25.1 Unaccounted–for difference

Supporting information

The volume recognised in the water accounting statements (69,952 ML) represents the total unaccounted-for difference for the Perth region for the 2012–13 year.

The unaccounted-for difference is the volume necessary to reconcile the opening water storage and closing water storage with the total water inflows and total water outflows reported in the water accounting statements. It is calculated according to the following table:


Calculation of the unaccounted-for difference for the 2012–13 year



Volume (ML)


Opening water storage (1 July 2012)



Total water inflows



Total water outflows



Closing water storage (30 June 2013)



Unaccounted-for difference


The unaccounted-for difference can also be calculated by summing the volumes necessary to reconcile (balance) the opening and closing storage with the water inflows and outflows of each of the separate water stores of the region, as shown in the following table.


Balancing item volumes for the Perth region for the 2012–13 year

Balancing item

Volume (ML)

surface water store


groundwater store


urban water system store


irrigation scheme store


Unaccounted-for difference



Detail of the balancing items calculation is given in the following Water resources and systems notes:

  • Surface water note (Table 4)
  • Groundwater note (Table 8)
  • Urban water system note (Table 12)
  • Irrigation scheme note (Table 16).

The second table above shows that the unaccounted-for difference volume is primarily explained by the balancing item of the surface water and groundwater store. The surface water balancing item is attributed to the fact that river and floodplain losses cannot be quantified, as well as uncertainty associated with runoff and river outflow (see Surface water note).

The groundwater assets of the Perth region could not be quantified (see Groundwater note). As a result, the calculation of the unaccounted-for difference in the first table above does not include opening or closing groundwater storage. Therefore, the groundwater balancing item volume of –180,342 ML represents the difference between known groundwater inflows and known groundwater outflows during the 2012–13 year (see Groundwater note).

Comparative year

The volume of unaccounted-for difference for the 2011–12 year has changed from that presented in the 2012 Account. This is attributed to changes in volumes associated with groundwater allocation to individual users in the region during the comparison year. Please refer to Restatement of comparative information in the 'Water accounting policies' for more information.