National Water Account 2017

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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 2017 were 297,520 ML, which represent the volume of water held in storages. 

Figure S1 Total storage volume in the region at 30 June 2017 compared with the previous 7 years

• Storage volumes increased from 31% full (262,516 ML) to 35% full (297,520 ML) during the year due to higher inflows and reduced diversions compared to the previous year.
• The higher inflows were the result of improved rainfall at the end of 2015–16 and start of 2016–17. The reduced diversions reflected low storage levels at the start of the year, as well as reduced summer demand related to wet summer conditions.
• Transfers of water from the urban supply system into the storages were much larger than previous years. The transfers, which occur to store groundwater and desalinated water produced during low demand periods, exceeded total diversions by 8,532 ML (see Urban water system note).
• While the record summer rain produced high streamflows in the Swan and Murray rivers, it generated little flow into storages, reflecting differences in catchment characteristics and the distribution of rainfall.

Figure S2 Percentage-full volume on 30 June 2017 for each storage

• The majority of the large storages in the region were less than 40% full reflecting many years of below-average rainfall.
• 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 2017 compared with the previous six years

• Increased annual rainfall from the previous year, including unusually high rainfall during the typically drier summer months, contributed to an increase in aquifer storage; this was the first annual increase in aquifer storage for the region in three years.

Water liabilities

• Total water liabilities for the Perth region was 110,896 ML, which refers to the volume of allocation remaining on licence entitlements at the end of the 2016–17 year.

• Most 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 allocations note.

Water asset increases

• Total water asset increases for the year was 3,246,829 ML. 

• The key items contributing to water asset increases in the region were recharge: landscape, runoff and inter-region inflow which together made up more than 80% of the total water asset increases. The 2016–17 volumes for all three items were higher than the previous year reflecting the higher rainfall at the start of the year and the unusually high summer rainfall (see Climate and water).
• Urban water system increases primarily represent wastewater collected and delivery: desalinated water.
• Delivery: desalinated water has consistently risen since the first desalination plant was commissioned in 2006 as the Water Corporation has progressively increased production in response to low surface water availability.
• 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 2,758,687 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 60% of the total water asset decreases.
• Surface water outflow was much higher than the previous year; about 30% of the surface water outflow occurred in the Swan and Murray rivers in February and March following the high summer rains (see Climate and water).
• Urban water system decreases primarily relate to supply system delivery: urban users.
• Irrigation scheme decreases represent the delivery of water to scheme customers.

Water flows

• Total water inflows to the region was 3,246,829 ML, equal to the volume of Water asset increases.
• Total water outflows from the region was 3,088,911 ML, higher than the volume of Water asset decreases (2,758,687 ML) as it includes flows corresponding to the accrual transactions—allocated diversions and extractions of water (see the Water liabilities section above).
• A detailed description on 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 volume is primarily attributed to the fact that groundwater storage cannot be quantified in the account. As a result the unaccounted-for-difference includes the change in groundwater storage over the year.