Perth: Climate and water

  • Total annual rainfall was below average for the second consecutive year.
  • Streamflows were below average following a relatively dry winter in 2019.
  • Groundwater levels at 30 June 2020 were lower than at the same last year.

Schematic representation of the Perth region. In 2019–20, rainfall was 575 mm, less than average for the second consecutive year. Streamflow in Swan River in 2019–20 was 95 GL compared to the average value of 279 GL. Groundwater levels were slightly lower than last year.

For further information on the region's climate and water conditions during the 2019–20 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 2019–20 year

 

  • Total area-averaged rainfall for the year was 575 mm, well below the mean value of 795 mm.
  • Rainfall was low during the latter half of 2019. The below-average rainfall throughout July–December 2019 was due to a positive phase of the Indian Ocean Dipole, one of the strongest on record, that influenced Australia's climate during this period (see the Bureau's 2019–20 Climate Report for more information).
  • There was some rainfall relief during February–March 2020 as tropical moisture associated with warmer than average sea surface temperatures off the northwest coast of Australia moved across the continent and combined with southern cold fronts to produce high rainfall across the region.

 

Figure C2 Total monthly rainfall for the Perth region during the 2019–20 year compared with the average and percentiles for the region

 

  • Rainfall was above average in February 2020 and slightly above average in March 2020. All other months had below average rainfall.

 

Actual evapotranspiration

Figure C3 Annual and monthly actual evapotranspiration deciles for the Perth region during the 2019–20 year

 

  • Total area-averaged actual evapotranspiration was well below average across almost the entire region.
  • Evapotranspiration was very low during the summer months due to low water availability following below-average rainfall throughout the entire latter half of 2019.

 

Soil moisture

Figure C4 Annual and monthly soil moisture deciles for the Perth region during the 2019–20 year

 

  • Soil moisture in the root zone (0–1 m depth) was below average across most of the region.
  • In July 2019, root-zone soil moisture was already average to below average following a relatively dry winter in 2019. Dry conditions during the latter half of 2019 meant that by January 2020, root-zone soil moisture was well below average over the entire region.
  • Soil moisture was above average over large parts of the region during March 2020 following higher rainfall in February 2020; however, by June 2020, soil moisture had returned to below-average conditions.
  • More information on soil moisture distribution across the Perth region is available in the Australian Landscape Water Balance.

 

Hydrological responses

Streamflow

Map of key gauging stations. Swan River at Walyunga, station number 616011, is in the northern part of the region about 40 km upstream of Perth. Murray River at Baden Powell Water Spout, station number 614006, is on the Darling Range in the region's south. Harvey River at Clifton Park, station number 613052, is in the southern part of the region on the coastal plain.
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 major rivers in the Perth region during the 2019–20 year compared with the average and percentiles

 

  • Streamflows in the region's major rivers were below average for the entire year. For example, total annual flow in the Swan River at Walyunga (Station 616011) was 95 GL compared to the average value of 279 GL.
  • In July–September 2019, which is when higher flows typically occur in the region, streamflow was below average following a relatively dry 2019 winter. 
  • Streamflow was very low during the latter part of 2019. During October–December 2019, flow in all three rivers was below the 10th percentile.
  • Despite some rainfall relief in early 2020, flows remained very low during the first half of 2020. There was no flow in the Swan River for a 5-month period between January–May 2020 and flow in the Murray River during February–April 2020 was the lowest on record.

 

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 2010–2020

 

  • Following the relatively dry winter in 2019, groundwater levels at 30 June 2020 were lower than at the same last year.
  • Although groundwater levels in Perth's aquifers remain low compared to the historical average, the improved rainfall conditions during the 3-year period between July 2016–June 2019 have led to an increase in water levels since 2016 in both the Gnangara and Jandakot mounds.
  • The rising trend has also been influenced by efforts to reduce and redistribute groundwater extraction.
  • 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.