Although Australia is likely to face more severe and frequent droughts as the climate warms, the capacity of warmer air to hold greater volumes of water means that when it does rain, extreme flooding will become more frequent.

“That is a real problem for anyone who owns a dam,” said Rory Nathan, professor of hydrology and water resources at the University of Melbourne and one of the authors of the study. “Average garden-variety rainfalls across the year for many parts of Australia will be lower but when there is a storm event, they are going to be more severe.”

The paper, funded by dam operators and to be published in the peer-reviewed Water Resources Research, applied the latest climate modelling to meteorological records to analyse the possible peak rainfall on 546 large dam catchments in Australia.

It is the first attempt globally to quantify the implications of climate change for major water storage infrastructure and exposes the risks to dams designed decades ago on outdated climate assumptions.

The dilemma faced by the operator of Eildon Dam is clear in data from Goulburn Murray Water. By the end of September, after the Bureau of Meteorology announced Australia would endure a third consecutive La Nina event, which would bring flooding to the east coast, Lake Eildon was already 98.9 per cent full.

This meant that, at the height of the Victorian flood crisis, the dam operator had to release more than 36,000 megalitres of water a day into the Goulburn River for six consecutive days. This was more than the entire monthly release in August.

Goulburn Murray Water emergency controller Peter Clydesdale said impacts on downstream communities were a “key consideration” in any water release from the dam. In the most critical week of the flood crisis, the dam helped mitigate floods by releasing only a third of the total water flowing into Lake Eildon.

“One of the challenges in managing a flood is knowing the amount of water that will flow into the dam and over what time period,” he said. “We continually assess the actual and estimated inflows to determine how quickly the storage will rise and how much water we need to release to safely operate the dam.”

Nathan said that while it was easy to say, with the benefit of rainfall data, that the dam should have released more water earlier, Goulburn Murray Water had a legal obligation as a storage manager to maximise the water it supplied for domestic, commercial and environmental use.

“If they started releasing water and with hindsight, found out they didn’t need to be, they would get whacked over the head,” he said. “They are super sensitive about wasting water because that water is effectively owned by other people.

“If they know a big flood is coming and are sure that the dam will be full they will do as much as they can to release water beforehand, but they are not going to make that decision too far out because of the uncertainty of weather forecasts.”

Nathan added that while the dam releases contributed a “minor part” of the Seymour and Shepparton floods, the principal cause was rainfall below the dam. “Had Eildon not been there, the flood would have been a heck of a lot worse,” he said.

On October 14, the day the Goulburn River peaked in Seymour and inundated more than 300 homes, the Eildon floodgates were opened for the first time this century.

By the end of the month, 591 gigalitres of water – more than the capacity of Sydney Harbour – had passed through the dam into the river system below. On Monday, with more floods forecast for the Goulburn Valley, the dam was still 97 per cent full.

Mitchell Shire councillor Bill Chisolm, whose constituency includes Seymour, said more needed to be done to protect his community from future floods.

“We definitely need more airspace in the dam to mitigate events like this,” he said. “Climate change is proving to be a challenge in a lot of ways and we have got to adapt our thinking.”

Opposition water spokesman Danny O’Brien said raising Eildon Dam’s wall was an idea worth exploring.

“Under a climate change scenario, it may well be that we have rain less often but more of it in concentrated periods,” he said. “We need to make sure we are doing our best to store it when it does fall. We are more than happy to examine whether there are opportunities to introduce more flood-mitigation options with our existing dams.”

Federal Water Minister Harriet Singh declined to comment, but a government spokesperson said Lake Eildon was not designed for flood mitigation. “If opportunities arise, it can be operated to absorb some floodwater during small to medium flood events.”

The Eildon Dam was completed in 1956 and its last significant upgrade was completed in 2005 when $52 million was spent on strengthening and refurbishment works.

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