NORTHERN Victorian irrigators may face a cut in water of at least five per cent under a revised cap on extractions as new forecasts indicate climate change will reduce water availability across the region.
Three CSIRO reports released on Monday forecast that under best estimate climate change conditions surface water availability will fall by between 14-18pc by 2030 in the Goulburn/Broken, Loddon/Avoca and Campaspe regions, water diverted for use by 5-6pc and end of stream flows into the Murray by 22-27pc.
If the dry conditions experienced over the past ten years become the norm, irrigators could see their irrigation water cut by a quarter as surface water availability across the three regions plummets by 41-54 pc, cutting flows into the Murray by 58-76pc.
The reports are the latest in the Sustainable Yields project intended to establish water availability across the Murray Darling Basin and underpin a Federal Government revision of the current Cap on extractions across the whole basin, due for completion by 2011.
As such they provide the first indication of the scale of cuts irrigators potentially face under a revision of the Cap, which was first set in 1997 based on 1993/94 usage levels, but is no longer regarded as sustainable.
But regional irrigators were quick to warn against relying solely on the CSIRO rainfall and runoff forecasts when setting a new Cap.
Merrigum orchardist John Corboy, one of the driving forces behind the $2.2bn plan to modernize the Goulburn Murray Irrigation District in return for handing a slice of the forecast 425GL water savings to the State Government and the environment, said the Goulburn was well suited to coping with rainfall variation.
“The key in the Goulburn is to come to grips with how the climate will change and the consensus is that there will be less water, but more extremes,” he said.
“In a system like the Goulburn, which is super efficient, a flood year could fill Eildon with 3 million megalitres and provide a buffer for three years.”
Mr Corboy said irrigators were already meeting the challenge of reduced rainfall by steadily improving water conservation on farm and productivity.
“We would want to ensure that any process to adjust the Cap would take into account all these factors,” he said. “If it is done purely on rainfall it may not accurately reflect how the catchment will respond.”
The Goulburn/Broken region covers just 2.1pc of the MDB area, but contributes 11pc of the total runoff, with one of the highest levels of rainfall and run off in the MDB.
CSIRO found current levels of surface water use relative to water availability to be extremely high in the Goulburn where 1606GL, or 50pc of the average 3233GL surface water available each year is diverted for use, a third of it transferred to the Campaspe, Loddon/Avoca and Wimmera regions via the Waranga Western Channel.
CSIRO project director Dr Tom Hatton said extensive modeling on groundwater systems had been done in conjunction with local authorities to assess available groundwater and how it interacted with surface water.
CSIRO has forecast an expansion of groundwater use by 10pc in the Campaspe, 50pc in the Loddon/Avoca and 67pc in the Goulburn/Broken.
This is expected to reduce streamflows in the three systems by 6GL, 17GL and 37GL a year respectively by 2030.