Perth
18.8 Non-allocated extractions to users

This line item refers to groundwater extraction from domestic non-licensed bores within the Perth region.

Department of Waters: Water Resource Licensing database, Australian Bureau of Statistics (ABS): domestic bore survey database.

Department of Water.

The Department of Water does not have sufficient metering information relating to actual extraction from domestic non-licensed bores to determine the total volume of water extracted. Consequently, the extraction was estimated based on ABS (2009) survey results for the Perth metropolitan area. These usage figures in the Perth metropolitan area were then extrapolated to also include the southwest area (around Mandurah). The estimated annual extraction per bore is 0.405 ML per year, which is based on the initial results of the Department of Water's domestic bore metering project for several dozen domestic bores across Perth.

The estimate for domestic non-licensed extraction is based on a number of assumptions, including:

• The ABS domestic bore survey is relevant for the southwest area, for which there is no alternative, independent research or evidence available.
• The usage based on the Department of Water's research for several dozen bores was used to estimate the usage of all non-licensed bores in the region.

The uncertainty estimate was not quantified.

The following table presents substantive changes in the line item reported in this 2011 Account with the corresponding line item in the 2010 Account.

 

Comparative year information for line item 18.8

2011 Account line item	2010 Account line item	2011 Account volume for the 2009–10 year (ML)	2010 Account volume for the 2009–10 year (ML)	Change from the 2010 Account to the 2011 Account
18.8 Non-allocated extractions to users	16.4.2 Groundwater extraction–other lumped basic rights	69,492	76,721	Methodology change: revised estimated extraction Change in line item name and number

The Department of Water used a change in methodology to recalculate non-allocated groundwater extraction for the 2009–10 year, which resulted in a 10% decrease in the line item volume from that reported in the 2010 Account. Although this change had a relatively minor impact on the water balance for the groundwater store, this line item is important for the management of the water store by the Department of Water. Therefore the comparative year value was restated in the water accounting statements.