National Water Account 2019

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Key findings

Climate and water

Water Accounting Statements

Supporting information

Region description

Reference information

Perth: Water stores

Surface water store

• The volume of surface water in the region increased by 6% from 390 GL at 1 July 2018 to 415 GL at 30 June 2019.
• The balancing item is approximately two thirds of the closing surface water store and is primarily attributed to uncertainties associated with the runoff and river loss estimates (see Methods).

Surface water flows

Figure S4 Water inflows and outflows for the surface water store during the 2018–19 year

• The largest surface water inflow in the region was runoff. Runoff was slightly higher than average reflecting the high 2018 winter rainfall across the region.
• The majority of the surface water inflows in the region discharged to the sea as river outflow.

Figure S5 Allocated surface water diversions during the 2018–19 year by allocation area

• Total surface water diverted in the region was 226 GL.
• Diversions to the Harvey Water irrigation scheme were 33% of the total and came from storages in the Harvey and Collie river catchments.
• Diversions to the urban system were 48% of the total, and included groundwater and desalinated water 'banked' in the storages. These diversions came from the storages in the Canning, Dandalup, Serpentine and Harvey river catchments.
• Diversions by individual users were 19% of the total and mainly came from unregulated rivers in the Murray River catchment.
• For a more detailed description of the water usage in the region, and the associated entitlements, see the Surface water rights note.

Groundwater store

• The volume of groundwater in the region could not be quantified due to a lack of available data; however, the change in groundwater storage during the year was estimated from groundwater models.
• It is likely that the balancing item is primarily attributed to uncertainties associated with the groundwater recharge and discharge, which are estimated from groundwater models, and with individual user water use volumes, most of which are estimated from licensed entitlements (see Methods).

Groundwater flows

Figure S6 Water inflows and outflows for the groundwater store during the 2018–19 year

• The majority of groundwater inflows came from recharge: landscape. Recharge has been relatively high during the last three years due to high winter rainfall.
• The largest groundwater outflows in the region were discharge to landscape and surface water. Discharge was the highest in five years due to the higher groundwater levels (see Climate and water).

Figure S7 Allocated groundwater extractions during the 2018–19 year by groundwater management area

• Total groundwater extracted in the region was 516 GL, about 84% of which was allocated (433 GL).
• More than 70% of the allocated extraction was for licensed individual users; the remainder was for the urban system.
• The Gnangara plan area had the highest allocated groundwater extraction, comprising almost half of the region's total.
• For a more detailed description of the water usage in the region, and the associated entitlements, see the Groundwater rights note.

Urban water system

• The volume of water in the region's urban system comprises water in the pipe network. As no major pipe network changes were made, this volume is assumed to remain unchanged throughout the year.
• The balancing item is likely attributed to metering inaccuracies, unaccounted losses, and uncertainties in the estimation of certain volumes (see Methods). The balancing item for the urban system is generally relatively small as the system is controlled and contained.

Urban water system flows

Figure S8 Water inflows and outflows for the urban water system during the 2018–19 year

Water supply inflows

Figure S9 Water sources used in the Perth region's urban supply system during the 2018–19 year compared with the previous eight years

• Total inflows to the water supply system was 287,533 ML, similar to the previous year.
• Very low surface water storage inflows in recent years meant the Water Corporation had to rely almost entirely on desalinated water and groundwater for urban supply during 2015–2018.
• In 2018–19, following a second succesive year of above-average winter rainfall and high storage inflows, net surface water diversions increased to more than 25% of total supply; the highest contribution in 10 years.
• The increased surface water contribution meant less desalinated water was required to meet urban demand. For the first time in several years, the region's two desalination plants were not running at near capacity.
• The supply strategy requires 'banking' of groundwater and desalinated water in the urban surface water storages during low demand periods to buffer peak period supply requirements.
• Groundwater extraction by the urban system was 7% less than last year, as banking of groundwater in the surface water storages was reduced due to improved storage inflows.

Wastewater and recycled water inflows

• Wastewater collected was 141,684 ML during the 2018–19 year.

Water supply outflows

Figure S10 Outflows from the water supply system

• The majority of water supply outflow was supply system delivery to urban users.
• Supply system discharge to surface water storages ('banking') decreased by 60% from the previous year due to improved storage inflows.
• 11% of the total outflow from the urban supply system was related to leakage and other losses.

Wastewater and recycled water system outflows

Figure S11 Outflows from the wastewater and recycled water systems

• About 9% of the treated wastewater was recycled, either through direct delivery to urban users or indirectly via aquifer recharge.
• Most of the remaining treated wastewater was discharge to sea.

Irrigation scheme

• The Perth region's irrigation scheme is the Harvey Water Irrigation Area which comprises three irrigation districts: Waroona, Harvey, and Collie River.

• The volume of water in the region's irrigation system comprises water in the pipe and channel network. As no major changes were made to this network, this volume is assumed to remain unchanged throughout the year.
• The balancing item may be due to a number of factors, such as metering inaccuracies, unaccounted losses and estimation errors (see Methods).

Irrigation scheme flows

Figure S12 Water inflows and outflows for the irrigation scheme during the 2018–19 year

• Diversions to the irrigation scheme were similar to the previous year, reflecting the higher allocations for the Waroona and Harvey irrigation districts and the improved water availability in the Harvey, Waroona and Logue Brook reservoirs over the last two years.
• Leakage represents losses from the open channel network in the Collie River irrigation district.

Figure S13 Water delivered for each irrigation scheme customer category during the 2018–19 year

• There are more than 1,600 customers in the Harvey Water irrigation scheme, most of whom receive water from the scheme for irrigation purposes.