Perth: Climate and water
- The region experienced its wettest July–August period in 17 years, and unseasonally high rainfall in January associated with ex-tropical cyclone Joyce.
- Annual rainfall exceeded the average for the first time in six years.
- Groundwater levels increased for the second successive year.
For further information on the region's climate and water conditions during the 2017–18 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 2017–18 year
- Total area-averaged rainfall across the region was 797 mm, which exceeded the average of 786 mm for the first time in six years.
- Rainfall was well above average in the normally dry months of January and December; it was particularly high in January due to unseasonal rain on 15 January 2018 associated with ex-tropical cyclone Joyce.
- On 15 January 2018, record daily rainfall of up to 145 mm was experienced at sites along the coastal parts of the region.
Figure C2 Total monthly rainfall for the Perth region during the 2017–18 year compared with the average and percentiles for the region
- During July–August 2017 period, typically the wetter months of the year, rainfall was above average, and the highest since 2000.
- Total rainfall in January was above the 90th percentile and more than three times the January average.
- Following the wet summer, the last five months of the year were drier than average.
Evapotranspiration
Figure C3 Annual and monthly actual evapotranspiration for the Perth region during the 2017–18 year
- Annual area-averaged actual evapotranspiration was close to the average, reflecting near-average annual rainfall conditions across the region.
- In January and February evapotranspiration was above average due to increased water availability following the unseasonal rainfall in mid-January associated with ex-tropical cyclone Joyce.
- In the latter part of the year, evapotranspiration was generally below average reflecting the below average rainfall.
Soil moisture
Figure C4 Annual and monthly soil moisture deciles for the Perth region during the 2017–18 year
- Soil moisture in the root zone (0–1 m depth) was average across most of the region.
- Rainfall and soil moisture are typically closely aligned. For example, well above average soil moisture in January and February was due to the unseasonal rainfall in mid-January. Below average soil moisture in the latter part of the year reflected relatively poor rainfall in this period.
- 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
- Streamflow in the region was close to average for most of the year reflecting the near-average rainfall conditions.
- In August 2017, streamflows in the Murray and Harvey rivers exceeded the 90th percentile for that month reflecting the wetter than average July–August conditions.
- The rainfall on 15 January 2018 associated with tropical cyclone Joyce caused unseasonal January flows in the Murray and Swan rivers. However the flows were still much lower than in the winter months due to the drier summer catchment conditions.
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 ecosytems 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 compared to average and percentiles
- Although groundwater levels in the Superficial aquifer remain low compared to the historical average, the higher rainfall of the past two years has resulted in water level rises in both the Gnangara and Jandakot mounds. In the Gnangara Mound this has resulted in the highest water levels in about ten years.
- The impacts of the wetter conditions have been less pronounced in the confined aquifers that are separated from the surface by aquitards (low permeability shale and clay layers). Fluctuations in these aquifers are related more to usage than climate; water levels remain low compared to the average and similar to the past few years.
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 three 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 late 2017; by June 2018, 7 GL of wastewater had been injected into the aquifers.
- 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 Myalup-Wellington project.
- Amongst its aims is arresting and reversing 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.
- The project received commitments of funding from the state government in January 2017 and from the federal government in May 2018.
- For further information refer to the Western Australian Government’s Water for Food website.