Perth
Water accounting policies

The information presented in the Perth region water accounting report is based on agreements between the Western Australian Department of Water, Water Corporation, Harvey Water, and the Bureau of Meteorology (the Bureau).

This report has been prepared using an accrual basis of water accounting, with the exception of the physical water flow information. The water attribute being quantified is volume, and the unit of account is megalitres (ML).

Water transfers to and from lakes and wetlands are included within water transfers to and from the water table aquifer. That is, lakes and wetlands are considered to be a part of the groundwater store on the Statement of Changes in Water Assets and Water Liabilities and the Statement of Water Flows because they are where the water table is exposed at the surface.

Only water that is held or managed by the State licensing authority or the urban utilities and is part of the entitlement system is defined as a water asset. Provided its volume can be quantified in a way that is complete, neutral, and free from material error, the volume of the water asset or water liability is recognised in the Statement of Water Assets and Water Liabilities.

Water held in storages that has already been abstracted from the entitlement system is deemed not to be part of the region (regardless of the size of the storage or its physical connection to the system). This is because the attendant water right has already been exercised.

The increases and decreases to the reported water assets and water liabilities consist of changes that can be quantified in a way that is complete, neutral, and free from material error. These volumes are presented in the Statement of Changes in Water Assets and Water Liabilities and the Statement of Water Flows.

Precipitation and evaporation are recognised only in relation to the surfaces of open storage volumes (9.1 Precipitation on surface water and 17.1 Evaporation from surface water). They do not represent all precipitation, evaporation and evapotranspiration within the region (notably, these processes occurring from the landscape are not captured).

That portion of runoff that flows into the surface water store (storages and rivers) is quantified and recognised on the Statement of Changes in Water Assets and Water Liabilities. This runoff includes precipitation captured in the catchment, less evaporation and infiltration, and is recorded as 9.4 Runoff to surface water.

Allocation announcements on regulated entitlements have been recognised as an increase in water liabilities. Note that the definition of regulated or unregulated entitlements for the purpose of the National Water Account may be different to the meaning applied by the jurisdiction (refer to 'Glossary'). Please refer to the Water rights, entitlements, allocations and restrictions note for more information on the allocations.

The volumes of several water assets identified in the Perth region were not reported in the water accounting statements but replaced by dashes ('–') as they 'failed the recognition criteria' specified in the Australian Water Accounting Standard 1, because:

• they could not be quantified in a way that is complete, neutral, and free from material error; and/or
  
• they were unlikely to provide a future benefit to the region or the region's stakeholders.

The descriptions of those water assets that failed the recognition criteria are given in the following line item notes:

• 1.2 Unregulated river
• 1.3 Regulated river
• 1.4 Lakes and wetlands
• 2.1 Water table aquifer
• 2.2 Underlying aquifers
• 3.2 Wastewater system
• 3.3 Recycled water system.

The volume of one change to water assets identified in the Perth region was not reported in the water accounting statements as it 'failed the recognition criteria' specified in the Australian Water Accounting Standard 1, because:

• it could not be quantified in a way that is complete, neutral, and free from material error; and/or
• it was unlikely to provide a future benefit to the region or the region's stakeholders.

The description of this change to water assets that failed the recognition criteria is given in the following line item note:

• 17.10 River and floodplain leakage, evaporation and errors.

In accordance with the Australian Water Accounting Standard 1, comparative year volumes must be restated in the statements if there is a prior period error or a change in line item presentation. Comparative year volumes are not required to be restated if there is a change in scope or methodology, or subsequent data availability for the quantification of a line item.

The following comparative year volumes in the water accounting statements were restated from the 2012 Account.

A miscalculation of the allocated extraction of groundwater to individual users within the Perth region during the 2011–12 year resulted in an incorrect volume for line items 18.11 Entitled extraction of allocated groundwater to users and 6.1 Groundwater allocation remaining in the 2012 Account (Table 1).

In addition, a change in line item presentation of allocated groundwater extraction to the irrigation scheme within the Perth region has resulted in a change of volume for line items 22.1 Groundwater allocation announcements and 14.1 Adjustment and forfeiture of groundwater allocation in the 2012 Account (Table 2). In last year's account, a volume of 652 ML was presented as a groundwater allocation announcement for the irrigation scheme. This allocation relates to the abstraction of water to an individual user, not the irrigation scheme. Consequently, the allocation announcement of 652 ML has been added to 22.1 Groundwater allocation announcements, and line items related to groundwater allocation for the irrigation scheme, which were presented in the 2012 Account, are no longer relevant to the Perth region and are not presented in the 2013 Account.

Both errors have been corrected in the comparative year information in the water accounting statements for this year and the differences are summarised in the tables below.