Is Perth really running out of water? Well, yes and no

The future of Perth’s urban wetlands is in doubt.
Orderinchaos/Wikimedia Commons, CC BY-SA

Don McFarlane, University of Western Australia

As Cape Town counts down to “day zero” and the prospect of its taps being turned off, there have inevitably been questions about whether the same fate might befall a major Australian city. The most striking parallels have been drawn with Perth – unsurprisingly, given its drying climate, rising evaporation rates (which increase consumption and reduce water yields) and growing population.

So is Perth really running out of water? The answer depends on what type of water is being considered, and what constitutes “running out”.

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Read more:
Cape Town is almost out of water. Could Australian cities suffer the same fate?

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When faced with this question most people think of drinking water, which is of course essential for household use.

It often ignores non-potable groundwater that is heavily relied upon in Perth to irrigate gardens, lawns, ovals, golf courses and market gardens. This water is also used by light and heavy industry, as well as being crucial to the health of wetlands and vegetation across the coastal plain.

Lake Jualbup in Perth’s western suburbs showing periods of low and high water level. Photos by Geoffrey Dean.
saveourjewel.org, Author provided

Perth’s drinking water supplies are largely safe, thanks to early investment in the use of groundwater and in technologies such as desalination. But somewhat ironically, as this recent book chapter explains, the future supply of lower-quality water for irrigation and to support ecosystems looks far less assured.

A drying climate

Perth’s annual rainfall has been declining by about 3mm per year on average, while the number of months receiving at least 200mm of rain has halved. Meanwhile, the annual mean temperature anomaly has increased by 1℃ in southwest Western Australia in the past 40 years and possibly by more in Perth, given the urban heat island effect.

Perth’s rainfall trend, as measured at Perth Airport’s rain gauge.
Bureau of Meteorology

The overall effect is that soils and vegetation are often dry, meaning that rainfall will be lost to evapotranspiration rather than running off into rivers and dams, or recharging underground aquifers.

At the same time, Perth has made major changes to its drinking water supply. The city now relies chiefly on groundwater and desalination rather than dams. For a variety of reasons, drinking water use per person has declined, most notably since the early 2000s when sprinkler restrictions were introduced. Some have switched to self-supply sources such as backyard bores, so for them total water use may even have increased.

Perth’s trends in runoff, population, and water supply.
Water Corporation

The reduction in per capita use of drinking water is just as well, because inflows into Perth dams have fallen from 300 billion litres a year to less than 50 billion. This disproportionate drop in stream flows, even against the backdrop of declining rainfall, means that evaporation from reservoirs can exceed inflows in very dry years.

Since the late 1970s, Perth has increasingly used groundwater rather than dam water. Seawater desalination has also grown to almost half of total supply. Even more recently Perth began trialling a groundwater replenishment scheme to recharge aquifers with treated wastewater.

With the declines in rainfall and streamflow predicted to continue, water security will continue to be an important policy issue over the next few decades. Although both are much more expensive than dam water, desalination and groundwater replenishment look set to secure Perth’s drinking supply, because seawater is virtually unlimited, and wastewater availability increases in line with the city’s growth.

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Read more:
This is what Australia’s growing cities need to do to avoid running dry

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Why are non-drinking water supplies less secure?

Boosting drinking water supplies with desalination or groundwater replenishment is unlikely to resolve the pressures on non-potable supplies. To understand why, it is necessary to understand Perth’s unusual hydrology.

Most of Perth is built on permeable sand dunes, which can soak up even the heaviest rainfall. This allows runoff from roofs and roads to be directed into nearby soak wells and absorption basins.

As well as cheap disposal of stormwater, the sands provide Perth with a place to store excess water from winter rains, which is then relied upon for summer irrigation. As a result, local governments have been able to provide many irrigated parks and sports ovals, and more than a quarter of Perth households use a private bore to water their gardens.

This arrangement isn’t as sustainable as it once was. Groundwater levels are falling under many parts of Perth, forcing the state government to reduce allocations and to introduce a range of water-saving measures such as winter sprinkler bans.

Unlike dam inflows, we don’t yet know the full scale of the reduction in natural groundwater recharge rates. But the question still remains: what can we do to halt the decline of this important water store, particularly as Perth’s population is expected to grow to 3.5 million by 2050?

About 70% of local road runoff and half of roof runoff already recharges the shallow unconfined aquifer, because it is the cheapest way to dispose of excess water in areas with sandy soils. As well as reducing discharge costs, this practice helps to ensure that bores do not run dry in summer.

Perth also has large main drains that are designed to lower groundwater levels in swampy areas and prevent inundation. Some of these waters could be redirected into the aquifer where there is a suitable site.

Don’t waste wastewater

About 140 billion litres of treated wastewater are discharged into the ocean every year in the Perth-Peel region. A further 7 billion litres are infiltrated into the sands as a means of disposal where there isn’t an option for ocean outfall. Recent investigations of these land disposal sites have shown them to be effective in protecting wetlands from drying and providing water for public and private irrigation.

Investigations have also shown that the quality of treated wastewater can be greatly improved when infiltrated through the yellow sands into the limestone aquifer in the western part of Perth. It is suitable for irrigation after a few weeks’ residence within the aquifer.

