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 2011–12 year
  Account

Volume

(ML)

  Opening water storage (30 June 2011) 2,230,533
add Total water inflows 6,500,579
less Total water outflows 4,918,948
less Closing water storage (30 June 2012) 2,491,853
  Unaccounted–for difference 1,320,311

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 2011–12 year
Balancing item

Volume
(ML)

Balancing item – surface water store 1,364,157
Balancing item – groundwater store 24,737
Balancing item – urban water system store (68,583)
Unaccounted–for difference 1,320,311

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 River Outflow from the 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 estimated surface water volume at the near-sea outlet for the relevant rivers, water diversion for the water treatment plant (WTP), wastewater disposal to the surface water within the catchment from the wastewater treatment plant (WWTP), change in storages, rainfall volume directly on to the storages and evaporation from the storages.