Melbourne's water supply is under threat from very high levels of bushfire fuel in its forested water catchments
Melbourne's water supply is under threat from very high levels of bushfire fuel in its forested water catchments.
The after-effects of intense bushfires could cause a dramatic, long-term decline in catchment runoff to Melbourne, experts have warned following a report that Canberra may have to build new storages sooner because of its 2003 forest fires.
Botanist Peter Attiwill said severe bushfires in Melbourne's catchment, north and northeast of the city, could cut streamflow by 30 to 40per cent as fast-regrowing trees monopolised the water.
If this happened, the city would have to rethink its water supply.
"It takes about 60 years for that downturn (in streamflow) to become an upturn again," Dr Attiwill, principal fellow in botany at Melbourne University, said yesterday. Veteran bushfire researcher David Packham agreed. "Melbourne could not withstand fires like (Canberra's) and its water catchments are so vulnerable," he said. "The fuel load is massive. It hasn't been burned since 1939."
After Victoria's 1939 Black Friday bushfires, the rapid and thirsty regrowth of mountain ash gum trees cut streamflow by 20 to 50per cent over many years, Dr Attiwill said.
He said fuel levels were very high and the broad-scale burning needed to mimic Aboriginal ecology had been neglected.
In Canberra's catchments runoff is projected to be 15per cent lower than its pre-bushfire level by 2020 before starting to recover, according to the water authority ACTEW Corporation.
Its latest report on Canberra's water supply cited the bushfire effect as one reason the city would soon have to choose between Australia's most severe water restrictions and building more storage.
Melbourne Water is expected to announce soon the deployment of its own fire-bombing helicopter so that it can quickly put out fires before they spread.
Hazard-reduction burns were carried out in "pockets" of the Melbourne catchment, a spokesman for Victoria's Department of Environment and Sustainability said.
"(But) the main focus is trying to keep fire out of the water catchment," he said.
He said Victoria was doing more hazard reduction burning generally, with 22,000ha having been burned in spring last year, compared with 6000ha in spring 2003 - although it was not a simple or easy task.
"We have to be very strategic about burning close to settlements," he said.
Mr Packham and other bushfire experts said that with the exception of southwest Western Australia, hazard-reduction burning had been in long-term decline, especially in Victoria, and any recent increase was trivial.
Sydney's catchment was quite different from Melbourne's but the effect of the 2001 bushfires on upland swamps and water quality and quantity was being studied, said George Dodds, general manager of catchment operations with the Sydney Catchment Authority.