulation, noise reduction, and added recreation-cultural value [17]. To evaluate these advantages, we studied the tolerance and performance of several wetland species under rooftop conditions. We then constructed a pilot-scale wetland to test its actual performance and features under rooftop conditions.
Section snippets
Research site
Field measurements were carried out on the rooftop of a six-story building (the College of Natural Sciences building; 37°27′ 31.01″ N, 126°56′ 53.67″ E; 100 m above sea level) at Seoul National University, Seoul, Korea. The concrete rooftop was recently covered with urethane elastic water proofing coatings (top layer is covered with a mixture of trimethylopropane-neopentyl glycol-1,6-hexanediol-phthalic anhydride-adipic acid copolymer, toluene, methyl methcrylate, butyl acrylate-2-hydrooxyethyl
Temperature, humidity and precipitation
The mean rooftop and wetland temperature and precipitation on the rooftop are shown in Fig. 2. The mean rooftop temperature during the experiment was 21.7 °C and the mean relative humidity was 46.3%. The average external temperature during the experiment was 21.1 °C. The highest rooftop temperature recorded was 38.0 °C on August 5, and the highest wetland temperature was 33.1 °C on the same day. The highest one-day mean rooftop temperature was 32.3 °C on August 3, and the lowest mean one-day
Discussion
Rooftop temperatures (ranging from 14.4 °C in October to 26.6 °C in August) fluctuated considerably more than those in the wetland (ranging from 22.1 °C in September to 25.8 °C in August). Although the wetland temperature was not measured in October, the average rooftop temperature in September was 19.8 °C, much lower than that of the wetland. The highest recorded rooftop temperature, at 38.0 °C, was five degrees warmer than that of the wetland, 33.1 °C. As water is a slow transfer medium for
Conclusions
The temperature of a constructed wetland on a rooftop was more stable than that of the ambient air of the rooftop, due to the high heat release and insulation ability of water. The highest rooftop temperature recorded during the study 38.0 °C, but was only 33.1 °C in the wetlands, clearly demonstrating he thermal benefits of the wetland system. The total evaporation of macrophyte plants during the study period was about 1700 l water/m2, indicating their ability for cooling. The constructed
Acknowledgments
This study was funded by the Rural Development Administration of Korea (Project No.3344-20120010).
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Contributed equally.