Wet winter belies true impact of declining streamflow to Perth dams

With recent rainfall across Perth and southern WA, it’s easy to assume our dams are full to brim. But the connection between rainfall and dam levels isn’t so simple. 

In fact, just 35 billion litres of runoff – or streamflow – has flowed into dams along the Darling Scarp this year, despite it being Perth’s wettest winter in 30 years.

That’s a far cry from the pre-1970s average of 420 billion litres of streamflow, and around half the 76 billion litres averaged annually since 2010.     

Water Corporation General Manager of Assets Planning, Evan Hambleton, said the decline was due to repeated years of below average rainfall in autumn and spring leaving dam catchments increasingly thirsty.

“Dam catchments are like massive sponges, and it’s only once the ground is saturated down to the groundwater or bedrock we start to see meaningful streamflow,” Mr Hambleton said. 

“Prior to 2000, we needed around 100mm of rain to generate 10 billion litres of streamflow. Now we need closer to 500mm for the same yield.

“Historically, we had much more consistent rainfall throughout the year, especially in the shoulder seasons of autumn and spring. Increasingly, however, the only material rain comes in winter. 

“Because our catchments are drier for longer, streamflow begins later and ends earlier - reducing the amount of drinking water we receive from natural runoff.”

Since the mid-1970s, a 15–20 per cent rainfall reduction has led to an 80 per cent decrease in streamflow. 

Today, rainfall makes up just 10-15 per cent of drinking water supplies, while most water in our dams actually comes from groundwater and desalinated seawater. 

Climate-independent desalination is a particularly important part of our supply mix, but it will never replace the need to be waterwise, according to Mr Hambleton. 

“Desalination is such a vital source because it doesn’t rely on rainfall, however, that doesn’t mean we can get complacent in how we use water,” he said.  

“Overwhelmingly, Western Australians heed the waterwise message. Since the early 2000s, average annual household water use across WA fell by 22 per cent. But lately, consumption has risen.

“It’s crucial we don’t view desalination as a licence to waste water. While it’s a vital part of our water supply strategy, it will never replace the need to be waterwise.”

To reinforce this message, Water Corporation has launched a new campaign urging people to “break up” with their water-wasting habits ahead of summer. 

It is underpinned by research revealing one in five people are unaware of sprinkler rosters, while 64 per cent don’t know showers should be under four minutes.

“A longer shower or watering the garden outside of rostered days might seem insignificant at a household level, but it has a big impact to collective water supply,” said Mr Hambleton. 

“Western Australians know the waterwise message better than most. But it’s time to be honest and ask ourselves if we’re still living that message every day.”