Adelaide
25.1 Unaccounted-for difference

Supporting information

The volume recognised in the water accounting statements (194,215 ML) represents the total unaccounted–for difference for the Adelaide region for the 2011–12 year.

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 for the 2011–12 year

 

Account

Volume (ML)

 

Opening water storage (30 June 2011)

228,582

add

Total water inflows

746,108

minus

Total water outflows

590,398

minus

Closing water storage (30 June 2012)

190,077

 

Unaccounted-for difference

194,215


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 volume for the Adelaide region for the 2011–12 year

Balancing item

Volume (ML)

Balancing item–surface water store

121,832

Balancing item–groundwater store

69,873

Balancing item–urban water system store

480

Balancing item–irrigation scheme store

2,030

Unaccounted-for difference

194,215


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, 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 that enters the sea.

The value of the groundwater extraction limit (95% of the total groundwater asset at 30 June 2012) is essentially constant from year to year and variations of the recharged water credit balance (5% of the total groundwater asset at 30 June 2012) have a relatively small impact on the total groundwater asset. Therefore, the 'balancing item – groundwater store' is primarily a reflection of the difference between groundwater inflows and outflows during the 2011–12 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 2011–12 year, as shown in the 'Groundwater' section of the 'Water resources and systems' note.

The unaccounted-for difference value in the 2011–12 year (194,215 ML) decreased significantly compared to the 2010–11 year (691,528 ML). This difference is due to the following factors:

  1. A decrease of the modelled surface water inflows in the 2011–12 year compared to the 2010–11 year. As not all the surface water outflows could be estimated in both years, the dryer conditions experienced in the 2011–12 year reduced the balancing item of the surface water store.
  2. A decrease of the difference between modelled groundwater inflows and outflows in the 2011–12 year compared to the 2010–11 year, which is the primary cause for the balancing item of the groundwater store.
  3. Improvements in the quantification approaches of the line items, in particular of 9.4 Runoff to surface water.

Causes 1 and 2 are the main factors that explain the decrease in unaccounted-for difference; they reflect the drier conditions experienced in the 2011–12 year compared to the 2010–11 year.

Comparative year

The 2010–11 year volumes of several line items used in the calculation of the unaccounted-for difference were restated in the 2012 Account. As a consequence, the volume of the 2010–11 year unaccounted-for difference has been restated from 720,542 ML as published in the 2011 Account to 691,528 ML in the 2012 Account. The difference between these two values is explained mainly by the restatement of the following line items due to changes in the quantification approaches:

  • 9.4 Runoff to surface water: the 2010–11 year volume was restated from 598,040 ML to 523,213 ML, equivalent to a decrease of net inflows of about 75,000 ML; and
  • 18.3 Discharge to landscape: the 2010–11 year volume was restated from 99,677 ML to 54,866 ML, equivalent to an increase of net inflows of about 45,000 ML.