A centre pivot boom in the Harvey Irrigation
District south of Perth (Photo: Robert Garvey)
South-West Western Australia Sustainable Yields report released
The most comprehensive assessment of water yields and demands to 2030 ever undertaken for south-west Western Australia was released by then federal Minister for Climate Change, Energy Efficiency and Water Senator Penny Wong in Perth in March.
Senator Wong said the findings of CSIRO's South-West Western Australia Sustainable Yields (SWSY) Project were sobering.
“It highlights the likelihood of a further reduction in Perth's water supplies by 2030, which is of considerable concern,” she said.
The research, which will inform key water planning and management decisions for Perth and the entire south-west of the state, found the region could face a 24 per cent reduction in surface water yields by 2030 under a median future climate, according to CSIRO project leader, Dr Don McFarlane.
Dr McFarlane said groundwater yields held up much better, with only a two per cent net reduction under a median future climate.
“Under a worst case scenario, surface water yields could drop by 49 per cent and groundwater by seven per cent,” he said.
Sustainable Yields Project Director Dr Tom
Hatton (CSIRO) delivers a copy of the final report
to Senator Penny Wong (Photo: Darryl Peroni)
The SWSY project was conducted over 18 months. It covered an area more than 62,000 square kilometres, extending from Geraldton, 424 kilometres north of Perth, to Albany on the south coast, and supports more than 1.9 million people.
Dr McFarlane said rainfall had already declined by 10 to 15 per cent in south-west Western Australia since 1975 and runoff into Perth dams had decreased by more than half. The projected surface water yield decrease by 2030 was in addition to this historical decrease.
Both of the historical and projected decreases were larger than is projected to occur in any other part of Australia by 2030.
Perth was fortunate to have been situated over a large reservoir of groundwater, which demonstrated less short-term response to climate change where watertables are within a few metres of the surface, Dr McFarlane said.
“The falling watertable results in less evaporative and drainage losses which reduce the rate of fall and provide more room for winter recharge. This means that in sandy areas that have been cleared and used for dryland agriculture, groundwater levels are expected to continue to rise even with reduced rainfall,” he said.
“However, groundwater yields in some important water supply areas are expected to reduce by between 20 and 50 per cent under a dry extreme future climate. These include Gnangara, Collie, the Blackwood Plateau and Albany, all of which have perennial vegetation over their recharge areas.”
Surface water dependent ecosystems were also at risk, especially those systems that depend on occasional flood flows. Many rivers in the project area were already ephemeral but the periods of ‘no flow’ were likely to increase by one month per year and up to three months under a dry extreme future climate.
For groundwater dependent ecosystems in the Southern Perth Basin, 40 per cent were likely to experience increased water stress where groundwater levels were projected to fall.
The project also looked at water yields and future demand for the south-west where water demand was expected to increase by about 35 per cent depending on population and economic growth.
Significant supply deficits in surface water were expected by about 2020 in areas where it was used for agricultural irrigation. Groundwater deficits were likely around Perth and to a lesser extent, Bunbury.
“Where both surface water and groundwater are available, groundwater will progressively substitute for surface water as a water source, as it has started to do in several areas,” Dr McFarlane said.
“The region overall may have a 250 gigalitre (GL) per annum water surplus under a median future climate and median demand scenario.
“However this may be a 250 GL per annum deficit if demand is high and the region has a dry extreme future climate.
“Even if there are water surpluses in some areas, the expense of moving water and localised water quality issues will result in a lack of water in some parts of the region.”
The SWSY project builds on CSIRO's successful Murray-Darling Basin, Northern Australia and Tasmania Sustainable Yields projects.
You can learn more at South-West Western Australia Sustainable Yields Project .
Dr Don McFarlane, CSIRO Land and Water