Heavy rain fails to boost dams

Perth's dams have received only a "tiny fraction" of run-off compared with the historical average so far this winter despite recent heavy rains, according to the Water Corporation.

With winter almost two-thirds over, figures provided by the State-owned water utility show there had been just 16.1 billion litres of rain- fall flow into Perth's dams as of Friday.

The figure is among the lowest recorded for this time of year and has left the city's reservoirs at 31 per cent of their capacity - or holding 197 billion litres.

But 110 billion litres of that is deemed unusable, meaning the true amount of dam water available is much lower.

Also, some of the water stored in the dams is desalinated seawater that is "banked" there by the Water Corp during winter, when demand is typically lower.

The Water Corp has previously said the dramatic fall in run-off has been caused by successive years of declining rainfall, which has led to a drop in groundwater levels of up to 10m in some areas within Perth's dam catchments.

It has likened catchments to sponges, saying that run-off begins only once they are sufficiently wet.

In its daily statement, the Weather Bureau noted Perth had received 149.2mm of rain this month up until 9am yesterday - still short of the long-term average of 168.5mm.

Despite the run-off figures - which contrast with Perth's pre-1975 dam inflow average of 394 billion litres a year - there is no risk to the city's scheme water supplies.

Perth, Mandurah and the Goldfields use almost 300 billion litres of drinking water a year, but up to half of this can be met by desalination plants at Kwinana and Binningup.

Much of the rest can be sourced from groundwater.

"Dams had, until the 1970s, met the city's entire drinking water needs," a Water Corp spokeswoman said.

"Dam inflows have dropped to a tiny fraction of that received historically, and the Water Corporation no longer relies on rainfall to supply our customers.

"In 2013, streamflow into dams totalled 80.5 billion litres, or 28.2 per cent, of the 285 billion litres supplied."

'The Water Corporation no longer relies on rainfall to supply our customers.'" *Water Corp spokeswoman *