The effect of water sensitive urban design and outdoor water-use practices on urban microclimate

Abstract

The excess heat that is stored in urban materials and created by human activities implies that people living in cities are particularly vulnerable to heat stress and heat related illness. In Australia, where extreme weather and prolonged drought are common, heat exposure in urban areas can be significantly exacerbated. The combination of increasing urban development, excessive urban heating, and lower water availability, alongside the impacts of future climate change could have damaging implications for the human health and well-being of urban dwellers. Water management in cities plays an important role in determining urban climates, but minimal work has been done that directly acknowledges these interconnected issues. Integrated water management approaches, such as water sensitive urban design (WSUD), provide a means for retaining water in the urban environment through stormwater harvesting and reuse. This study examines the potential for WSUD to provide cooling benefits and reduce human exposure to heat stress and thermal discomfort. A high resolution observational field campaign, measuring surface level microclimate variables and remotely sensed land surface characteristics, was conducted in a mixed residential suburb containing WSUD in Adelaide, South Australia. At Mawson Lakes microclimate variability was measured, including air temperature and thermal comfort, to investigate the microclimate effects of WSUD features, including man-made lakes, bio-filtration wetlands, and irrigated open space. Additionally, the SURFEX (SURFace EXternalis´ee in French) model was used to simulate the potential for irrigation to reduce exposure to heat stress and provide more thermally comfortable conditions during heatwaves. Clear evidence was found that WSUD approaches and irrigation can provide cooler air temperature and more comfortable thermal urban environments. It was found that the microclimate conditions that people experience in Mawson Lakes vary significantly over small spatial scales. The effects of WSUD features were seen to be highly localised, having little effect beyond 50 m from the source. Air temperature variability was influenced by land surface differences, such as proximity to water bodies. Sites that there were influenced by water bodies were up to 1.8 ◦C cooler during the day. However, human thermal comfort ii (HTC) was less influenced by land surface characteristics, and was mostly determined by contrasts in shading and airflow in the environment. The modelling analysis showed that there is high potential for irrigation to cool air temperature during heatwave conditions; with irrigation a 2.8 ◦C maximum reduction in daily average temperature was predicted. From this research some specific recommendations for the implementation of WSUD have been developed. These include consideration of the local wind regime when implementing WSUD; the use of smaller distributed features throughout the urban landscape at targeted locations; the use of irrigation during heatwaves to provide cooling; and the retention of trees in urban areas to provide shade, and augment the thermal comfort benefits provided by WSUD features. Overall, this research argues that distributed WSUD features can be used to capture, store, and treat stormwater, simultaneously providing ecological benefits and thermal benefits. Further, harvested stormwater can be re-integrated into the urban environment, thereby helping to maintain vegetation and providing distributed evaporative cooling benefits.

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