Adelaide
25.1 Unaccounted-for difference

In the Adelaide 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:

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.

Detail 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 12)

• Irrigation schemes note (Table 18).

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

The groundwater asset of the Adelaide region is quantified as the average long-term groundwater volume available for extraction without adversely impacting the system. This value is essentially constant from year to year. The groundwater asset is also only defined for a few of the total aquifers in the Adelaide region. While there are limitations in estimating the groundwater inflows and outflows, the large balancing item for the groundwater store is not representative of this error. It rather reflects the fact that groundwater assets and flows are calculated in ways that do not allow them to reconcile.

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, in the quantification of rainfall runoff there is uncertainty in the meteorological inputs into the model and the model structure. Similarly, for river outflow to sea, there is uncertainty associated with the rating curves and the ungauged runoff which enters the sea.

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:

In the 2010 Account, the groundwater balancing item was the biggest contributor to the unaccounted-for difference reported for the 2009–10 year. That volume was attributed to uncertainty in the inflows and outflows arising from the quantification approaches and the extent to which assets and flows in the water accounting statements represent the physical system.

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.