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Does logging need to stop in Melbourne’s largest water catchment?

Logging in Melbourne’s water catchments could reduce the city’s water supply by 35GL a year by 2050, according to a study from the Australian National University (ANU).

Researchers from ANU’s Fenner School of Environment and Society calculated water lost due to logging in the Thomson Catchment. They found past logging in the catchment’s ash forests had reduced the water yield (the amount of water flowing through the catchment) by 15GL each year.

If logging continues at the current rate, this could increase to 35GL per year by 2050. Based on an estimated water use of 161L per person per day, this equates to the yearly water use of about 600,000 people.

Thomson became a designated water catchment after the completion of the Thomson Reservoir in 1983, which makes up about 60% of Melbourne’s total water storage capacity.

According to the researchers, catchments covered with old-growth ash forests yield almost twice the amount of water each year than those covered with younger trees. This is because evapotranspiration, where trees transpire water into the atmosphere, is higher in young forests.

“Intense competition between young trees results in rapid growth rates along with increased evapotranspiration,” Professor David Lindenmayer and Research Fellow Chris Taylor wrote in The Conversation.  

“As the forest matures, the trees thin out, and after 200 years an ash forest can have less than 50 trees per hectare. These older ash forests release more water back into the catchment.”

In their report, entitled Resource Conflict Across Melbourne’s Largest Domestic Water Supply Catchment, the researchers said the frequency of timber harvesting, which occurs every 60 to 120 years, meant large areas of forests were kept in a high evapotranspiration stage of growth.

“Given this catchment contributes to over 40% of total stream runoff across the water catchment network for Melbourne, this has significant ramifications for both water security and wood supply,” they wrote.

“Thus, our investigation is a classic example of resource competition; that is, competition for land and forest for logging versus water production.”

Although they acknowledged the economic arguments for logging and the loss of jobs if operations were to cease, the researchers called on the Victorian Government to stop logging in the catchment.

“Guaranteeing water quality and water quantity demands long-term planning for infrastructure and the maintenance of water catchment integrity,” they wrote.

“The work we have reported … demonstrates the need to maintain the integrity of forest cover in a key water supply catchment for the city of Melbourne by excluding native forest logging.”