Issue 12 February 2001
Stream flow: One of the weirs built in North Maroondah to measure water yield
Research on mountain ash forests in experimental mini-catchments has examined the impact of bushfires on water yield.
The imposing mountain ash eucalypt forests that are the heart of Melbourne's protected catchments provide a glimpse of history. Now they are also helping predict an important aspect of the future.
The mountain ash forests, which are a living example of the countryside before European settlement, hold the key to Melbourne's water supply.
Mountain ash, among the world's largest hardwood trees, are confined to the wetter parts of the highlands of Victoria and Tasmania. In Victoria, mountain ash grow at altitudes of 200 to 1000 metres.
Some 80 per cent of Melbourne's water comes from ash-type forests, which cover about half of Melbourne's water supply catchment area.
Researchers are working to increase their understanding of water yield in the catchments, particularly in the event of a major bushfire, which have the potential to halve streamflow into the reservoirs.
The forested catchments, which were set aside solely to provide water for Melbourne and surrounding areas, have escaped large-scale fires since Black Friday in 1939. It is estimated that only 10 per cent of the catchment area is older than 1939.
Imposing and stately: About 80 per cent of Melbourne's water comes from ash-type forests
Pat O'Shaughnessy, one of the pioneers of the research on the mountain ash catchments, said that on average, the ash forests reach their maximum water yield after 120 years following a minimum of about 30 years (when the trees are soaking up a lot more water).
This means that for the regrowth forest in the catchments, maximum yields are still 60 years away. After that, the trees would continue to age for another 300 years or more, during which time water yield would not fall.
Mr O'Shaughnessy, a Research Forester with Melbourne Water from 1967 to 1993, wrote in a 1991 report: "If no wildfires occur, then the yield from the existing catchments is expected to increase at a rate of 2000 to 3000 megalitres per year over the next 50 years."
The Melbourne and Metropolitan Board of Works (now Melbourne Water) Catchment Hydrology Research Program began in 1948 and in the early 1950s a major study began at Coranderrk, near Healesville, in predominantly old-growth forest. This was complemented in 1969 with a project in North Maroondah. A further study was initiated in 1976 at Crotty Creek, near Narbethong.
A total of 18 experimental, mini-catchments were established to research water yield under a range of conditions, 15 in North Maroondah. Some 13 of these North Maroondah catchments were in 1939 mountain ash regrowth forests and the other two were in old-growth forest.
Thinning, patch cutting and other techniques were trialled to determine how yield was affected by reducing the density of stands. Weirs were built to measure yield and the research involved engineers, scientists, field operatives, mathematicians and statisticians.
Melbourne Water's current fire suppression policy revolves around fast, initial attacks preventing severe fires.
If extensive bushfires were to hit the catchments, the rapidly regenerating mountain ash seedlings would soak up copious amounts of water, leaving significantly less water to go through the soil into streams and, eventually, reservoirs.
Researchers have discovered that mountain ash seedlings thrive only in exposed soil with direct sunlight. In the absence of bushfires, mountain ash would disappear from a site within 500 years, according to Mr O'Shaughnessy's report.
"But lightning strikes alone provide enough fire to keep the species going," he said. "Very few trees of that age would be left in the catchments.
"The other thing to remember is that running fires in mountain ash forests tend to be very fierce and there's not always much you can do about stopping them once they take hold."
However, Mr O'Shaughnessy said fire suppression techniques had improved dramatically in Victoria since the 1950s, and a mass conflagration involving entire catchments was less likely now.
Melbourne Water's fire suppression policy aims to prevent severe fires developing through fast, initial attacks.
If extensive bushfires were to hit the catchments, the rapidly regenerating mountain ash seedlings would soak up copious amounts of water, leaving significantly less water to go through the soil into streams and, eventually, reservoirs.
