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Running on empty: dams dry up

  • Peter Ker
  • May 21, 2009
Sugarloaf Reservoir this month.

Sugarloaf Reservoir this month. Photo: Justin McManus

The long dry is sapping Melbourne's dams and raising questions about the viability of the city's water supply.

It's a sight to raise the spirits. The dam is full to the lip, its body is swollen. For weeks now, water has rushed down a spillway at the side of the dam wall and cascaded down the creek in a torrid, white flurry.

Serious flooding alert for NSW

As Queensland braces through the worst flooding in 30 years, large falls are set to continue to hit NSW over the next few days.

It sounds like a vision of yesteryear, or a postcard from somewhere far away from Melbourne's drying climes. Yet this has been a daily event near the Victorian town of Warburton over the past month, at one of the nine dams that supply Melbourne's drinking water.

Not only is O'Shannassy reservoir bucking the trend for Melbourne's dams - such as Sugarloaf (above), which now resemble bare canyons - it seems to be capable of the impossible. According to official statistics published by Melbourne Water, yesterday the reservoir was holding 86 million litres more water than its total capacity. That's full, in anyone's language.

But the terrible irony is that O'Shannassy is by far the smallest of the city's dams - with its 3 billion litres making up only 0.17 per cent of total storage capacity. It is 356 times smaller than the Thomson - the largest dam - which is only 17 per cent full, but carries more than half the total burden of Melbourne's water capacity.

Since the network of dams was built several decades ago, Melbourne has never had less drinking water. The record low-water mark was broken in mid-April, when storages reached 28.3 per cent of capacity.

With consumption outstripping inflows, a lower record has been set every few days since then, and yesterday the city's storages were at 26.8 per cent.

O'Shannassy's spill is a quirk owing to size and location. It's also surrounded by a patch of forest that's arguably more productive for water collection than any of the catchments surrounding Melbourne's other dams.

Truth be told, the glory of O'Shannassy's spill - all of which is recovered at dams downstream - is little more than a heart-warming exception to an increasingly grim tale.

After a season of broken records, Victoria and its capital city are closing in on a couple more. If dry conditions persist for another 10 days, Victoria could notch up its driest five-month start to a year.

The Bureau of Meteorology says the statewide average rainfall since January 1 has been 99 millimetres - perilously close to the 99.9 millimetres recorded in the first five months of 1967.

The bureau's head of climate analysis, Dr David Jones, says he expects enough rain will fall to protect the 1967 record - but only just.

"For Victoria as a whole, it will almost certainly be the second driest start to a year on record, and that's for records going back to 1900," he says.

By comparison, in the watershed year of 2006 - when rainfall was lower than the amount projected under even the most dire long-term climate-change forecasts - 172 millimetres fell over the first five months.

That was the year that revolutionised attitudes to water in Victoria and convinced a State Government that believed no new sources of supply were needed to spend $4.9 billion on a desalination plant, a northern pipe and the like.

The Melbourne city rainfall gauge - that humble building on the corner of La Trobe and Exhibition streets - is also threatening to set a record this year.

The gauge has captured 94.2 millimetres since January 1, putting it within striking distance of its record low for the first five months - 98.3 millimetres, also recorded in 1967.

Rain gauges in Melbourne's eastern fringe are more relevant to the dams than the city gauge, but Bureau of Meteorology spokesman Blair Trewin says most of the gauges tell similar tales - it has been a wickedly dry start to the year.

In the search for answers, a look around the country can be instructive.

Until 48 hours ago, Perth had enjoyed an Indian summer. The western capital endured its driest start to autumn since records began, and the fine weather was accompanied by temperatures above 25 degrees through much of this month.

South Australia has also endured a crippling drought, leaving Adelaide's closest catchments empty and the city almost totally reliant on the Murray River for its drinking water.

But this band of southern Australia is only half the story, and distant ports to the north and south are enduring cruelly different conditions. A rocky outcrop halfway between Australia and Antarctica, Macquarie Island hasn't always excited mainlanders. According to Commonwealth publications, Captain Douglass sailed past on his ship Mariner in 1822 and described the island as "the most wretched place of involuntary and slavish exilium that can possibly be conceived".

