National Water Account 2018

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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 31% from 297,520 ML at 1 July 2017 to 390,226 ML at 30 June 2018.  
• The balancing item is mostly attributed to uncertainty associated with the estimates of runoff and river losses (see Methods). 

Surface water flows

Figure S4 Water inflows and outflows for the surface water store during the 2017–18 year

• The largest surface water inflows in the region were runoff and groundwater discharge, which together comprised 79% of the total.
• Runoff was near average, and about 30% higher than the average over the past decade, reflecting the high winter rainfall compared to recent years. 
• 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 2017–18 year by allocation area

• Total surface water diverted in the region was 196,763 ML. 
• Diversions to the Harvey Water irrigation scheme were 37% of the total and came from storages in the Harvey and Collie river catchments.
• Diversions to the urban system were 44% 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 2017–18 year

• The majority of groundwater inflows came from recharge: landscape. Recharge was the highest in four years due to the high rainfall compared to previous years.

• The largest groundwater outflows in the region were discharge to landscape and surface water, both of which were higher than the previous several years due to the higher groundwater levels (see Figures C6 and S24).
• Managed aquifer recharge increased by about 7,000 ML from the previous year with the commencement of Perth's Groundwater Replenishment Scheme in November 2017 (see Major water reforms).
• Urban system groundwater extractions declined by 5% from last year, as 'banking' of groundwater in the surface water storages was reduced due to improved storage inflows.
• The combined effect of reduced urban extraction and increased managed aquifer recharge was a 10% decline in net groundwater withdrawals by the urban system.

Figure S7 Allocated groundwater extractions during the 2017–18 year by groundwater management plan area

• Total allocated groundwater extraction in the region was 431,468 ML. 
• About two thirds of the allocated extraction was for licensed individual users and one third for the urban system.
• The Gnangara plan area had the highest groundwater extraction, comprising about half of the region's total and split 57% for individual users and 42% for the urban system.
• 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 2017–18 year

Water supply inflows

Figure S9 Water sources used in the Perth region's urban supply system during the 2017–18 year compared with the previous 7 years

• Total inflows to the water supply system was 283,782 ML, similar to the previous year.
• Following very low surface water storage inflows in recent years, the Water Corporation has moved to relying almost entirely on desalinated water and groundwater for urban supply. In 2017–18, these two sources made up 99% of the urban supply. 
• 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.
• In 2017–18 discharge to the urban surface water storages ('banking') was approximately balanced by diversions from the storages, such that net surface water diversions were less than 0.5% of total supply.
• Delivery of desalinated water was similar to the previous year reflecting the region's two desalination plants running near capacity over the year.
• Groundwater extraction by the urban system was 5% 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 141,185 ML during the 2017–18 year.

Water supply outflows

Figure S10 Outflows from the water supply system

• The majority of water supply outflow was delivery to urban users.
• Supply system discharge to surface water storages ('banking') decreased by 15% from the previous year due to improved storage inflows.
• 8% of the total outflow from the urban supply system was related to losses.

Wastewater and recycled water system outflows

Figure S11 Outflows from the wastewater and recycled water systems

• About 10% of the treated wastewater was recycled, either through direct delivery to urban users or indirectly via aquifer recharge.
• Managed aquifer recharge increased about sixfold from the previous year due to the start-up of Perth's Groundwater Replenishment Scheme (see Major water reforms).
• Most of the remaining treated wastewater was discharged to the 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 2017–18 year

• Diversions to the irrigation scheme were 23% more than the previous year, reflecting higher allocations for the Waroona and Harvey irrigation districts enabled by improved water availability in the Harvey, Waroona and Logue Brook reservoirs.
• 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 2017–18 year

• There are about 1,621 customers in the Harvey Water irrigation scheme, most of whom receive water from the scheme for irrigation purposes.