Researchers concluded that the catchments got off relatively lightly in 2009, but one of the study’s authors said a similarly big fire in the Upper Yarra and Thomson catchments could decimate supply, leaving Melbourne with just a few months of usable water.

And while the mammoth Thomson Dam was lauded on its completion nearly 30 years ago for having “drought-proofed” Melbourne, only a project that can produce clean water any time such as the desalination plant can truly guarantee the capital’s water supply.

“No matter what happens in the catchments we can still turn on the tap,” reassured Associate Professor Patrick Lane, of Melbourne University.

The vitriolic debate over the need for — and cost of — the Wonthaggi plant, which has sent Melburnians’ water bills sharply higher, has well overshadowed the merits of the facility.

But Dr Lane, principle research fellow in the Department of Forest and Ecosystem Science, recalled just how close Melbourne came to losing its water supply during the devastating 2009 fires. 

“The catchments got off lightly: 30 per cent was burnt, but mostly lightly,” he said. “Neither the Upper Yarra or the Thomson were burnt and that’s where the action is.” 

The equivalent of two Sydney Harbours, the Thomson reservoir (pictured) accounts for 60 per cent of Melbourne’s water storage capacity. 

“The water from the Thomson is piped to the Upper Yarra,” said Dr Lane, outlining potential risks. “If the Upper Yarra catchment burnt . . . and water quality was too poor to supply Melbourne, it would be very difficult to get water from the Thomson.

“Now we have the insurance of the desal plant. This is a game changer compared to four years ago.”

In their study, published in the Journal of Hydrology, the researchers estimate that between 1200 billion and 2530 billion litres (1.4 to 2.8 per cent) less water will flow into Melbourne’s reservoirs over the next 100 years because of the extra water needed to regrow the forests burnt in 2009.

Dr Lane said the reduction was less than expected because of the kinds of trees that were burnt in the Black Saturday fires.

“Mixed Eucalypt forests don’t die: over a few years they recover,” he said.

But Mountain Ash forests, which make up 80 per cent of the catchment, are more vulnerable to fire.

“If burnt at high temperature they die,” said Dr Lane.  

Researchers have found that up to 50 per cent less water flows into reservoirs during the regrowth of burnt or clear-felled forests dominated by Mountain Ash as compared with established “old growth” Mountain Ash forests. 

Climate change and population growth are also expected to strain further Melbourne’s water supply.

“Melbourne’s storages were developed on the premise that every now and then you get a really wet year which pretty much fills them up,” Dr Lane said.

“But in the 2000s it just kept getting drier and drier . . . Will wet years become further apart?

“Modeling says it will be warmer and drier and we will have more extreme bushfire conditions more often. 

“Melbourne’s population is increasing but the catchment is not getting any bigger and rainfall is likely to be less, so there will be less water per person.”

On a positive note, Melburnians are now using less water than in the past.

The  manager of water resources at Melbourne Water, Bruce Rhodes, said in the 1990s water use was 478 billion litres a year on average, while in the last five years that had dropped to 358 billion litres.

“As restrictions have been eased, we have seen water use generally trending at the same level, which indicates that a lot of the behaviours adopted during the drought have been maintained,” he added.

The Wonthaggi desalination plant can produce 150 billion litres of water per year, not quite half of the current annual consumption.

* Pictures courtesy of Melbourne Water.