Professor Sharma said one cause of the reduced run-off into dams was the already observed reduction in small flooding events, a trend seen across the globe. Large floods, which tend to fill and spill dams, are less useful for reservoir operators.
Rising temperatures also play a role by making the ground drier when rains fall, requiring heavier falls to have the same effect on filling rivers or creating other run-off.
“When the rains fall, it used to saturate the ground,” he said. “It gets dry faster now.”
Compounding the problem will be the likelihood that cities will lift consumption of water for parks, gardens and other uses.
“Because of that increased demand, you’ll see a greater reduction in reliability [of storage],” Professor Sharma said.
Dam levels plunged in the past year across the Murray Darling Basin, but also for Sydney, where they have fallen faster than during the Millennium Drought.
On Wednesday, Sydney’s dams were 42.6 per cent full, down about a third in the past 12 months. Melbourne’s dams were 62.5 per cent full, up from a year ago, but the second-lowest for a January since 2011.
For NSW, the dive in storage levels has prompted the Berejiklian government to announce it would “fast track” a doubling of Sydney’s desalination plant, a facility that already provides almost a sixth of the city’s supplies.
NSW Water Minister Melinda Pavey declined to comment on what climate studies were telling the government about the long-term reliability of storage to meet demand.
“The NSW government is building dams to increase supply and is exploring other options, including further desalination and recycled water to build in more climate resilient supply,” Ms Pavey told the Herald and The Age.
Complicating water security issues are the ongoing bushfires, which have burnt out almost all of the catchment for Warragamba Dam, the reservoir that makes up 80 per cent of Sydney’s supply.
Benjamin Henley, a University of Melbourne researcher who has done work on Melbourne Water’s climate risks, said the technical study by Professor Sharma’s team was “an interesting report”.
“In parts of southern Australia, we’ve been seeing steep declines in run-off, mostly to do with rainfall,” Dr Henley said.
“A certain amount of rainfall will [also] not give you the same amount of run-off,” he added. “It’s not going in the right direction.”
With water supply “such a key critical question for cities and regional areas, I’d be surprised if everybody’s not looking at it”, Dr Henley, who formerly worked for Hunter Water, said.
Professor Sharma said that, while authorities could find further ways to reduce demand, at some point a threshold would be crossed and governments would have to make bigger investments, much as they have done for transport and other large infrastructure.
Peter Hannam writes on environment issues for The Sydney Morning Herald and The Age.