Perth: Climate and water
- Total annual rainfall was below average, despite the 2018 winter being the wettest since 2000.
- The wet winter contributed to high streamflows during July–August 2018.
- Groundwater levels increased for the third successive year.
For further information on the region's climate and water conditions during the 2018–19 year scroll down this page or click on the links below:
About the region
- The Perth region lies within southwestern Australia, which generally experiences dry, hot summers and wet, mild winters. Most of the rainfall in the region occurs during a 5-month period from May–September.
- The seasonal flow characteristics of the region's rivers reflect the distribution of rainfall over the year, with most of the streamflow and storage inflows occurring during the winter and early spring months (June–October).
Climate conditions
Rainfall
Figure C1 Annual and monthly rainfall deciles for the Perth region during the 2018–19 year
- Total area-averaged rainfall for the year was 655 mm, below the mean value of 786 mm.
- Rainfall was very low during September 2018 and May 2019 across the entire region; some coastal areas south of Perth experienced their driest May on record.
- The below-average rainfall throughout the spring 2018 months was likely due to a positive phase of the Indian Ocean Dipole that influenced Australia's climate during this period (see the Bureau's 2018–19 Climate Report for more information).
- High rainfall during July–August 2018 contributed to the region's wettest winter (June–August) since 2000.
Figure C2 Total monthly rainfall for the Perth region during the 2018–19 year compared with the average and percentiles for the region
- Rainfall was below average for a 9-month period between September 2018–May 2019.
- The monthly rainfall in September 2018 and May 2019 was well below the 10th percentile. It was the driest May in more than 70 years and the fourth lowest rainfall in September on record.
- Above-average rainfall occurred during July–August 2018 and June 2019, the typically wetter months of the year. Rainfall in June 2019 was above average for the first time in 10 years and the highest in more than 20 years for that month.
Actual evapotranspiration
Figure C3 Annual and monthly actual evapotranspiration for the Perth region during the 2018–19 year
- Total area-averaged actual evapotranspiration was below average across most of the region.
- Evapotranspiration was generally below average due to the dry conditions and low water availability across the region throughout most of the year.
Soil moisture
Figure C4 Annual and monthly soil moisture deciles for the Perth region during the 2018–19 year
- Soil moisture in the root zone (0–1 m depth) was below average across most of the region.
- Rainfall and soil moisture are typically closely aligned. For example, very poor soil moisture conditions across the entire region during the 9-month period from October 2018–June 2019 is due to well below-average rainfall that occurred during most of that period.
- Above-average soil moisture conditions across the region only occurred in August 2018 following high rainfall during July–August 2018.
- More information on soil moisture distribution across the Perth region is available in the Australian Landscape Water Balance.
Hydrological responses
Streamflow
Figure C5 Key flow gauging stations along the main rivers within the Perth region
- The Swan, Murray and Harvey rivers are the largest rivers in the region. The Swan River's catchment extends to the east of the Perth region where the river is known as the Avon River, while the Murray and Harvey river catchments lie completely within the Perth region.
- The Swan and Murray rivers are unregulated whereas the Harvey River is regulated by the Harvey Reservoir.
Figure C6 Total monthly flow for key Perth region rivers compared to average and percentiles
- Total annual flows in the region's major rivers were close to average. For example, total annual flow in the Swan River at Walyunga (Station 616011) was 297 GL compared to the average value of 283 GL.
- In July–August 2018, streamflows were generally above average reflecting the wetter than average rainfall during the winter months (June–August).
- Streamflows during the remainder of the year (September 2018–June 2019) were below average, which was influenced by the relatively low rainfall and poor soil moisture conditions experienced across the region during this period.
Groundwater levels
- The Perth region includes a number of important aquifers. The shallow Superficial Aquifer is an important source of public and self-supply and supports numerous groundwater-dependent ecosystems in the Gnangara and Jandakot mounds. The deeper confined aquifers (e.g. Leederville and Yarragadee) are major sources of urban supply.
Figure C7 Groundwater levels for key bores within the Perth region from 2010–2019
- Although groundwater levels in Perth's aquifers remain low compared to the historical average, the higher winter rainfall of the past three years has resulted in water level rises in both the Gnangara and Jandakot mounds.
- Following the high rainfall in winter 2018, groundwater levels in the Gnangara and Jandakot mounds were the highest in more than 9 years.
- Groundwater levels in the confined aquifers that are separated from the surface by aquitards (low permeability shale and clay layers) remain low compared to the average. Fluctuations in these aquifers are related more to usage than climate.
Major water reforms
Groundwater Replenishment Scheme
- To secure the region's water supply in the face of large declines in reservoir inflows since the 1970s, the state government has pursued climate-independent water sources including desalination and managed aquifer recharge.
- In 2013, following a successful 3-year trial, the government initiated Perth's Groundwater Replenishment Scheme. The scheme includes injection of treated wastewater to Perth's confined aquifers at Beenyup in Perth's north.
- The project is being developed in two stages, each with 14 GL/year of injection.
- Stage 1 started operation in 2017 and stage 2 is planned to be completed at the end of 2019.
- For more information refer to the Water Corporation webpage.
Harvey Water irrigation expansion
Figure C8 Proposed expansion of the Harvey Water irrigation area
- Harvey Water is participating with Collie Water in the development of the $400 million Myalup-Wellington project.
- One of the project's aims is to arrest and reverse the increased salinity in Wellington Reservoir. Wellington Reservoir is the second largest surface water storage in Western Australia and salinity levels are currently recorded at about 1,200 mg/L.
- As part of the project, the open channels in the Collie River Irrigation District will be replaced with gravity pressure pipes.
- The water saved by piping the Collie River district will be used to expand production at the Myalup precinct.
- Construction is planned to start in 2020 with project delivery between 2021–25.
- For further information refer to the Western Australian Government’s Water for Food website.