25.1 Unaccounted-for difference - surface water

Supporting Information

In the Perth region, 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 2010–11 year



Volume (ML)


Opening water storage (30 June 2010)



Total water inflows



Total water outflows



Closing water storage (30 June 2011)



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 2010–11 year

Balancing item

Volume (ML)

Balancing item – surface water store


Balancing item – groundwater store


Balancing item – urban water system store


Balancing item – irrigation scheme store


Unaccounted-for difference



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

  • Surface water note (Table 3)
  • Groundwater note (Table 7)
  • Urban system: System's interactions with the region note (Table 10)
  • Irrigation schemes 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 (–433,972 ML) thus represents the difference between known groundwater inflows and known groundwater outflows during the 2010–11 year (see Groundwater note).

Comparative year

Substantive changes in the unaccounted-for difference for the 2009–10 year reported in the 2011 Account with the corresponding value reported in the 2010 Account are presented in the following table:

Comparative year information for the unaccounted-for difference

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



Methodology change: several of the physical water inflows and outflows for the Perth region were restated due to new models and revised datasets used


In the 2010 Account, the surface water balancing item was the biggest contributor to the unaccounted-for difference reported for the 2009–10 year. That volume was attributed to inaccuracies in the estimation of rainfall-runoff, which is the largest contributor to total surface water inflows.

Improvements in the rainfall-runoff model used in the 2011 Account (see line item 9.4 Runoff to surface water) is the main reason the restated value of unaccounted-for difference for the 2009–10 year has reduced from that reported in the 2010 Account.