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Stormwater drainage system helps keep street trees hydrated

Conditions are changing for street trees and other vegetation in urban areas. Increased coverage by impermeable surfaces reduces the infiltration of rainwater into the soil, while urban demands on water resources increase. In addition, global warming may bring raised temperatures and decreased annual precipitation, both of which can prove inhospitable for the growth of urban vegetation.

Street trees and other urban vegetation make many important contributions to urban life, and are a welcome sight in cities. They help to mitigate heat islands, can help reduce air and ground temperatures through shading and evapotranspiration, and contribute to the mental and psychological wellbeing of inhabitants. Yet tree cover is decreasing in many cities due to environmental factors and stressors such as pollution.

Researchers from Flinders University and the University of South Australia have been involved in assessing the benefits of different water sensitive urban designs (WSUD) that could be employed to improve conditions for street trees and other vegetation. In particular, a study published in the journal Frontiers in Climate, shows how the use of a stormwater harvesting device, the TREENET Inlet, has helped relieve water stress for trees in an area of Adelaide, Australia, over the past five years.

“Increased land surface sealing due to urbanization and building homes and infrastructure has decreased rainfall infiltration to the soil, decreased vegetation cover and increased demand on mains water resources,” said lead researcher 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.”

Urban areas often have to get rid of stormwater discharge which may be destructive to ecosystems and present risks to communities living downstream. The TREENET Inlet system intercepts stormwater runoff from roads and soaks it into the soil through a porous well around street trees. The system includes curbside inlets that are designed to deliver passive irrigation to street trees to decrease water stress during summer, to improve tree health, and to enhance urban cooling. 

The current study aimed to test whether the TREENET Inlets increase water availability for street trees, and improve tree leaf gas exchange and growth during the dry season. The study was conducted in a part of suburban Adelaide where the local government authority had installed curbside inlets in a 17.5 ha stormwater sub-catchment area. These inlets were linked to “leaky wells” in the root zones of white cedar (Melia azedarach) trees, with each leaky well able to contain around 130 liters of water. In previous research, TREENET Inlets had been found to intercept between 1.8 and 4.5 kiloliters of stormwater in a year.

White cedar trees of around 60 years of age were present in the study area, and saplings of the same species had been planted there in 2015 by the local authority. Trees just outside the catchment area did not have inlets and leaky wells in their root zones. The researchers investigated mature tree water use, sapling leaf gas exchange rates and sapling growth in trees with and without stormwater inlets.

The results of the research showed that stormwater harvesting and infiltration by TREENET Inlets provides significant benefit to white cedar trees growing in a suburban street. Mature trees transpired 17 percent more water, on average, for more than a year, and 21 percent more during dry seasons. The stormwater inlets provide 20 percent more water for cedar tree transpiration in summer.

Trees that transpire more water help mitigate heat islands and keep both the atmosphere and ground cooler in the region of the trees. The authors calculate that a mature white cedar tree can transpire 200 liters of water per day which, considering water’s latent heat of vaporization, transfers ~500 MJ of heat energy per day. This is equivalent to running a 12 KW evaporative air conditioner for 12 h per day.

The saplings with stormwater inlets grew 65 percent more in height and 60 percent more in diameter at breast height over a three-year period than saplings without stormwater harvesting. This supports the researchers’ finding that saplings with access to stormwater showed 106 percent greater stomatal conductance and up to 169 percent higher photosynthesis rate in dry seasons.

“It’s clear this passive irrigation directly into street tree root zones greatly benefits mature trees,” said study lead author 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.”

By Alison Bosman, Staff Writer

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