Significant water events
There are three primary river systems within the Perth region:
More detail on these river systems, including location, is provided in the Surface water section of the 'Contextual information'.
Consistent with the short-term mean annual flows over the last ten years, total annual flows in the Swan, Murray and Harvey rivers during the 2011–12 year were well below the long-term mean annual flows for these rivers. However, total annual flows were two to six times larger than the record low flows observed in these rivers during the 2010–11 year.
During the wetter months of the year (July–October and May–June), monthly flow in the three primary rivers was almost entirely below the 50th percentile (Figures W1–W3). No flow was observed in the Swan River during a 3-month period between March and May 2012 (Figure W1).
Rainfall during the 2011–12 year was equivalent to average conditions (see Climate overview). Therefore, the below average flows in these primary river systems are indicative of the unsaturated soil conditions in the region as a result of the record low rainfall observed the previous year (see the 2011 Account).
Figure W1 Graph of total monthly flow along the Swan River during the 2011–12 year compared with the long-term percentiles
Figure W2 Graph of total monthly flow along the Murray River during the 2011–12 year compared with the long-term percentiles
Figure W2 shows that total flow during December 2011 in the Murray River was an order of magnitude greater than both the mean monthly flow and the 90th percentile for that month. Total flow during January 2012 was also higher than the 90th percentile for that month. This reflects the record high rainfall observed during December (see Climate overview) and, given similar high flows were not observed in the Swan and Harvey rivers (Figures 1 and 3 respectively), indicates that much of this rainfall occurred over the upper reaches of the Murray River catchment.
Figure W3 Graph of total monthly flow along the Harvey River during the 2011–12 year compared with the long-term percentiles
Major water initiatives
The region has experienced a decline in rainfall since the 1970s (Perth–Peel Regional Water Plan, 2009). 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 were pursued during the 2011–12 year in the region, including the construction of a second desalination plant and a groundwater replenishment trial.
The Southern Seawater Desalination Plant is located near Binningup in the southern part of the Perth region (see Figure P3 in General description). The first stage of the plant was commissioned in September 2011 and is capable of producing 50,000 ML of water for the Perth metropolitan area per year.
The second stage of the plant was commissioned in January 2013 and will double the plant's capacity to 100,000 ML per year. Desalinated water will then provide more than half the urban water needed for the Perth metropolitan area.
This water resource is part of the Water Corporation's long-term water security solution for the Perth region. For more information on desalinated water in the Perth region, see the Water Corporation website.
The groundwater replenishment trial has been conducted in the Perth region since November 2010. Treated wastewater from the Beenyup wastewater treatment plant is further treated to drinking water standards and recharged back into the groundwater store for future use. This process is referred to as managed aquifer recharge in the 2012 Account (see line item 10.9).
For more information on the groundwater replenishment trial, see the Water Corporation website.