17.11 Entitled diversion of allocated surface water to users
This line item refers to the total surface water diverted for all water supply in the Perth region not related to urban water system or irrigation schemes. It includes water diverted by individual users in the mining, industrial and agricultural sectors.
The volume recognised in the water accounting statements (24,975 ML) represents the total volume of surface water diverted for these licence entitlements during the 2011–12 year.
The Department of Water 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 2011–12 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 2011–12 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 2011–12 year, the volume of abstraction is estimated to be the full licensed allocation.
- For licences that expired during the 2011–12 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 2011 until its expiry date.
- For new licences that were created during the 2011–12 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 2012.
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 2011–12 year.
Assumptions, limitations, caveats and approximations
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.
This line item corresponds to line item '17.11 Surface water allocation diversion' reported in the 2011 Account.