Australia — Fresh modelling shows that timber harvesting in Melbourne’s water catchment has a minimum impact on water yield and could even increase water supply, according to a report.
The results are produced by a Department of Sustainability and Environment (DSE) model that investigates the interaction of logging and fire with water yield in Melbourne’s catchments. The model has been applied to the Thomson and Tarago river catchments.
The results are in the Review of Timber Production in Melbourne’s Water Catchments, written by forestry consultants Mark Poynter and Gary Featherston for the Victorian Association of Forest Industries.
Timber production is permitted in 12% (18,490 hectares) of Melbourne’s 157,000 hectares of water catchment, with annual harvest averaging 306 hectares, or 0.19%, of the catchment, over the past 10 years, the report says.
No “old growth” is harvested. The trees logged are 69-year-old regrowth from the 1939 bushfires. The report identifies high-intensity bushfires as the biggest threat to Melbourne’s water supply catchment.
For example, the 2003 north-east Alpine fire, which burnt more than 1 million hectares of forest in 59 days, would reduce in-flows into the Murray River headwaters by 430 billion litres a year within 20 years, the authors say.
The DSE model concludes that under a base scenario, where available forests in the Thomson catchment are logged as planned, water yield will steadily rise by 20% over the next 125 years despite ongoing harvesting. If timber harvesting was completed within the next five years and then stopped, average water yield would decline 7% by year 10 compared with the base scenario, and would then start to recover.
If the catchment was completely burnt within the next five years, the DSE model says water yield would decline 39% by year 10, and then start to recover.
The DSE model supersedes an earlier model, the generalised Kuczera curve, that estimated that regrowth caused a 50% reduction in water yield over 20-30 years before a gradual rise in yield back to pre-logging levels over the next 100-150 years.
The consultants’ report says water yield could be improved by thinning regrowth timber. This had been shown by 20 years’ of research. In Western Australia, a 12-year thinning and fuel reduction program would increase Perth’s Wungong catchment yield by 4-6 billion litres a year, the authors say.
The report says studies by URS Forestry and MBAC Consulting, for DSE, show that hardwood plantations could not replace the native timber from Melbourne’s catchments because there are not enough hardwood plantations, and these are mainly planted for woodchips; little local land is available and it is expensive; and funding of sawlog plantations is also problematic.
“Their relatively long rotation lengths and high management costs make them less attractive to investors,” the authors say.