Keeping streets cool and green with stormwater

Urban heat is an increasingly dangerous killer – and it’s only going to get worse as the climate warms. The good news is that tree cover is one of the most effective ways to cool a neighbourhood: a single tree can transpire hundreds of litres of water in a day, providing natural cooling better than many air conditioners – as long as it has enough water to thrive.

Now, a team of Australian researchers have demonstrated that a trick for harvesting stormwater can make trees even more effective.

The study revolves around a device, called the TREENET Inlet, which can harvest stormwater runoff from roads and soak it into nature strips. This device provoked saplings to grow 65% more in height and 60% more in diameter than control saplings, on strips without the devices.

The saplings with stormwater feeding were then 169% better at photosynthesis during dry months.

According to Associate Professor Huade Guan, a hydrologist at Flinders University, trees with the inlet installed also had cooler canopy temperatures – meaning they were less “stressed”.

A paper describing the research is published in Frontiers in Climate.

The TREENET Inlet is a pipe, surrounded by a porous material, about 60 centimetres tall and 30 centimetres wide. It’s installed into a hole dug in a nature strip.

“Water will be stored there temporarily, and slowly soaked into the surrounding soil,” explains Guan.

The inlet is connected to the gutter via a small pipe, so it can collect stormwater.

The researchers tested the inlets in Mitcham Council, in southern Adelaide, on white cedar trees (Melia azedarach).

While the saplings grew better with stormwater, mature white cedars could also transpire more water. On average, trees near installed inlets transpired 17% more water – and this rose to 21% in dry periods.

Guan says that the inlets should work similarly well for other types of tree.

“All trees need water,” he says. “TREENET actually provides more water to support tree growth and functioning.”

The researchers point out that this device, which is now being trialled by other councils, is only one way that stormwater could be used to cool the streets and benefit urban greenery.

“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 PhD candidate Xanthia Gleeson.

“TREENET is good for the street,” says Guan.

“But I think that in Adelaide, we should harvest more water. We should use other structures and other space – for example, residential gardens. Everybody can install something to retain the stormwater in our landscape.”