The information presented in the Perth region water accounting report is based on agreements between the Department of Water, Water Corporation 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).
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 accurately, the volume of the water asset or water liability is recognised in the Statement of Water Assets and Water Liabilities. Where a water asset or water liability does not meet the recognition criteria, it is reported in the Statement of Water Assets and Water Liabilities with a dash (-). Water held in storages that has already been abstracted from the entitlement system is deemed not to be part of the region (no matter their size or connection to the system). This is because the attendant water right has already been exercised.
While water within 1.2 Unregulated channel and 1.3 Regulated channel are vital water assets for the conveyance of water within the region, it cannot be quantified accurately due to a lack of available data and thus is not recognised as a water asset.
The volume of water within 1.4 Lakes and wetlands—connected cannot be quantified accurately. The volume of water held in 1.4 Lakes and wetlands—connected is relatively small in size and its omission does not have a material impact on the credibility of this report.
The volume of water within 2.1 Water table aquifer and 2.2 Underlying aquifers cannot be quantified accurately. The Gnangara Mound is the largest and most important groundwater resource in the region. The Department of Water is currently working on a method to calculate the storage volume of water in the aquifer necessary to maintain and protect groundwater-dependent ecosystems (the ‘extractable limit’ water level). Preliminary results have shown that there are some areas of the Gnangara Mound where the aquifer levels are above this extractable limit and other areas where the aquifer levels are below this limit. Water above the extractable limit is considered to be a water asset. The gap between the water and the extractable limit is considered to be a water liability to the environment (i.e. water is owed back to the water table to raise it to the extractable limit). However, the volumes of water asset and water liability cannot yet be measured. This is the case for all other aquifers in the Perth region.
The increases and decreases to the reported water assets and water liabilities consist of changes that can be quantified accurately. Where a change to a water asset or water liability cannot be quantified accurately it does not meet the recognition criteria and the volume is reported in the Statement of Changes in Water Assets and Water Liabilities and the Statement of Physical Water Flows with a dash (-). These increases and decreases contribute to 30 Unaccounted-for difference and are explained in the notes section.
Precipitation and evaporation are recognised only in relation to the surfaces of open storage volumes (13.1 Precipitation on connected surface water and 14.1 Evaporation from connected 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 connected surface water store (reservoirs 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 13.4.1 Runoff to connected surface water.
The definition of regulated or unregulated for the purpose of the National Water Account may be different to the meaning applied by the jurisdiction. Please refer to Section 1.4 - Water rights, entitlements, allocations and trade for the definition used in the National Water Account 2010.