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Losing our cool: How water can help combat urban heat

Hotter cities are now becoming the daily norm, with global mercury levels spiking by as much as four degrees celsius in recent years thanks to factors such as the buildup of greenhouse gases and the Urban Heat Island (UHI) effect.

The UHI effect, where air and surface temperatures are significantly higher than in rural areas, is largely the result of a shift away from natural, vegetated landscapes to urbanised spaces.

That cities absorb and release more heat than country areas has been known for over a century, but only recently have we come to understand its dire consequences – contribution to extreme weather events, demand on utilities and increased mortality during heatwaves – and sought ways to mitigate it.

“Urban Heat Island is the most documented phenomenon of climate change,” UNSW High Performance Architecture professor and lead researcher of the Cooling Western Sydney study Mattheos Santamouris said.

Conventional wisdom has held that planting more trees was the solution, but Professor Santamouris’s study showed it to be more complex, requiring a multi-faceted approach that includes the use of water technology and high solar reflectance materials on roofs, building facades and pavements.

Professor Santamouris said the recent discovery that heat waves are associated with precipitation of previous months has furthered the need for research and development around large-scale mitigation projects.

“When the moisture content of the soil is low, we don’t have evaporation, and so we have a stronger heat wave,” Santamouris explained. “But once we are using water to increase the humidity of the soil, then we may decrease the strength of heat waves.

“Water technologies have a very high potential and a big advantage, that when used in the proper way may decrease the ambient temperature, not just the surface temperature.

“When there is a radiant temperature of urban surfaces, this has a huge impact on comfort that can be felt immediately.”

Just add water

Comfort is key to the vision of Alan Hoban, a director of engineering firm Bligh Tanner, that is part of the Greater Parramatta to the Olympic Peninsula (GPOP) project that is moving a new CBD to Parramatta. Here, the number of days seeing temperatures soar to over 35 degrees has increased 250% in the last 50 years due to climate change and urbanisation — a perfect UHI storm, resulting in maximum summer temperatures up to nine degrees more than coastal suburbs.

“We are looking at a range of schemes which would tap into the sewer network, draw that water up and treat it to standard, and then supply that to streetscape planning and to green systems on the facades of buildings,” Hoban said.

“You are getting water cycle benefits, using up some of the wastewater load, and creating greenery. The irrigation demands become quite significant.”

Hoban said the challenges and opportunities of urban cooling will change the way that we see water utilities. “Rather than simply being the provider of clean water and treating wastewater, water suppliers are increasingly going to be seen as a provider of urban amenities.”

With this in mind, Hoban said it is important to think about features such as shade and water more holistically to optimise cooling, and the particular demands of climates in individual Australian cities when it comes to having a mitigation strategy.

“As you reach more humid climates, like Brisbane, the shade becomes more significant than evaporative cooling, which is key in places like Melbourne and Adelaide.”

Ready for takeoff

It was flying into Adelaide that SA Water Environmental Opportunities Business Development Manager Greg Ingleton had an idea for a world-first trial using the wastewater facility next door to irrigate the land surrounding the airport.

“We irrigated every second or third night, using the same amount of water as you would use to keep your backyard green over summer,” he said.

“We were getting a three degree or more difference between the irrigated and unirrigated areas.”

With planes unable to take off in very high temperatures, he said such a reduction could be the difference in both the ability to fly and the amount of fuel consumed, on top of reducing energy use in airport cooling towers.

Ingleton then took the program to the next level, transforming the newly-irrigated buffer land into productive land by growing the crop lucerne, a common livestock feed.

“You can’t just throw water on an area in most situations if you want to get the maximum cooling effect,” Ingleton said.

“Utilising crops with a high density and leaf-space is the better way to go. And it even pays for the cost of water.

“The message we’re trying to get across is don’t use more but be smarter in where and when you use your water.”

Domestically, Ingleton recommended increasing education on how houses work, such as growing gardens near air conditioning units and windows, watering grass after work and opening windows, and irrigating at night ahead of hot weather conditions.

Life and death

Hot weather conditions resulted in Victoria’s Black Saturday bushfires in 2009, killing 173 people. But few realise that the extreme heat wave that ushered in the bushfires resulted in the deaths of a further 432 people.

“A recent report suggests that extreme heat is Australia’s deadliest form of natural disaster, and is responsible for more deaths than cyclones, floods and bushfires combined,” explained Sydney Water Research Direction and Value Manager Dr Michael Storey.

“It adds a large burden to our healthcare system and presents a wide array of socio-economic challenges,” Dr Storey said.

Aged care facilities, in particular, are rushing to incorporate green design to mitigate risk and improve amenities, but the cycle of strain on existing infrastructure during hot weather is less easy to fix.

“Energy consumption nearly doubles in western parts of Sydney when compared to cooler eastern parts of the city, and peak energy demand also doubles when temperatures rise from 20 degrees celsius to 40 degrees celsius,” Dr Storey said.

“As we prepare for an increasing population and a drier and more variable climate, we need to look at ways of mitigating urban heat beyond conventional approaches currently used.”

Hotter cities are now becoming the daily norm, with global mercury levels spiking by as much as four degrees celsius in recent years thanks to factors such as the buildup of greenhouse gases and the Urban Heat Island (UHI) effect.

Greener pastures

While the benefits of urban cooling are clear, the question of how to articulate its value, and who will pay for it, has been less apparent.

“An initial barrier to action and implementation is measurement: quantifying the impact of urban heat and the effectiveness of mitigation measures,” Alluvium Consulting Engineer Dan O’Halloran said.

“The CRC has developed a GIS based tool to help designers measure heat within the urban environment and select and locate interventions. As more of these type of tools and the research behind them grows, designers like us can mainstream them into the development plans we provide to councils, authorities, and land developers.”

The investment required for action on urban heat is significant, but the weight of inaction is far greater.

O’Halloran said businesses should be applying this knowledge immediately to avoid future costs of retrofitting. Given the febrile climate when it comes to the issue of climate change, public engagement on the consequences of urban heat and the benefits of mitigation is also critical.

One project giving hope to experts is the Citizen Science Project, a collaboration between RMIT and UNSW, which is using citizens to help measure urban heat islands, overheating, and local climate change.

In asking some 12,000 members of the community to gather the data, it aims to help others understand, mitigate and adapt to extreme heat, and assist policymakers in predicting future health and utility needs as they plan the cities of the future.

First published in Current magazine May 2018.