Use it or lose it: Turn on Desal Plant in Wet Years to Beat Next Dry, Expert Says

Use it or lose it: Turn on Desal Plant in Wet Years to Beat Next Dry, Expert Says

Melbourne's water storages could collapse during the next big drought unless the city's dams are regularly fed with desalinated water, even during times of good rainfall, a senior water planner says.

 By Adam Carey

Chris Williams, the manager of integrated planning at Melbourne Water, rejected the notion that Victoria's multibillion-dollar desal plant should only be used when the city's dams start to run low.

Victoria's desalination plant. in Wonthaggi.

Victoria's desalination plant. in Wonthaggi. Photo: John Gollings

He warned that such a policy would risk a repeat of the collapse of Melbourne's water storages in 2006, at the peak of the so-called Millennium drought, when Melburnians saw dam levels fall 20 per cent in just one year.

The accumulative effect of 13 years of low rainfall and low stream flows saw Melbourne lose 360 gigalitres of water – about 20 per cent of the city's storages – in 2006.

The desalination plant is capable of producing up to 150 gigalitres of water in a year, well under half of what the city lost at the height of the drought.

"If you are just relying on watching the levels in storages gradually diminish, and then turn on the desal plant as an 'in case of emergency break glass' option, then you're going to fail," Mr Williams said.

Addressing a public forum on national water reform, Mr Williams said the vast Thomson dam, east of Melbourne, was the city's best defence against drought.

When full it contains 60 per cent of Melbourne's stored water.

The Productivity Commission released its draft report into national water reform on Friday.

It found Australia's cities need to plan better to ensure water supply is secure in a time of rapid population growth and climate change.

Consumers in Melbourne and Perth would pay between $3.2 billion and $4.2 billion over 20 years for desalinated water, due to rushed and poorly planned desal projects, the Commission found.

Mr Williams said to guard against Melbourne's next drought it made more sense to use desalinated water and preserve the water in the Thomson Dam.

"It really underpins water security and our ability to be resilient against those long-term droughts," Mr Williams said.

"Using all the tools that you have, including the Victorian desalination project, can help us keep that bank of water back in the Thomson Dam to withstand [drought]."

Modelling by Melbourne Water found that Melbourne could face chronic water shortages within 10 to 12 years, under a high population growth, low rainfall scenario.

This could trigger a plan to boost the size of the Wonthaggi desalination plant to 200 gigalitres a year.

This year the Andrews government set a new threshold for Melbourne's water authorities, including Melbourne Water, to begin to prepare for drought.

Storages are currently 68.6 per cent full.

Should the city's dams drop below 60 per cent full by the end of spring, a range of drought-proofing measures, including potentially turning the desalination plant to full capacity, would be triggered.

The previous threshold was 54.1 per cent.

In May the government ordered 15 gigalitres of water from the desalination plant.