Perth: Climate and water overview
Climate conditions
Rainfall
The total area-averaged rainfall over the Perth region during the 2014–15 year was 614 mm, which is much less than the long-term area-averaged rainfall of 799 mm (based on the 1900–2015 period). Rainfall ranged from more than 900 mm in the southern central part of the region to less than 400 mm in the far southeast (Figure C11).
Figure C11 Total annual rainfall for the Perth region during the 2014–15 year
Rainfall across the majority of the Perth region was generally below average to very much below average for the 2014–15 year (Figure C12). These conditions represent a decrease in rainfall compared to the 2013–14 year total of 777 mm (see the 2014 Account). This decrease in rainfall during the year contributed to decreased flows in the major river systems in the region (see Streamflow).
Figure C12 Annual and monthly rainfall deciles for the Perth region during the 2014–15 year
Generally below average rainfall conditions were experienced across much of Australia's southwest coast during the 2014–15 year. Climate across Australia was largely influenced by persistent near El Niño conditions in the Pacific Ocean from late-2014, and the Bureau of Meteorology declared an El Niño event in May 2015.
During the 2014–15 year, below average rainfall occurred for the majority of months during the year (Figure C13). Above average rainfall occurred during February–April 2015; however, the impact on annual rainfall was minimal as these months are typically low rainfall months.
Below average rainfall was experienced during the typically wetter months of July–August and May–June, which had much more substantial impacts on total annual rainfall. The low rainfall during these periods contributed to the below average streamflow conditions in the region (see Streamflow).
Figure C13 Total monthly rainfall for the Perth region during the 2014–15 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 2014–15 year was 1,821 mm, which is slightly above the long-term area-averaged potential evapotranspiration of 1,805 mm (based on the 1911–2015 period). Potential evapotranspiration was higher in the southern parts of the region and lowest along the coast and in the north (Figure C14).
Figure C14 Total annual potential evapotranspiration for the Perth region during the 2014–15 year
Potential evapotranspiration during the 2014–15 year was average across most of the Perth region and above average in the western part of the region (Figure C15).
Figure C15 Annual evapotranspiration deciles for the Perth region during the 2014–15 year
Soil moisture
Figure C16 shows that annual soil moisture, up to 1m depth (root zone) for the 2014–15 year was below average across most of the Perth region and average in the southern central part of the region (compared with the 1911–2015 period).
Figure C16 Annual and monthly soil moisture deciles in the root zone (0 to 1m depth) for the Perth region during the 2014–15 year
Figure C16 also shows monthly soil moisture deciles in the root zone (0 to 1m depth) during the 2014–15 year. Comparison of monthly rainfall and soil moisture deciles show the response of soil moisture to changes in rainfall and align closely for the majority of months in the 2014–15 year.
The combination of the generally below average rainfall conditions experienced across the region (see Rainfall) with the below average soil moisture observed in the upper reaches of the primary river catchments (Figure C16), contributed to the below average streamflow in the Swan, Murray, and Harvey rivers (see Streamflow).
More information on soil moisture distribution across the Perth region is available in the Australian Landscape Water Balance.
Streamflow responses
Significant water events
There were no significant water events in the Perth region during the 2014–15 year.
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 C8 in the 'Physical information'.
Consistent with the short-term mean annual flows over the last ten years, total annual flows in these three river systems during the 2014–15 year were well below the long-term average. Annual flows were approximately 60–85% of the total annual flows observed during the 2013–14 year.
During the typically wetter months of the year (July–September and May–June), monthly flow in the three primary rivers was entirely below the long-term average (figures C17–C19). These flow conditions reflect the below average rainfall conditions generally experienced across the region (see Rainfall).
The above average rainfall experienced during February–April 2015 did not result in above average streamflows in these rivers during this period, presumably because soil water stores and groundwater levels were relatively low following a very dry summer period.
Figure C17 Total monthly flow for the Swan River during 2014–15 compared to the long-term average and percentiles for the river
Figure C18 Total monthly flow for the Murray River during 2014–15 compared to the long-term average and percentiles for the river
Figure C19 Total monthly flow for the Harvey River during 2014–15 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. A major water initiative during the 2014–15 year was the commencement of the construction of a full-scale groundwater replenishment scheme.
Groundwater replenishment
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 the full-scale Perth Groundwater Replenishment Scheme commenced during the 2014–15 year and is on track for completion by the end of 2016. For more information on this groundwater replenishment scheme, see the Water Corporation website.