IN RECENT times, however, as its rainfall has soared, Macquarie Island has excited more interest from abroad. Rainfall on the island has risen by about 15 per cent since the 1950s.

"That's certainly consistent with what is being seen at similar latitudes in the northern hemisphere," says Trewin.

Australia's northern extremes, including Townsville, have also experienced a wetter trend in recent years. Heavy rains have caused havoc and endangered lives in parts of northern NSW and Queensland this year, and the rare spectacle of water filling Lake Eyre has occurred for the first time since 2000.

While it can't be said that Queensland's drenching is the balance of Victoria's drought, the Bureau of Meteorology's Dr Jones says that, on a global scale, rainfall increases in one location are offset by decreases in others. "The world always produces about the same amount of rain, so if it's dry in some regions, its usually wet in others. So on a global scale, there is definitely a balancing," he says. "If you look at global patterns over the last 30 years, what we've seen is the high latitudes (the Arctic and the Southern Ocean) getting wetter, and they are getting wetter quite quickly. The most robust change we see in a warming planet is an expansion of the dry subtropical regions toward the poles."

That means Melbourne, Adelaide and Perth - once linked only by a love of Australian football and a transcontinental train line - now belong to the band that is drying up so as to supply increased rainfall elsewhere.

Pinpointing the causes of lower rainfall in this region is both difficult and politically controversial. What is known is that westerly winds influenced by the Roaring Forties - the famous trade winds that drove ships around the bottom of Africa - now reach the Australian continent with less force.

"What we've seen over the past decade is that the westerlies have been much weaker over southern Australia," says Jones. "They historically have bought most of the rainfall to southern Australia (and) … that dominant producer of rainfall in the autumn, winter and spring periods has weakened substantially."

So could Victoria enjoy relief from drought any time soon? The bureau doesn't think so. Under its long-term forecast to July 31, the bureau believes there is a 60 per cent chance of Victoria experiencing lower than average rainfall. It also believes there is now a 50 per cent chance of an El Nino weather event - the sort that typically brings drying conditions - hitting eastern Australia in coming months. That doesn't augur well for Melbourne's dams.

Several months ago, Melbourne Water installed special pumps on the Thomson dam in case water needed to be extracted from the very bottom. The dam is now about 17 per cent full and the pumps are expected to be used if it falls to 15 per cent.

Melbourne Water supply manager John Woodland says it is still too early to say whether the pumps will be needed before storages start their climb through winter and spring.

"The pumps are in and tested and ready to go … it's really too early to say," he says.

"We are reaching our natural low point in our storages before we get into the winter and spring filling season."

Melbourne's nine dams now hold 475 billion litres, which in crude terms is the equivalent of about 475 days' water consumption for Melbourne. The north-south pipe is set to supply its first drops this time next year, and will bring 75 billion litres to Melbourne by December 31, 2010.

Complete water security for Melbourne won't arrive until a year later, when the desalination plant becomes operational.

"The fact our dams are at 26.8 per cent, that's still ample reserves to get us through to the augmentations (desal etc)," says Woodland.

Water security appears to be on the horizon. Let's hope it doesn't turn out to be a mirage.

Peter Ker is water and environment reporter.

Setting a low-water mark

- Melbourne's dams are at 26.8% capacity overall. The largest, the Thomson (left), is 17% full, holding 183 billion litres.

- Melbourne's reservoirs can hold 1.7 trillion litres, of which the Thomson would store 60% - 1.07 trillion litres, if full.

- Cardinia is the second-largest dam, with a capacity of 287 billion litres. It is now at 40% capacity.

- The two reservoirs full or close to capacity are O'Shannassy (100%, or 3 billion litres) and Silvan (86%, or 34 billion litres). They contribute only 0.17% and 2.2%, respectively, to Melbourne's total water storage capacity.

- Melbourne's water storages have, for the most part, dropped progressively over the past 12 years. In 1997 they were at their highest level, starting at about 95% full.

- During 2000, they fell below 50% in autumn. At the end of 2006, storages were below 40% capacity. They first dipped under 30% in mid-2007, before rallying.

- At present, storage levels are consistently below 30% capacity.

- Melbourne consumed 968 million litres yesterday (including Western Water).

SOURCE: MELBOURNEWATER.COM.AU

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