Perth
17.11 Surface water allocation diversion

This line item refers to the total water abstracted for all surface water supply in the Perth region not related to urban water system or irrigation schemes. It includes water abstracted by individual users in the mining, industrial and agricultural sectors. 

The water supply licences are divided into licence entitlement purposes. The total allocation diversions for each licence purpose during the 2010–11 year are given in the following table. The following figure shows the percentage of the total volume of water abstracted for each licence category. 

Graph of the percentage of the total volume of surface water abstracted for each licence category

The Department does not have sufficient metering information relating to actual licensed abstraction to determine the total volume of water abstracted. Consequently, the allocation diversion during the 2010–11 year was estimated based on the total licensed allocation.

The volume of abstraction is estimated to be the full allocation, multiplied by the proportion of days that each licence was active during the 2010–11 year. The number of days the licences were active is calculated using the issue date of each individual licence.

• For licences that expired and were renewed during the 2010–11 year, the volume of abstraction is estimated to be the full licensed allocation.
• For licences that expired during the 2010–11 year (and were not renewed), the volume of abstraction is estimated to be the full allocation, divided by the number of days from 1 July 2010 until its expiry date.
• For new licences that were created during the 2010–11 year, the volume of abstraction is estimated to be the full allocation, divided by the number of days from the licence issue date until 30 June 2011.

Some licences are issued in perpetuity and do not have a finalisation date. In these cases, the volume of abstraction is estimated as the amount of one full year's allocation for licences in force during the 2010–11 year.

There is not sufficient information relating to actual abstraction to provide more accurate estimates of abstraction for all licences. The pro-rata estimates of abstraction assume that the full annual allocation is abstracted each year, and that the rate of abstraction is uniform throughout the year. It is unlikely on both counts that this will be the case for all licences.

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