Climate change impacts on the South West
Climate modelling has shown that rising temperature and falling rainfall trends are expected in the 21st century for the south west of Western Australia. On average, the dry scenario indicated that the region would experience a 14 per cent reduction in average annual rainfall by 2030 and a 0.7°C rise in temperature, relative to the baseline period.
The median scenario showed a 5 per cent reduction in rainfall and the wet scenario showed only a 2 per cent reduction for the same period. In the Perth region, the dry scenario showed a distribution in annual rainfall similar to the last decade. For the South West, the range of rainfall captured by the scenarios is consistent with previous studies and also with the observed trends over much of the region.
Impacts to groundwater
Urban centres located on the coast between Geraldton and Augusta rely heavily on groundwater from the Perth sedimentary basin for drinking water, agriculture, horticulture, commercial purposes as well as irrigating parks and gardens. Shallow groundwater, which is widely used for irrigating parks and gardens, is not available in some new urban areas planned for Perth, Mandurah and Greater Bunbury, so alternative water sources are being identified.
Groundwater taken from the deep aquifers is mainly for drinking water supplies and can be sustained if in balance with natural recharge and abstracted from suitable locations, or in some cases increased, by offsetting with groundwater replenishment using treated wastewater. The Northern Perth Basin, which underlies the western part of the Mid West and Wheatbelt regions, has more groundwater available for future use than the rest of the Perth Basin, which underlies the Perth, Peel and South West regions west of the Darling Scarp.
Increased groundwater abstraction to make up for reduced inflow to dams, and the decline in recharge from rainfall has resulted in falling groundwater levels in parts of the Gnangara groundwater system, so total use needs to be reduced. In the South West region, some groundwater from the South West Yarragadee aquifer will be used to supply growth of local towns and is no longer being considered as a water source option to meet demand in other regions.
Impacts to drinking water supply
One short to medium-term strategy for increasing the drinking water supply for the Perth and Peel regions is groundwater replenishment, which helps to support continued usage of the Gnangara groundwater system. As the Perth and Peel populations grow in the long term, along with demand management strategies additional water sources will be needed to secure the regions’ drinking water supply. These are most likely to include local groundwater resources along the northern coast of the metropolitan area, further groundwater replenishment using highly treated wastewater, expansion of seawater desalination plants as well as new seawater desalination plants.
The decreasing reliability of surface water and future population growth mean new water sources are now being planned for urban areas in the southern coast and southern inland regions, including Denmark, Walpole and towns supplied by the Great Southern and Lower Great Southern regional water supply schemes.
Options to secure water supplies to meet the longer-term demand for Denmark include integrating the town into the Lower Great Southern Towns Water Supply Scheme, providing greater flexibility and certainty of water supply. To improve water security for the Great Southern Towns Regional Water Supply Scheme, the Water Corporation will pipe water from Stirling Dam to supplement the current source at Harris Dam.
The Water Corporation and the department are investigating an expansion of the existing South Coast borefield to secure water supplies for Albany and other towns connected to the Lower Great Southern Towns Water Supply Scheme. In combination with water efficiency savings, the prospective additional groundwater could secure the water supply for towns serviced by the scheme until after 2030. Seawater desalination is currently the most likely option to increase supply beyond this timeframe.
Groundwater use from Perth’s Gnangara groundwater system
The Gnangara groundwater system is a basin of water-holding sands and gravels interspersed with clays. It underlies Perth between the hills and the coast, and from the Swan River to Gingin Brook. It supports the health of our natural environment and our urban amenity, and we also abstract this water for drinking water, agriculture, ovals, parks and gardens.
In 2016–17, about 287 GL was allocated from the Gnangara system – enough water to fill Optus Stadium 287 times or 114 800 Olympic swimming pools. Over 40 per cent of this went into the Water Corporation’s Integrated Water Supply Scheme (IWSS) and about 45 per cent was used for local parks and grounds, horticulture and businesses. Householders using domestic bores for gardens took around 13 per cent.
Our community, local governments and the Water Corporation have taken steps to respond to a drying climate by being more efficient and seeking alternative supplies. However, more adjustment is needed to rebalance the system as a whole.
- horticulturalists and farmers take more than 60 GL of groundwater a year to irrigate locally grown vegetables and fruit
- local councils, schools and sporting clubs take about 45 GL a year to irrigate parks, sports ovals and other public open spaces
- local businesses and other commercial industries take over 10 GL a year to irrigate grounds or to use in production of goods and services, such as construction.
An estimated 70 000 individual households take about 36 GL a year through domestic bores to irrigate gardens and for use by livestock (exempt from water licensing).
To maintain groundwater as a viable resource for ongoing use and other public as well as environmental benefits, we need to stabilise groundwater levels and enable some key areas to recover. There are many ways to do this. To help achieve a better balance, some local governments and businesses are already investing in water-efficient technology, investigating managed aquifer recharge and applying water-sensitive urban design.
New urban developments can also be an opportunity to promote water sensitive urban design and consider alternative, local water supply solutions that are fit-for-purpose, especially for public open space. All water users will need to consider options like these and use water more efficiently to adjust to reduced groundwater availability.
Perth’s drinking water
Perth’s drinking water supply is delivered by the Water Corporation through the Integrated Water Supply Scheme (IWSS) – which is a system of pipes, dams, groundwater bores and desalination plants that delivers water to over 2 million people in Perth, the Goldfields and some parts of the South West each year.
Reduced rainfall has already had a large impact on Perth's water resources. Inflow to metropolitan dams has reduced by about 60 per cent since the 1970s. This has led to increased reliance on groundwater and desalinated water from the Perth and Southern seawater desalination plants.
In 2015–16, water supplied into the IWSS came from the following sources:
- 7 per cent surface water (from our dams – this water can also contain a proportion of water that originated from groundwater or desalination, as Water Corporation banks water from these sources in our dams)
- 46 per cent groundwater
- 47 per cent desalinated seawater.
In addition, stage 1 of the Water Corporation’s Groundwater Replenishment Scheme is underway and has the capacity to recharge up to 14 GL of recycled water into groundwater supplies every year. When stage 2 of the scheme is complete in 2019, it is anticipated that this annual capacity will double to 28 GL, which will provide a new climate-independent water source.
Changing inflows to Perth metropolitan dams
With the south west of Western Australia becoming hotter and drier due to climate change, there is less inflow to dams, as well as less recharge to groundwater. Being located on one of Australia’s best groundwater resources has allowed Perth to become a modern, vibrant and green city that provides a lifestyle we all enjoy. Now, with reduced rainfall and significant use, the groundwater system has shifted out of balance and the once healthy system is under strain.
Water allocation planning for the Gnangara groundwater system
Declining rainfall has also led to a decline in water recharging the Gnangara groundwater system under northern Perth. This system has experienced a steady decline in groundwater levels over the last 40 years. Groundwater played an essential role in responding to the water shortage caused by decreased inflow to dams.
The amount of groundwater for Perth's drinking water is no longer as high as it was, and groundwater for agriculture, plantations and parks and gardens was capped and reduced slightly. However with less rainfall, groundwater use it still out of balance with recharge.
To make sure we have Gnangara groundwater for now and the future, the department is working on the next Gnangara groundwater allocation plan, and consulting with water users to find practical pathways to bring the system back into balance by 2030.