Cool way to keep streets green

Reports of tree canopy coverage dwindling in Australia’s suburbs is increasing pressure on local governments and other authorities to improve the health of urban street trees.

Flinders University research has found stormwater interception and infiltration that allows water to soak into tree root zones is proving effective for a shady tree canopy in concrete urban environments where extreme heat occurs regularly.

Associate Professor Huade Guan, from the National Centre for Groundwater Research and Training at Flinders University.

The new study, published in Frontiers of Climate, examines how growth, water use, leaf-level gas exchange and productivity of white cedar (Melia azedarach) trees has benefited from a popular stormwater harvesting device, the TREENET Inlet. This intercepts stormwater runoff from roads and soaks it into the soil through a porous well around street trees.

“Increased land surface sealing due to urbanisation and building homes and infrastructure has decreased rainfall infiltration to the soil, decreased vegetation cover and increased demand on mains water resources,” says lead researcher, Flinders University Environmental Science graduate and PhD candidate Xanthia Gleeson.

“As a result, city water management projects using stormwater harvesting and infiltration are increasingly combined with urban greening to support adaptation and resilience to the changing climate,” she says.

The study shows that stormwater harvesting and infiltration by TREENET Inlets provides significant benefit to white cedar trees growing in a suburban street in the City of Mitcham, with mature trees transpiring 17% more water on average for more than a year, and 21% more during dry seasons.

White cedar saplings with stormwater harvesting grew 65% more in height and 60% more in diameter at breast height over a three-year period than saplings without stormwater harvesting.

This is consistent with observed 106% greater stomatal conductance and up to 169% greater photosynthesis rate in dry seasons for saplings supported by harvested stormwater.

Flinders University College of Science and Engineering researcher PhD candidate Claire Moore, chief pilot Robert Keane and Geospatial Information Systems Associate Professor David Bruce (a licensed CASA remotely piloted aircraft operator) conducting aerial research over St Marys, in the City of Mitcham council area in March 2022. The Flinders University drone surveying uses heat sensor and LiDAR technology to record which areas are creating cooler conditions with healthy tree cover, which is an important part of the council’s future planning in the climate change crisis.

The stormwater inlets not only provide 20% more water for cedar tree transpiration in summer but assist root zone moisture at night, when moisture update accounts for about 25% of total daily tree water use.

To comprehend the benefits of water sensitive urban designs (WSUD) for improving our environment, Flinders University researchers are investigating how various WSUD implementations may have helped relieve tree water stress in the City of Mitcham over the past five years.

“It’s clear this passive irrigation directly into street tree root zones greatly benefits mature trees,” says lead author, Flinders University researcher Associate Professor Huade Guan, from the National Centre for Groundwater Research and Training.

“It makes sense because increased stormwater discharge presents risks to marine and other ecosystems, and infiltration in-situ is a low-cost and sustainable alternative.

“Quick urban drainage exacerbates the heat island effects which is raising the stakes on the health and lifestyle risks of extreme climate events under climate change. We need to do more to mitigate the problems as we face more extreme climate events, with summer maximum temperatures likely to reach 50ºC by 2050,” Professor Guan says.

The study team will compare airborne thermal images captured in March 2022 with others taken in 2016, together with satellite images between the two airborne thermal captures.  The drone survey and thermal images from a handhold camera will be included in the analysis.

The work is part of the Green Adelaide campaign to monitor urban heat patterns and tree canopy coverage.

The article, Enhanced passive stormwater infiltration improves urban Melia azedarach functioning in dry season (2022) by Xanthia Gleeson,  Tim Johnson, Gobert Lee, Yifei Zhou and  Huade Guan has been published in Frontiers in Climate DOI: 10.3389/fclim.2022.783905

Did you know:

  •  Urban trees can transpire large amounts of water in dry weather. For example, a mature white cedar tree can transpire 200 litres or more a day (compared to 300 litre average per capita mains water consumption in Adelaide), transferring heat equivalent to running a 12 KW evaporative air conditioner for 12 hours. Natural air conditioning does not have a cost to create more amenable street microclimate.
  • Street stormwater can be harvested and stored to support tree functioning and growth, which otherwise would run into the marine environment (e.g. Gulf St Vincent in SA), with possible harm to coastal water ecosystems with nutrients and contaminants.

    Before and after photos of the City of Mitcham rain garden constructed in Harvey Hayes Reserve, Daw Park receives stormwater from as far as Goodwood Road, soaking it into the reserve’s soil to reduce the potential for flooding downstream, while any overflow is filtered through the pond’s sedges and rushes.
  • Managed aquifer recharge (MAR) is emerging as an leading way to harvest stormwater in constructed wetlands to filter pollutants and pathogens before returning to groundwdater. Stormwater harvesting via aquifer storage and recovery now accounts for about 10% of Adelaide’s water supply. Dozens of MAR schemes operate in Adelaide with a combined recharge capacity of more than 20 million cubic metres a year. They have low costs and high public acceptance.

Acknowledgements: Adelaide and Mount Lofty Ranges (now Green Adelaide) Natural Resources Management Board and the City of Mitcham.

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