Most of Melbourne's forested water catchments are conserved in national parks. But about 13 per cent - mostly in the Thomson Dam catchment - is available for sustainable harvesting by the timber industry. About 0.2 per cent of the total catchment area is annually logged and regenerated.
With Victoria in the grip of long-term drought, it is being claimed that this annual timber harvest is denying Melburnians access to significant water resources because post-logging regrowth uses excessive volumes that would otherwise flow into storages.
While it is well known that regrowth uses more water than mature forests, predictions that ending logging would increase flows into storages are dubious; they are based on all catchment forests reaching maturity and staying that way forever.
Given Victoria's history of severe bushfire, it is almost certain that Melbourne's catchments will always contain substantial areas of post-fire regrowth that will prevent attainment of theoretically optimum levels of water production, regardless of whether or not logging continues.
Advanced regrowth from the January 1939 Black Friday bushfires already occupies about 40 per cent of Melbourne's catchments. It will continue to have a far greater impact on inflows into storages than the relatively small amount of regrowth from logging staggered across decades.
Severe bushfires that burn everything in their path over huge areas are unquestionably the greatest threat to Australian water supplies. They can initiate massive regrowth that reduces stream flows for decades and can dramatically degrade water quality.
The most stunning recent example was in 2003 when two million hectares of alpine and mountain forest in south-east Australia burnt over a two-month period. It has been estimated that regrowth initiated in the areas most severely burnt by this fire will absorb 430 billion litres of water a year for the next 50 years - water that would otherwise have flowed into headwater tributaries of the Murray River.
In addition, hugely increased sediment loads entering streams in the years following the fires significantly reduced the quality of town and city water supplies, including Canberra's.
Despite being portrayed as a villain, timber harvesting in the form of thinning can substantially counteract the impact of fire regrowth on water yield. The benefits of regrowth thinning have been widely studied throughout Australia. In Melbourne's catchments, strip-thinning trials have shown that up to 2.5 million litres a year of additional run-off can be generated from each hectare of thinned regrowth. A program of thinning the 1939 regrowth could add billions of litres of water to our storages.
Western Australia has been quicker to take advantage of thinning as a water management tool. Earlier this year, a $20 million, 12-year thinning program was initiated in a substantial segment of Perth's catchment following four years of exhaustive public and stakeholder consultation. Every 1,000 hectares thinned is expected to deliver an additional one billion litres of run-off into the Wungong Dam a year.
Although this involves substantial public expenditure because of forests mostly unsuited to timber production, the thinning of older and larger regrowth would most sensibly involve the production of timber that can fund the operation.
Compared with simply locking up catchments, active management of fire regrowth to increase water flows potentially offers a range of other benefits, including stronger imperatives for fire protection and improved stream health.
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