25.1 Unaccounted-for difference

Supporting information

The volume reported (370,953 ML) represents the total unaccounted–for difference for the Adelaide region for the 2012–13 year.

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 unaccounted-for difference for the 2012–13 year
 Account Volume (ML)
Opening water storage (at 1 July 2012)
Total water inflows 873,167
Total water outflows (514,986)
Closing water storage (at 30 June 2013)
Unaccounted-for difference 370,953

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 for individual water stores for the Adelaide region for the 2012–13 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 Water resources and systems notes:

  • Surface water note (Table 4)
  • Groundwater note (Table 8)
  • Urban water system note (Table 13)
  • Irrigation schemes note (Table 17).

The second table above shows that the unaccounted-for difference volume is mainly explained by the balancing item of the surface water store and the groundwater store.

The balancing item for the surface water store is due to a combination of line items that were wholly or partly not quantified, and due to errors and uncertainty associated with the quantification methods. In particular, the modelled runoff volume, has a level of uncertainty associated with it and is likely to be an overestimate of the volume that actually entered the surface water store. For river outflow to sea, there is uncertainty associated with the rating curves and the ungauged runoff that enters the sea (refer to the 'Surface water' section of the Water resources and systems note for further details).

The value of the groundwater extraction limit (95% of the total groundwater asset at 30 June 2013) is essentially constant from year to year, and variations of the recharged water credit balance (5% of the total groundwater asset at 30 June 2013) have a relatively small impact on the total groundwater asset. Therefore, the groundwater store balancing item is primarily a reflection of the difference between groundwater inflows and outflows during the 2012–13 year. An indication of the accuracy of the calculated groundwater inflows and outflows is given by comparing them with the measured changes in groundwater storage during the 2012–13 year, as shown in the 'Groundwater' section of the Water resources and systems note.

Comparative year

The volume of unaccounted-for difference for the 2011–12 year has changed from that presented in the 2012 Account. This is attributed to restatement of 2011–12 volumes due to (a) changes in classification and (b) prior period error corrections. Refer to Restatement of comparative information in the 'Water accounting policies' for more information.