South East Queensland
25.1 Unaccounted-for difference

Supporting Information

In the South East Queensland (SEQ) 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 reporting in the water accounting statements. It is calculated according to the following table.

Calculation of the unaccounted-for difference for the 2010–11 year
  Account

Volume

(ML)

  Opening water storage (30 June 2010) 2,108,190
add Total water inflows 13,413,380
less Total water outflows 13,010,965
less Closing water storage (30 June 2011) 2,231,797
  Unaccounted-for difference 278,808

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 South East Queensland (SEQ) region for the 2010–11 year
Balancing item Volume
(ML)
Balancing item – surface water store 226,189
Balancing item – groundwater store 23,950
Balancing item – urban water system store 28,669
Unaccounted-for difference 278,808

Details 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)

The previous table shows that the unaccounted-for difference volume is primarily explained by the balancing item of the surface water store. It is likely that this was due to errors associated with the rainfall-runoff (a large source of surface water increase, line item 9.4 Runoff to surface water) and outflow to outside region (a large source of surface water decrease, line item 17.2 Outflow to outside region). The rainfall-runoff volume is derived from a rainfall-runoff model and it is reasonable to expect an uncertainty around the estimated runoff volume that could account for the balancing item. The surface water outflow to sea was based on flow gauge data and regression analysis. The significant floods that occurred in December 2010 and January 2011 may have resulted in inaccurate gauge data as the river levels may have resulted in backwater effects, unmeasured overbank flows and breakouts previously unrecorded.

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
1,469,223 414,737 Methodology change: Several of the physical water inflows and outflows for the South East Queensland region were restated due to new models and revised datasets used. This was all due to rainfall–runoff methodology change.

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 was 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) are the main reason the restated value of unaccounted-for difference for the 2009–10 year has increased from that reported in the 2010 Account.