VICTORIA'S stressed water storages and rivers — both at record lows — could face a nightmarish year of severe drought as the odds firm of an El Nino weather event hitting Australia within months.
Such an event could force Melbourne onto stage 4 restrictions, with the Yarra River unable to bear further reductions in flows to support storages upstream.
According to the Bureau of Meteorology, half of its long-range climate models are predicting an El Nino, while the other half are predicting neutral conditions.
"They have steadily been going towards an El Nino throughout the year," a senior climatologist at the bureau, Dean Collins, said.
"Half of the models are going for an El Nino. It's a concern; it's something we're watching."
The El Nino-La Nina cycle, a natural part of the climate cycle, is generated by the temperature of the Pacific Ocean. Warmer waters can trigger an El Nino, which brings drought; cooler waters summon a La Nina, which brings increased rainfall.
Mr Collins said an El Nino would come at a time when large parts of the nation, including Victoria, were struggling to cope with low rainfall. "Because we've had 12 years of average to below-average rain, the impact (of an El Nino) is going to be magnified."
Mr Collins said it could be several months before the bureau knew whether the nation was in the grip of an El Nino event. "You have to be in an El Nino-La Nina to actually define it as an event."
Melbourne's water storages usually increase only in the second half of the year as inflows outstrip water demand. But during previous El Nino years, such as 1997 and 2006, rainfall has been so poor that levels have actually fallen in this period.
Melbourne's storages are now at 27 per cent, the lowest ever. If levels drop as they have in previous El Nino years, they could be below 20 per cent by January.
With the desalination plant not open until 2012, Melbourne is depending on rainfall to boost dams by 300 million litres until then for the city to avoid running out of water.
The State Government broke a 2006 pledge to release an extra 17 billion litres of water into the Yarra in order to prop up storage levels.
Leonie Duncan, healthy rivers campaigner with lobby group Environment Victoria, said the Government had only promised not to extract any more water if flows fell below a certain level, equivalent to 200 megalitres a day. It had not promised to replenish the river at any stage. "There's no obligation to put water in; it's only that they're not allowed to take it down any further."
Ms Duncan said that meant an El Nino drought could stress the river even more.
"The Yarra's flows are down to about 20 per cent of the long-term average, which represents about 12 per cent of natural flows.
"Streamflows are less than 10 per cent of average in catchments across Victoria, particularly in the west and north."
Melbourne Water declined to talk on the record about the impact of an El Nino but issued a statement saying that "one of the reasons Melbourne has been able to make it through the climate variability over the past decade is because we have such a large storage capacity. Even with reservoirs at 27 per cent we still have about 480 billion litres of water in storage, which is significant."
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