Warrnambool's water authority says a 30 per cent reduction in the city's main water source could occur by 2055 if nothing is done to steer the climate away from a worst case scenario.
Wannon Water has released its Urban Water Strategy, a two-year analysis of the region's supply systems based on projected declines in seasonal flows.
Branch manager of asset planning Peter Wilson said trends towards a Representative Concentration Pathway of 8.5 - a release of greenhouse gasses at 8.5 watts per square-metre - would have a dramatic impact.
"RCP 8.5 is the worst case scenario - if we don't act now and do what we should then that's not a very good position to be in," he said.
"That particular scenario in 2055 sees a reduction in flows in the Gellibrand River - where we draw water from - of about 30 per cent, so that's a significant reduction in flows."
Climate modelling for the Great South Coast released by the Department of Environment, Land, Water and Planning warns a rise in temperature and reduction in rainfall could increase pressure on water supply.
It could also prompt more saline intrusion to the water table and reduce both stream flow volume and shallow aquifer levels.
Taste is already a source of complaint among Port Fairy residents.
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While the report notes annual rainfall totals in the Gellibrand River catchment have been typically low across the past decade, Mr Wilson said Warrnambool was well-positioned.
Wannon Water hasn't implemented water restrictions in the Otway system since the '80s.
Flows in the Gellibrand River and tributaries have been sufficient (combined with use of the Carlisle River bores) to avoid the need to limit use.
Based on medium climate impacts and high demand growth for the system, the earliest date of a required augmentation (an increase in system capacity) is 2031.
Wannon Water is already implementing roof water harvesting infrastructure in Warrnambool to supplement supply.
Other options include expanding existing storage on the North Otway pipeline at Ewan's Hill which would increase the system yield by 300 megalitres.
If necessary, a groundwater bore in the South Otway pipeline at Curdievale could be accessed. That bore will be brought online after Ewen's Hill's augmentation in 2035 and will add 2150 megalitres to the system.
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