National Water Account 2018

Menu

Key findings

Climate and water

Water Accounting Statements

Supporting information

Region description

Reference information

Perth: Statement details

Water assets

Surface water assets

• Surface water asset volumes in the Perth region at 30 June 2018 were 390,226 ML, 99% of which is water held in storages. 

Figure S1 Total storage volume in the region at 30 June 2018 compared with the previous 8 years

• Storage volumes increased from 34% full at 30 June 2017 to 45% full at 30 June 2018 due to higher inflows, low urban diversions and 'banking' of desalinated water in the surface water storages. The end of year storage volume of 45% full was the highest in more than 20 years.  
• Because of the large soil water store in the storage catchments, inflows are affected by antecendent catchment conditions relating to rainfall in previous years. The higher inflows in 2017–18 are a consequence of high winter rainfall combined with wetter antecedent conditions produced by rainfall in the previous two years. 
• The low diversions and banking reflect the Water Corporation's reduced reliance on surface water in recent years. This is as a consequence of their adoption of a range of demand management strategies and continued development of climate-independent water sources in response to the long-term decline in storage inflows (see Urban water store note). Banking represented about 30% of the total water input to the urban storages in 2017–18. 

Figure S2 Percentage-full volume on 30 June 2018 for each storage

• Storage levels peaked in October at over 50% full following above-average rainfall in July–September. This is the first time storages exceeded 50% full since 2014. 
• About 60% of the surface water stored in the region was for urban supply and 40% for irrigation.
• More detailed information on the individual storages within the region is available on the Bureau of Meteorology's Water Storage website.

Groundwater assets

• Volumes of water stored in aquifers cannot be quantified due to a lack of available data; however, information on the total annual change in aquifer storage is available.

Figure S3 Total annual change in aquifer volume in the region at 30 June 2018 compared with the previous 7 years

• The near average annual rainfall contributed to a second consecutive year of increasing aquifer storage in 2017–18. This was reflected in rising groundwater levels (see Figures C7 and S24).

Water liabilities

• Total water liabilities for the Perth region was 149,038 ML, which refers to the volume of allocation remaining on licence entitlements at the end of the 2017–18 year.

• Many of the water supply licences for the region do not have a water management year that ends on 30 June; therefore, the allocation remaining is the unused component of the annual allocation for these entitlements.
• A more detailed description of water allocations and associated water rights in the Perth region is given in the Water rights and use note.

Water asset increases

• Total water asset increases for the year was 3,956,818 ML. 

• The key items contributing to water asset increases in the region were recharge: landscape and runoff, which together made up about two thirds of the total water asset increases. These items were high relative to the previous several years reflecting the higher rainfall across the region (see Climate and water).
• Urban water system increases primarily represent wastewater collected and delivery: desalinated water.
• Following a decade of increasing desalinated water production since the first desalination plant was commissioned in 2006, desalinated water production has stabilised at about 150,000 ML/year reflecting the region's two desalination plants operating near capacity.
• Wastewater collected changes little from year to year as it is not strongly influenced by climate (see Urban water store note).

Water asset decreases

• Total water asset decreases for the year was 3,373,882 ML. 
• The largest items contributing to water asset decreases for the region were surface water outflow and groundwater discharge: landscape, which together made up about half of the total water asset decreases. Both were high relative to the previous several years reflecting the above average rainfall.
• Urban water system decreases primarily relate to supply system delivery: urban users, which change little from year to year.
• Irrigation scheme decreases represent the delivery of water to scheme customers.

Water flows

• Total water inflows to the region was 3,737,510 ML, lower than the volume of Water asset increases (3,956,818 ML). Total water inflows include allocated surface water diversions and groundwater extractions by the urban water system and irrigation scheme; total water asset increases include the annual allocation for surface water and groundwater for the urban water system and irrigation scheme.  
• Total water outflows from the region was 3,779,865 ML, higher than the volume of Water asset decreases (3,373,882 ML). Total water outflows include the flows corresponding to the accrual transations–allocated diversions and allocated extractions. Total water asset decreases include the forfeiture of allocations. 
• A detailed description of all the water flows associated with the surface water store, groundwater store, urban water system and irrigation scheme is provided in the Water stores note.

Unaccounted-for difference

• The unaccounted–for difference is the volume necessary to reconcile the opening and closing water storage with the total water inflows and outflows reported in the water accounting statements.

• The unaccounted-for difference is mostly attributed to uncertainties associated with the estimates of runoff and river losses (see Methods).