25.1 Unaccounted-for difference

Supporting Information

In the Melbourne 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 2010–11 year



Volume (ML)


Opening water storage (30 June 2010)



Total water inflows



Total water outflows



Closing water storage (30 June 2011)



Unaccounted-for difference


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 Melbourne region for the 2010–11 year

Balancing item

Volume (ML)

Balancing item – surface water store


Balancing item – groundwater store


Balancing item – urban water system store


Unaccounted-for difference


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

  •  Surface water note (Table 3)
  • Groundwater note (Table 5)
  •  Urban system, System's interactions with the region note (Table 10).

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 unaccounted-for difference for groundwater is estimated using a water balance approach and the considerably high unaccounted for volume can be attributed to a number of factors:

  •  The groundwater asset of the Melbourne region is based on the long-term groundwater volume available for extraction – the permissible consumptive volume (PCV). The PCV is essentially constant from year to year and is therefore insensitive to groundwater level fluctuations. In contrast, groundwater inflows and outflows consider the annual fluctuations in groundwater levels.
  • The PCV is defined for water table and underlying aquifers of key groundwater management units (GMUs) only. The influence of the remaining groundwater areas in the Melbourne region is therefore not well represented.
  •  Several of the groundwater inflows and outflow volumes were modelled. In particular recharge from landscape represents the volume of water that could potentially reach the water table given the rainfall, land use, and soil type in the region. This modelled volume is not a direct measure of groundwater recharge as it does not take into consideration the time lag that occurs between the rainfall infiltrating into the soil and actually reaching the water table.

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



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

In the 2010 Account, the surface water balancing item was the biggest contributor to the unaccounted-for difference reported for the 2009–10 year. That volume was attributed to inaccuracies in the estimation of rainfall-runoff, which is the largest contributor to total surface water inflows.

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 reduced from that reported in the 2010 Account.