25.1 Unaccounted-for difference

Supporting Information

In the Canberra 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 volume for the Canberra region for the 2010–11 year

Balancing item – surface water store



Balancing item – groundwater store



Balancing item – urban water system store



Unaccounted-for difference




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

  • Surface water note (Table 2)
  • Groundwater note (Table 4)
  • Urban system, System's interactions with the region note (Table 7)

The second table above shows that the unaccounted-for difference volume is primarily explained by the balancing item of the surface water  store. This may be due to an under estimation of outflows form the region following flood events, water leaving the channels and not returning or an over estimation of rainfall runoff from the region.

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 Canberra 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 increased from that reported in the 2010 Account.