Perth
Water overview
Significant water events
Soil moisture
Figure W1 shows that soil moisture was generally average to below average in the Perth region during the 2013–14 year (compared against the 1970–2014 period). The soil moisture is based on a simple conceptual representation of soil water storage and transfer processes averaged over a 5 km x 5 km grid cell derived from the Australian Water Resources Assessment system landscape model (AWRA-L) version 3.0. The model produces a soil moisture grid for the region at a daily time step, which is then averaged over the entire year to produce an annual soil moisture grid. The annual grid represents the mean daily volume of soil moisture across the region during the year.
Figure W1 Annual soil moisture deciles for the Perth region during the 2013–14 year
Despite the generally average rainfall conditions experienced across the region during the 2013–14 year (see Climate overview), streamflow in the Swan and Murray rivers was below average (see Streamflow). This is likely to be attributed to the generally dry conditions in the soil in the Swan River catchment and the upper reaches of the Murray River catchment (Figure W1). Similarly, average soil conditions across the Harvey River catchment has contributed to the average streamflow in the Harvey River.
Streamflow
There are three primary river systems within the Perth region:
- Swan–Canning
- Murray
- Harvey.
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 and Murray rivers during the 2013–14 year were approximately 40% below the long-term average for these rivers; however, total annual flow in the Harvey River during the year was equivalent to the long-term average flow. These below average to average streamflows can be attributed to the soil moisture conditions across the region during the 2013–14 year (see Soil moisture).
Annual flows in all three rivers during the 2013–14 year were greater than that observed during the previous year.
Figures W2–W4 show that monthly flow in the three rivers were below average for all but two months of the year: September–October in the Swan and Murray rivers and August–September in the Harvey River. These flow conditions in the early part of the 2013–14 year reflect the above average rainfall conditions experienced across the region during August–September (see Climate overview). The well above average rainfall in May 2014 did not result in above average streamflows in these rivers, presumably because soil water stores and groundwater levels were relatively low after the dry summer period.
Figure W2 Total monthly flow along the Swan River during the 2013–14 year compared with the long-term average and percentiles for the river
Figure W3 Total monthly flow along the Murray River during the 2013–14 year compared with the long-term average and percentiles for the river
Figure W4 Total monthly flow along the Harvey River during the 2013–14 year compared with the long-term average and percentiles for the river
Major water initiatives
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 the region during recent years to secure water supply in the long term. Two major water initiatives which have continued during the 2013–14 year include the completion of the second desalination plant and planning for a full-scale groundwater replenishment scheme.
Desalinated water
The Southern Seawater Desalination Plant is located near Binningup in the southern part of the Perth region (see Figure P3 in 'Physical information'). The plant was designed to be constructed in two stages, each able to produce 50,000 ML of water per year for the Integrated Water Supply Scheme. The second stage of the plant was fully commissioned in late 2013. Desalinated water can now provide almost 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.
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. Planning has now commenced for construction of Australia's first full-scale groundwater replenishment scheme in the region. For more information on Perth's groundwater replenishment scheme, see the Water Corporation website.