Perth
Water accounting policies

Introduction

The information presented in the Perth region water accounting report is based on agreements between the Bureau of Meteorology and:

  • the Western Australian Department of Water (Department of Water)
  • the Water Corporation of Western Australia (the Water Corporation)
  • Harvey Water.

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). Where a reported volume is negative, it is shown in brackets.

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 in 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.

 

Recognition of water assets and water liabilities

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.

 

Recognition of changes in water assets and water liabilities

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. 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 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.

 

Water assets and water liabilities that fail the recognition criteria

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.

Water assets that failed the recognition criteria include rivers, lakes and wetlands (see Surface water note), groundwater aquifers (see Groundwater note), and the wastewater and recycled water systems (see Urban water system note).

 

Changes to water assets and water liabilities that fail the recognition criteria

The volume of two changes to water assets 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; and/or
  • they were unlikely to provide a future benefit to the region or the region's stakeholders.

The line items that failed the recognition criteria describe the river and floodplain leakage, evaporation and errors (see Surface water note) and evaporation from the urban water system (see Urban water system note).

 

Restatement of comparative year information

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 2013 Account.

 

Surface water storage

A change in presentation of surface water storage within the Perth region has resulted in a number of changes of volume for line items in the 2013 Account (Table 1). This year, water within the Mundaring Reservoir is no longer considered a surface water asset for the region as it primarily provides water to the Goldfields and Agricultural region located outside the Perth region boundary. Consequently, the opening and closing balance of net water assets in the 2013 Account has decreased by the volume of water stored in the Mundaring Reservoir at the start and end of the reporting year.

Line items related to surface water allocation from the Mundaring Reservoir to the Goldfields and Agricultural region, which were presented in the 2013 Account as an inter-region commitment, are no longer relevant to the Perth region and are not presented in the 2014 Account.

In addition, water transferred to Mundaring Reservoir from Perth's urban water system is now considered to be transfer of water outside the region; water diverted from Mundaring Reservoir to Perth's urban water system is considered to be an inter-region inflow.

Comparative year information in the water accounting statements for this year and the differences are summarised in the table below.

 

 

Table 1  Changes in line item volumes from the 2013 Account due to change in line item presentation

Account

Correct 2013 volume in 2014 Account
(ML)		Incorrect volume in 2013 Account
(ML)		Difference
(ML)
Surface water store   
Storages159,492183,415(23,923)
Precipitation on surface water17,97820,317 (2,339)
Evaporation from surface water 28,71933,883 (5,164)
    
Urban water system
   
Delivery of water from outside region to urban water system1,93901,939
Transfer of water outside of region29,0101,259 27,751
    
Unaccounted-for difference70,65769,952705


 

Transactions within the region

Transactions between water assets and water liabilities within the region (e.g., between the surface water store and the groundwater store) are not reported in the water accounting statements because they do not affect the region's total water assets and water liabilities.

This information is reported in the Water resources and systems note, which contains water accounting statements for each water store within the region. These statements represent all transactions that occur for each water store, including internal transactions that were excluded from the consolidated water accounting statements.