Perth
Water accounting policies

The information presented in the Perth region water accounting report is based on agreements between the 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.

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 and water liabilities 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;
• in the case of water assets, they were unlikely to provide a future benefit to the region or the region's stakeholders; and
  
• in the case of water liabilities, they were unlikely to result in a decrease in the region's water assets or an increase in another water liability when the obligation was discharged.

The descriptions of those water assets and water liabilities 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 volumes of several changes to water assets and water liabilities identified in the Perth region were not reported in the water accounting statements 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;
• in the case of changes to water assets, they were unlikely to provide a future benefit to the region or the region's stakeholders; and
• in the case of changes to water liabilities, they were unlikely to result in a decrease in the region's water assets or an increase in another water liability when the obligation was discharged.

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

• 9.5 Point return from irrigation scheme
• 20.5 Return of water to surface water.

These line items appear in the Water resources and systems note. They are not reported in the water accounting statements as they refer to a flow that occurs between stores.

Several comparative year volumes in the water accounting statements were restated from the 2011 Account in order to provide more useful information to the users. Restatements may be due to:

• error in the 2011 Account (referred to as a 'prior period error')
• change in the presentation of the water accounting statements, where line items were generally simplified since the 2011 Account
• change in scope (e.g. to include more of the region's area in the definition of a line item in order to present more complete information)
  
• improvement in quantification methodology
  
• increase in the data available to quantify a line item.

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, a change in line item presentation, or change in scope.

According to the Standard, comparative year volumes are not required to be restated if there is a change in methodology or subsequent data availability for the quantification of a line item; however, for this account, these volumes were restated if the change was considered material. In particular, any change affecting the volume of allocation remaining was restated.

The notes attached to the line items of the water accounting statements give the details of all the restatements made.