By Tom Cowie
Melbourne’s dams are full to overflowing, but the arrival of an El Nino weather pattern and the likelihood of a hot and dry summer could see water stores drop quickly.
The city’s drinking reservoirs are chock-a-block after a wet winter in 2022 and some recent heavy rainfall, with total water storages at 95 per cent capacity.
The Thomson Dam in West Gippsland, which holds more than half of Melbourne’s water supply, is sitting at 99.3 per cent full – close to where it was a year ago when it spilled for the first time in over two decades.
Water catchments are still soaked after three years of La Nina – which resulted in wet and cooler summers – meaning that any excess rain has quickly flowed into dams.
Last month, the Bureau of Meteorology declared that an El Nino was underway in the Pacific Ocean and expected to last until late summer or early autumn, raising concerns about a more intense bushfire season.
It’s the first El Nino since 2018-19, and is combined with a positive Indian Ocean Dipole, another climatic driver which typically leads to reduced spring rainfall for central and south-east Australia.
Most of Australia’s major droughts – including in 1982, 1994, 2002, 2006 and 2015 – have coincided with El Ninos, according to the Australian Research Council’s Centre of Excellence for Climate Extremes.
Climate change is also causing hotter temperatures, which increases water evaporation from dams, as well as reducing the amount of rainfall to replenish water storages.
Professor Stuart Khan, an urban water supply expert from the UNSW school of civil and environmental engineering, said that Melbourne’s full dams were “money in the bank” with the arrival of El Nino.
“But it’s in the bank for this reason,” he said.
“When you look at the historical trend for Melbourne, it’s quite reliably seasonal, it’s up and down every year. And then, every now and then, you get a year where that fill doesn’t come and then that’s when you see a significant drop.”
Melbourne’s water levels last dropped below 50 per cent in May 2019 after several years when the dams failed to refill during winter and spring.
The millennium drought of the 2000s was even more dire – and led to permanent water restrictions – as storages dropped to a low of 26 per cent in mid-2009.
While summer naturally led to a fall in Melbourne’s water levels, Khan said it would be what happens after the hot weather is over that will have a greater impact.
The Thomson Dam spillway overflowing last year.Credit: Nine News
“If the next six months are dry enough that the catchment dries out, that makes it harder to fill in the second half of the year because rainfall is less likely to translate into significant runoff,” he said.
“People often underestimate the importance of how much water is in the catchment and how wet it is.”
Victoria’s desalination plant has not been in operation since April this year due to the high dam levels, with no water orders expected until the middle of 2024.
The desalination plant has contributed 455 billion litres to Melbourne’s supply since 2017, around a quarter of the city’s water needs.
“It takes more than one year of drought to get Melbourne into water shortages,” said Khan.
“But troubles come if you have a July to November period that’s not wet. Then it could get down to 75, even 60 per cent [full]. That’s when people get more concerned.”
Wenju Cai, chief research scientist at the CSIRO and an expert in El Nino-La Nina, said that El Ninos brought hotter and dryer conditions to south-eastern Australia.
It’s still too early to tell, but Cai said that this El Nino could be a weaker pattern, which means that its effects can last longer than one year.
The Thomson Dam in 2008.Credit: Craig Abraham
The El Nino in 2018-19 was weaker and corresponded with the devastating Black Summer bushfires in 2019-20.
“Those consecutive dry conditions in the Pacific affected the cumulative rainfall and also made the temperature oppressively high before the fires could occur,” he said.
A Melbourne Water spokesperson said water storages were almost at the same highs experienced last year, putting the city in a good position ahead of summer.
“Despite our strong water storage levels, under El Nino weather conditions we know storages can drop quickly, by as much as 20 per cent in a single year,” they said.
“With September rainfall over our four major harvesting catchments 43 per cent below the last 30-year average, it is likely that we’re already experiencing the effects of El Nino, so we ask Melburnians to continue using water wisely and make every drop count.”
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