25.1 Unaccounted-for difference

Supporting information

In the Sydney 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 2011–12 year



Volume (ML)


Opening water storage (30 June 2011)



Total water inflows



Total water outflows



Closing water storage (30 June 2012)



Unaccounted-for difference



The unaccounted-for difference of 208,300 ML represents 9% of the total water storage for the Sydney region and 2% of the total water inflows. The unaccounted-for difference in the Sydney account found in both the urban water system and the surface water can be attributed to:

  • modelling errors, particularly in reference to line item 9.4 and line item 17.2
  • uncertainty in estimating leakage (as in line item 19.2)
  • errors in measurement equipment and/or meter accuracy on gauging stations and flow meters.

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 Sydney region in the 2011–12 year
Balancing item Volume (ML)
Balancing item–surface water store 220,386
Balancing item–groundwater store
Balancing item–urban water system store 7,566
Unaccounted-for difference

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

  • Surface water note
  • Groundwater note
  • Urban system


Comparative year

There have been several restatements from the 2011 Account. As a result, the volume stated in this line item has been restated from 218,095 ML (as published in the 2011 Account) to 140,542 ML.