Inflows to reservoirs plunging as cities’ thirst for water intensifies
Reservoirs across Australia are recording dwindling inflows as the climate warms and dries, a trend that is likely to continue and force cities, including Melbourne and Sydney, to bolster the security of water supplies.
A new study by University of NSW scientists, published in the
journal, examined streamflow data for 222 catchments and applied six of the latest climate models. All models forecast drops in supply.
‘‘We are looking at an average of 20 per cent reduced reliability in the future across all the catchments considered,’’ said Ashish Sharma, a professor at the university’s school of civil and environmental engineering, and an author of the report.
‘‘While this might not matter a lot up north where you have lower demands compared with inflows, this is pretty serious down south where the demands are high and we are already seeing impacts of the drought.’’ Sharma said one cause of the reduced runoff 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, requiring heavier falls to have the same effect on filling rivers or creating other runoff.
The rain 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 will see a greater reduction in reliability [of storage],’’ 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 New South Wales, the dive in storage levels has prompted the Berejiklian government to announce it will 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,’’ Pavey told the Sydney Morning Herald and 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.
Dr Benjamin Henley, a University of Melbourne researcher who has done work on Melbourne Water’s climate risks, said the technical study by Sharma’s team was ‘‘an interesting report’’.
‘‘In parts of southern Australia, we have been seeing steep declines in runoff, mostly to do with rainfall,’’ Henley said.
‘‘A certain amount of rainfall will [also] not give you the same amount of runoff,’’ he added. ‘‘It is not going in the right direction.’’
With water supply ‘‘such a key critical question for cities and regional areas, I would be surprised if everybody is not looking at it’’, Henley, who formerly worked for Hunter Water, said.
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.