http://www.examiner.com.au/s,
ory.asp?id=193103
A new report sounds the alarm about water flow from forest plantations, says FRAN VOSS. ST HELENS could be without water in 20 years if plantation forestry in its catchment is not reduced, a report has found.
The report was compiled by Hobart-based David Leaman, a geohydrologist with 40 years' experience.
Dr Leaman examined catchments on the Blue Tier, Groom and Ransom rivers which provide about 40 per cent of the catchment of the George River system, the water source for St Helens.
Conversion of native forests to plantations may lead to a substantial loss of water yields from which the catchment may never recover, the report claims.
The water demand of the growing trees can exceed that of old- growth forest by as much as 50 per cent during the peak growing period, which occurs at the time the new forest achieves canopy coverage at around 30 or 40 years.
Even after the peak, trees will continue to draw extra water until they are mature, which could be between 100 and 150 years.
When trees are harvested on a 20-to-30-year rotation and replanted, the groundwater store has no time to recover before the cycle begins again.
"It's a continual drying process and it's never in balance," Dr Leaman said.
Commercial requirements for fast-growing trees mean plantations are most often grown in high-rainfall areas, generally upper catchments.
Trees and vegetation draw water directly from the sub-surface store, altering the volume returned to the surface and into river systems.
These groundwater storages normally carry streams through dry periods, Dr Leaman said.
Forestry Tasmania Bass district forest manager Steve Manson conceded that plantations took up more water than old-growth.
"However, the quantity of it we don't know, and we don't know many people that do," he said.
Mr Manson believes the best information to date comes from a Melbourne Metropolitan Water Board study that has been in progress since the 1939 bushfires.
"The only way you can get accurate information like that is to get that long-term data."
Dr Leaman said, "The fact is no one really knows - and why should they go ahead if they don't know the long-term impacts?
"Forestry Tasmania is flying blind in terms of water yield issues, with potentially catastrophic consequences for Tasmania."
But Mr Manson said water uptake by a growing plantation forest was balanced by the increased run- off from adjacent clearfelled coupes.
Dr Leaman said extra surface run-off was short-lived since the coupe was quickly replanted and the new trees grew rapidly.
The peak time for drawing water is much longer than the period of extra run-off.
Mr Manson said research by Forestry Tasmania begun in 1999 at its Warra site in the South had shown water quality was not affected by forestry. But these trials make no assessment of losses or gains in water quantity.
The Forest Practices Code deals only with quality and omits any mention of quantity, according to Dr Leaman, who said it did not address the significance of groundwater either, or its impact on base flows and sustainability.
The code restricts Forestry Tasmania to logging no more than 5 per cent of a catchment a year. Mr Manson says only about 2.5 per cent a year is ever logged.
But 5 per cent a year adds up to 50 per cent over 10 years.
Dr Leaman's results confirm findings in a report on Launceston's water catchment, commissioned by the city council and released in August 2002.
Carried out by the CSIRO's land and water division and the Cooperative Research Centre for Catchment Hydrology, the study showed that maintaining current logging practices in the North Esk catchment could reduce water yields by up to 20 per cent.
This raised the prospect of a dam on the St Patricks River to help supply Launceston.
The project manager, Launceston City Council hydraulic modelling systems manager Steve Ratcliffe, has since clarified the estimate. He said that, if the catchment were logged at 2 per cent a year, water flow could be reduced by 20 per cent by the second rotation in 50 years' time.
Dr Leaman says logging the Blue Tier could be the death knell for the St Helens catchment. "Most of the George River catchment has already been used for forest activities, so the Blue Tier face is the remainder of the catchment."
His modelling shows that changed forest conditions in the Groom and Ransom river catchments would make both dry for several months in summer.
The total output from these two significant tributaries of the George River would be virtually zero for a large part of the year.
"In Tasmania-wide terms the massive conversion of upper catchments, especially to single-age rotational forestry, must lead to significant reduction in water availability in lower catchments," Dr Leaman said.
The reduced flows might not be apparent for a generation - by which time the situation might not be recoverable within 100 years.