Perth
25.1 Unaccounted–for difference

The volume recognised in the water accounting statements (447,320 ML) represents the total unaccounted–for difference for the Perth region for the 2011–12 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:

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.

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 groundwater store. 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. The groundwater balancing item volume (256,302 ML) thus represents the difference between known groundwater inflows and known groundwater outflows during the 2011–12 year (see Groundwater note).