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Read more:
‘Drought-proofing’ Perth: the long view of Western Australian water

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Without these kinds of measures, local governments will struggle to water parks and sports ovals, to protect Perth’s remaining wetlands, and to safeguard the trees that help keep us cool.

So while drinking water supplies for an affluent city like Perth are reasonably secure, our vital non-drinking water supplies need to be augmented using some of the water we currently discharge into the ocean. As Perth gets even hotter and drier, and green spaces and wetlands are needed to provide much-needed cooling, we can no longer afford to let any water go to waste.

Don McFarlane, Adjunct professor, University of Western Australia

This article was originally published on The Conversation. Read the original article.

Advertisement

Hidden depths: why groundwater is our most important water source

Deep dive: water flows from a bore in Birdsville, Queensland.
Lobster1/Wikimedia Commons, CC BY-SA

Emma Kathryn White, University of Melbourne

Vivid scenes of worried Cape Town residents clutching empty water vessels in long snaking queues are ricocheting around the globe. Everyone is asking, “How did this happen?” Or, more precisely, “Can it happen in my city?” The importance of effective water management has been shoved, blinking, into the limelight.

In Australia we’re watching somewhat nervously, grateful to have been spared the same fate – for now, at least. Experts tell us that the key is “water divestment” – that is, don’t put all your eggs in one basket (or, perhaps more appropriately, don’t get all your water from the same tap).

Perth is held up as a shining example of Australia’s success in water divestment. The city now relies partly on desalination and crucially gets almost 70% of its supply from groundwater.

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Read more:
The world’s biggest source of freshwater is beneath your feet

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Groundwater, the great salvation of parched cities and agricultural development, is the world’s largest freshwater resource. The volume of fresh water in all the world’s lakes, rivers and swamps adds up to less than 1% of that of fresh groundwater – like putting a perfume bottle next to a ten-litre bucket.

What’s more, because it’s underground, it is buffered somewhat from a fickle climate and often used to maintain or supplement supply during times of drought.

Yet caution is required when developing groundwater. Sinking wells everywhere, Beverley Hillbillies style, is unwise. Instead, robust groundwater management is required – defining clearly what we want to achieve and what are we prepared to lose to get it.

Despite the common perception of its abundance, groundwater is not inexhaustible. Its management is fraught with minefields greater and more enigmatic than those of surface waters. It is, after all, much easier to spot when a reservoir is about to run dry than a subterranean aquifer.

Subsidence can be surprisingly rapid, as in the case of this example in California’s San Joaquin Valley.
USGS

Only when aquifer depletion is already quite advanced do we begin to see the tell-tale signs at the surface: metres and metres of subsidence, huge cracks in roads, and dried-up wetlands clogged with dead trees and dried-out bird carcasses.

For the most part, however, groundwater remains out of sight, hidden beneath many metres of soil and rock. We only remember it is there when something goes wrong, such as a drought, at which point people begin raving about groundwater, location, yield, salinity, stygofauna – wait, what?

Actually hardly anyone cares about stygofauna; most people have never heard of these tiny subterranean creatures, and you will certainly never see one as a state emblem. Mound springs? What are they? Clearly being underground has left groundwater with an image problem.

There was much media coverage of water theft from the Murray River, with broadcast journalists reporting breathlessly from tinnies, and dramatic footage of huge pumps sucking swirling brown water from a sluggish river. Film of groundwater pumps sedately slurping water is much harder to get, because bores tend to be on private property, often hidden inside little tin shacks and kind of boring, really.

Groundwater just doesn’t capture the public imagination. Great reservoirs and rivers are evocative of wilderness and adventure; they almost make you want to build a little raft and float lazily away, Huck Finn style. But the thing is, groundwater feeds many great rivers, supplying base-flow, so when we suck water out of wells, in many instances we may as well be sucking out of rivers.

Despite this connectivity, in many regions groundwater and surface water are managed separately. This is akin to treating to your left hand as a separate entity to your right. Regulation of groundwater lags behind that of surface water and, in many parts of the world including the United States, China, India and Australia, groundwater is overexploited and pumped prolifically, leading to severe social and environmental impacts.

Mound springs support unique and endemic ecosystems and bubbling clear cold water, a welcome sight for dusty travellers. And as for the aforementioned stygofauna, well, what could be cooler than a blind cave eel?

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Read more:
Squeezed by gravity: how tides affect the groundwater under our feet

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Groundwater will become increasingly important as a water source as we grapple with growing cities and burgeoning populations, not to mention climate change, which is projected to reduce rainfall across eastern Australia.

It is crucial that we ensure our groundwater management is effective and robust in the face of drought. It is no longer enough just to write management plans; we must put them to the test by running our groundwater models through a range of future climate and management frequency scenarios. We need increased investment in groundwater management planning, and for management to be conducted in conjunction with surface water management.

With many cities’ water supplies drying up before our eyes, we also need to remember to think about the water we cannot see.

Emma Kathryn White, PhD Candidate, Infrastructure Engineering, University of Melbourne

This article was originally published on The Conversation. Read the original article.

Peak Water & Food Supplies

The link below is to an article that takes a look at what is being called ‘peak water,’ as groundwater supplies begin to dry up around the world and its impact on global food supplies.

For more visit:
http://www.guardian.co.uk/global-development/2013/jul/06/food-supply-threat-water-wells-dry-up