Adelaide
25.1 Unaccounted-for difference

Supporting Information

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:

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

 

Account

Volume (ML)

 

Opening water storage (30 June 2010)

199,925

add

Total water inflows

1,444,750

minus

Total water outflows

696,959

minus

Closing water storage (30 June 2011)

227,174

 

Unaccounted-for difference

720,542


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 Adelaide region for the 2010–11 year

Balancing item

Volume (ML)

Balancing item – surface water store

211,658

Balancing item – groundwater store

489,179

Balancing item – urban water system store

16,025

Balancing item – irrigation scheme store

3,680

Unaccounted-for difference

720,542

 

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.

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:

Restatement of 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

548,707

220,348

Methodology change: several of the physical water inflows and outflows for the Adelaide region were restated due to new models and revised datasets used

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.