EXCLUSIVE: REPEATED fires are likely to deliver more water to the Murray Darling Basin's dams, according to one of Australia's leading catchment hydrologists.
Research in North East Victoria's fire-damaged catchments has shown flows into the region's dams initially surged by up to 70 per cent following the 2003 fires.
Normally this surge would decline within about five years as dense mountain and alpine ash regrowth sucked up rainfall, cutting flows by 20-30 per cent.
However, examination of catchments, burnt out in both the 2003 and 2006 fires, shows that alpine ash regrowth was destroyed in the second fire and replaced by bracken and other understory species.
University of Melbourne forest hydrologist Patrick Lane said initial evidence from 100,000 ha hit by both fires, some of which was in the Kiewa Valley, showed water yields were likely to rise.
"It all depends on the frequency of the fires, but we know repeated fires in alpine ash forests are likely to lead to higher yields.
"The more frequent the fire then the less chance there is for the vigorous regrowth that has been seen after fires in ash forests."
Dr Lane said the evidence was that alpine ash regrowth from the 2003 fires, which was subsequently burned out in 2006, did not regrow.
"The seed bank is gone and what we've seen is stands of dead trees and bracken," he said.
However, Dr Lane said the long-term impact of repeated fires on many of Victoria's catchments was still poorly understood.
"The more frequent the fires the less time there is for vigorous regrowth that uses more water. We would get more water, but at what cost to water quality and the environment."
Dr Lane's colleague Gary Sheridan has found that fire can have a dramatic impact on water quality.
While there is an initial risk to water supplies from ash, there are longer-term problems such as erosion.
He said the university's work raised questions about past assumptions on the impacts of fire on water yields in the Victorian alps.
Most of the current forecasts are based on the impacts of the 1939 fires on mountain ash forests in Melbourne's catchment areas.
But Dr Lane said most of North East Victoria's forests consisted of mixed-species eucalypts and some areas of alpine ash.
"Only 16 per cent of the forest burnt out in the 2003 fires was alpine ash, the rest was other eucalypt species."
"I'm reasonably sure we over predicted the impact of fire (in reducing water yields) as a result of applying these yield models for ash forests to mixed species forests."
