Mimicking nature: urban metabolism frameworks guiding decision making

Could a city be as water-efficient as a human body? What would such a city look like? And more importantly, how could we get there?

These are questions occupying the University of Queensland’s Dr Steven Kenway, team member of Catchment scale landscape planning for water sensitive city-regions in an age of climate change research (Project B1.2) at the CRC for Water Sensitive Cities (CRCWSC), and Francis Pamminger, Manager of Research and Innovation at Yarra Valley Water in Melbourne.

Steven and Francis are interested in how understanding the metabolism of cities (seen as organisms) can help us plan for their future water needs. “Urban metabolism gives us a different perspective,” explains Francis. “Using the analogy of a human body, you could argue that a water company is only responsible for drinking enough water, and for how the kidneys and liver work. But they are part of a larger organism, so it’s about looking at what’s best for the whole.”

Using metabolism to measure efficiency

In April this year Steven was the invited speaker on Radio National’s Ockham’s Razor program. “Most of us know a little about the metabolism of our bodies,” he pointed out. “We consume water and food. We use oxygen to convert the food into energy so our cells and organs can function. And we produce wastes.” Likewise, cities consume water and food, use fuel to generate energy, and produce wastes. “Urban metabolism provides a framework through which we can compare the efficiency of cities – not just against each other, but also with natural systems – and identify efficient design.”

This comparison highlights how inefficient the current design of our cities is, for water management. “During the Millennium Drought, Australian cities reused only 1–4% of the water they consumed. In contrast, a human body recycles water 19 times.”

With Australia’s population continuing its rapid growth, water efficiency is crucial. But it’s difficult to design efficient systems without really understanding the movement of water through cities. Analysing Sydney, Melbourne, and Brisbane during the Millennium Drought, Steven identified large flows of unused water: more wastewater and stormwater flowed out of those three cities than the amount of freshwater used, and the water demand was less than half of their annual rainfall.

“There was so much political pressure on the reliability of water during the Millennium Drought that there was no long-term vision. It was just ‘How do we secure our demands now with supply?’ ” Steven recalls. “But if we had designed our cities to capture, store, and use rainfall, there would have been far less pressure on centralised water systems.”

A powerful conceptual framework

Steven argues that our mental models are critical to how we approach problems. “It would have been a challenge sending people to the moon if we thought Earth was at the centre of the universe,” he observes. But present thinking about urban water is just as out of date, and limits the available solutions: “The current conceptual model is about supply equalling demand. You add up the water from all supply points – groundwater, surface water, desalination, rainwater tanks – and make sure supply equals demand. While that’s important, metabolism forces us to do something quite different. A central principle is using a mass balance to account for all flows into, out of, and stored within something.”

The new model, which identifies how the city impacts on each flow, is far more powerful for measuring, understanding, and simulating how our cities will perform in the future. But what exactly is a mass balance?

Imagine a glass. Two taps are pouring water in, and three straws are sucking it out. The critical feature of mass balance is that inputs minus outputs equals the change in water stored. It must all balance mathematically. Accounting for all water flows is very different from a supply–demand balance. “Water mass balances are crucial for understanding how well our cities manage water,” says Steven. “But we’ll need to radically change both the way we evaluate water performance, and the way we compile and manage water data.”

Urban metabolism in decision making

Part of the challenge of water in cities is that issues are complex, interconnected, and changing. Urban metabolism frameworks clearly describe interconnections and trade-offs, thus exposing problems and opportunities.

Steven and Francis argue for urban metabolism’s potential as a powerful decision-making tool. Although not yet substantially applied to real-world problems, two examples illustrate how it might be used.

For Yarra Valley Water, the trade-off between water and energy was crucial in deciding whether to implement water recycling. With a tenfold variation in the energy required to provide water and sewerage services across the jurisdiction, recycling made sense in areas where more energy was required to pump fresh water than was required to recycle wastewater.

Steven and his colleagues assessed the impacts of various water servicing alternatives during a research synthesis activity for the new Ripley Valley development, west of Ipswich in Queensland. “One option was creating lots of impervious surfaces that would shift the flood profile,” explains Steven. “Opportunities also existed for wastewater reuse, and rainwater and stormwater harvesting. That all these options effectively tied together the water providers and wastewater side was seen as valuable.”

From theory to practice

The traditional gulf between managing water supply and managing stormwater, Steven believes, is a kind of administrative compartmentalisation that has kept urban metabolism largely within the realm of academia. “Science has compartmentalised not only our knowledge but how we manage complex systems like cities,” he says. “It’s reduced metabolism to the workings of one cell – rather than how humans interact with their environment. We use energy to solve water shortage problems without thinking about the implications for energy. We lose the systems perspective, so issues like climate change are very difficult to address.”

Yet Steven is optimistic that changes – both at governance and technical levels – are opening doors to unleash the potential of urban metabolism. “As of this year we’ve got, for the first time, a federal agency – the Bureau of Meteorology – with responsibility for much of the critical data for a water mass balance approach. We’re starting to benchmark a couple of cities in collaboration with the Bureau. And we’re looking at how we use metabolism to evaluate scenarios and underpin a vision for our cities. Ultimately, we need good examples of what I think will be the massive and multiple benefits of urban metabolism frameworks.”

Returning to the human body analogy, Francis highlights how easily we can get lost in the detail of how to stay healthy. “Complex advice about what to eat or how much to exercise can paralyse action,” he says. Meanwhile, an actuary uses simple variables to predict health, such as weight and cholesterol levels. What are those critical variables for cities? “I hope urban metabolism will help us identify the key things that every council and water company should focus on to improve cities’ health and liveability.”

Nicola Dunnicliff-Wells for the Mind Your Way team