Perth: Climate and water
- Total annual rainfall was below average for the third consecutive year.
- Streamflows were below average following a relatively dry winter in 2020.
- Groundwater levels at 30 June 2021 were lower than at the same last year.
For further information on the region's climate and water conditions during the 2020–21 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 2020–21 year
- Total area-averaged rainfall for the year was 712 mm, below the mean value of 794 mm. Over the past 40 years, the total area-averaged rainfall was 718 mm which has significantly decreased from the long term average.
- Rainfall during the 2020 winter months was below average for the second consecutive year.
- In November 2020, rainfall was well above average across the entire region. Some areas in the central and northern parts of the region, including the Perth metropolitan area, experienced their wettest November on record.
- Heavy rainfall associated with a tropical low that impacted the region in early-February 2021 and a weak low pressure system in early-March 2021 contributed to above-average rainfall over the entire region during that 2-month period.
Figure C2 Total monthly rainfall for the Perth region during the 2020–21 year compared with the average and percentiles for the region
- Rainfall was above the 90th percentile in November 2020 and February–March 2021.
- The above-average rainfall during March–May 2021 contributed to the region's wettest autumn since 2016.
Actual evapotranspiration
Figure C3 Annual and monthly actual evapotranspiration deciles for the Perth region during the 2020–21 year
- Total area-averaged actual evapotranspiration was average to below average across the region.
- Evapotranspiration was below average across most of the region during July–October 2020 due to low water availability following a dry winter.
- Evapotranspiration was average to above average in March–May 2021 due to increased water availability during a wet autumn period.
Soil moisture
Figure C4 Annual and monthly soil moisture deciles for the Perth region during the 2020–21 year
- Soil moisture in the root zone (0–1 m depth) was average across most of the region.
- Soil moisture was below average over most of the region during July–October 2020 following a dry winter.
- In February–March 2021, soil moisture was well above average over most of the region following higher rainfall during those two months; however, by June 2021, 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
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 dam.
Figure C6 Total monthly flow for major
rivers in the Perth region during the 2020–21 year compared with the average and
percentiles
- Streamflows in the region's major rivers were below average for most of the year. For example, total annual flow in the Swan River at Walyunga (Station 616011) was 91 GL compared to the average value of 276 GL.
- In July–September 2020, when higher flows typically occur in the region, streamflows were below average following a relatively dry 2020 winter.
- Streamflow was well above average in the Swan and Murray rivers in March 2021 following very high rainfall during February–March 2021; however, by June 2021 streamflows were below average after a dry start to the 2021 winter.
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 2010–2021
- Following the relatively dry winter in 2020, groundwater levels at 30 June 2021 were lower than at the same time 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.