Perth: Climate and water
Annual rainfall in the region was below average. The combination of relatively poor rainfall and soil moisture conditions, particularly during July–December, contributed to well below-average streamflows in the river systems. Annual streamflows in the Harvey and Murray rivers were the lowest on record.
Introduction
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. Most of the streamflow and storage inflows occur during the winter months (June–August), and usually very little streamflow and storage inflow occurs during summer and autumn (December–May).
See Water resources in 'Region description' for more information.
Climate conditions
Rainfall
Generally below-average rainfall conditions were experienced across much of southwestern Australia during the 2015–16 year. Australia's climate was largely influenced by strong El Niño conditions in the Pacific Ocean and a positive phase of the Indian Ocean Dipole; however, poor rainfall across southwestern Australia was more influenced by a lack of cold frontal activity, particularly during the winter months (June–August) when most of the rainfall usually occurs. The lack of cold frontal activity is attributed to dominant high pressure systems that displaced the cold fronts and lows from their typical path across southwestern Australia.
The total area-averaged rainfall over the Perth region during the 2015–16 year was 649 mm, which is less than the long-term area-averaged rainfall of 797 mm (based on the 1900–2016 period). Rainfall ranged from more than 900 mm to less than 600 mm across the region (Figure C1).
Figure C1 Total annual rainfall for the Perth region during the 2015–16 year
Rainfall across the majority of the region was generally below average to very much below average for the 2015–16 year (Figure C2). Below-average annual rainfall has occurred in three of the past four years.
Figure C2 Annual and monthly rainfall deciles for the Perth region during the 2015–16 year
During the 2015–16 year, the Perth region experienced below-average rainfall for seven months of the year, including a 5-month period from July–November 2015 (figures C2–C3). Total rainfall across the Perth region during the summer period (December–February) was above average. Total rainfall in autumn (March–May) was also above average, particularly in the northern part of the region (Figure C2) where several locations in the area recorded the highest autumn rainfalls in more than 20 years (see the Bureau of Meteorology's Climate webpage).
Figure C3 Total monthly rainfall for the Perth region during the 2015–16 year compared with the long-term average and percentiles for the region
Evapotranspiration
The total area-averaged potential evapotranspiration over the Perth region during the 2015–16 year was 1,819 mm, which is marginally above the long-term area-averaged potential evapotranspiration of 1,808 mm (based on the 1911–2016 period). Potential evapotranspiration was higher along the coast and in the north of the region and lowest in the southern parts of the region (Figure C4).
Figure C4 Total annual potential evapotranspiration for the Perth region during the 2015–16 year
Potential evapotranspiration across the entire Perth region was generally average for the 2015–16 year (Figure C5).
Figure C5 Annual evapotranspiration deciles for the Perth region during the 2015–16 year
Soil moisture
Soil moisture in the root zone (0–1 m depth) for the 2015–16 year was below or very much below average across most of the Perth region, compared with the 1911–2016 period.
Figure C6 Annual and monthly soil moisture deciles in the root zone (0–1 m depth) for the Perth region during the 2015-16 year
Comparison of monthly rainfall and soil moisture deciles show the response of soil moisture to changes in rainfall align closely for the majority of months in the 2015–16 year. For example, below average soil moisture during July–November 2015 (Figure C6) reflects the well below-average rainfall that occurred across the region during this 5-month period; average to above average soil moisture during the summer and autumn months (December–May) reflects the above-average rainfall that occurred in December 2015–January 2016 and March–April 2016.
More information on soil moisture distribution across the Perth region is available in the Australian Landscape Water Balance.
Streamflow responses
Streamflow
There are three primary river systems within the Perth region:
- Swan–Canning
- Murray
- Harvey.
The location of key gauging stations at Walyunga (Station 616011), Baden Powell Water Spout (Station 614006), and Clifton Park (Station 613052) on these river systems is provided in Figure R8 in 'Region description'.
Total annual streamflows in these three river systems during the 2015–16 year were well below the long-term average, which is consistent with recent trends in rainfall and streamflow decline. Annual streamflows in the Murray and Harvey rivers were the lowest on record.
The 3-month period from July–September is usually when the majority of the annual flow occurs in these rivers. In response to the very low rainfall observed during these typically wetter months (Figure C3), total flow during this 3-month period was well below the long-term average (figures C7–C9), which contributed to the overall well below-average annual flow during the year.
Above-average monthly flows only occurred during March–May 2016 in the Swan River (Figure C7), reflecting the above-average rainfall observed during this period over the upper reaches of the Swan River catchment area (Figure C2). Flows are, however, typically low during this period of the year. Similar flow responses in the Murray and Harvey rivers were not observed, which can be attributed to a combination of less rainfall over the southern part of the region during this period and the relatively lower soil moisture conditions across the southern portion of the region following a very dry winter and spring period (Figure C6).
Figure C7 Total monthly flow for the Swan River during 2015–16 compared to the long-term average and percentiles for the river
Figure C8 Total monthly flow for the Murray River during 2015–16 compared to the long-term average and percentiles for the river
Figure C9 Total monthly flow for the Harvey River during 2015–16 compared to the long-term average and percentiles for the river
Major water reforms
The region has experienced a decline in rainfall since the 1970s (Department of Water 2009a). During this time, rainfall has declined by more than 10% during the average wet season (May–October). The result has been a 50% reduction in streamflow into the urban water supply storages and a substantial reduction in aquifer recharge. As a result, a number of major water initiatives have been pursued in recent years to secure the region's long-term water supply, including the construction of the full-scale Groundwater Replenishment Scheme.
Groundwater Replenishment Scheme
A three-year groundwater replenishment trial in the Perth region was completed in December 2012. In the trial, treated wastewater from the Beenyup Wastewater Treatment Plant was further treated to drinking water standards and recharged back into the groundwater system for future use.
As a result of the trial, the State Government announced in August 2013 that groundwater replenishment would continue in the region as an alternative water source for public water supply. Construction of Perth's full-scale Groundwater Replenishment Scheme commenced during the 2014–15 year. The first stage is on track for completion by the end of 2016 and will have capacity to recharge up to 14,000 ML of water into the groundwater system each year. In July 2016, the State Government announced an expansion of the Groundwater Replenishment Scheme by a further 14,000 ML. The expansion work is expected to be complete within three years (in 2019) and raises the scheme's total capacity to 28,000 ML per year (approximately 20% of the total wastewater collected in the region).
For more information on the Groundwater Replenishment Scheme, see the Water Corporation website.