Murray-Darling Basin
25.1 Unaccounted-for difference

 In the Murray–Darling Basin (MDB) 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.

Balancing item – surface water store

Line item 17.10 River and floodplain leakage, evaporation and errors was calculated applying a water balance approach. Any difference on the water balance calculation for the surface water store was included in that line item. Therefore, contribution from the surface water to the total unaccounted-for difference should be zero. However, a 2 ML contribution is reported that is due to rounding-off errors associated with surface water inflow and outflow volumes. Information on all water flows to and from the surface water store including between store flows is available in the Surface water note.

Balancing item – groundwater store

The balancing item – groundwater store is the portion of the unaccounted-for difference attributed to the calculation of the groundwater terms. For the groundwater store, opening and closing storage are defined as opening and closing balance of the water asset. Therefore, the balancing item – groundwater store represents the volume necessary to reconcile the opening and closing balance of the groundwater asset with all the water inflows and outflows to and from the groundwater store.

Groundwater asset (see line item 2.5 Other groundwater assets) was quantified as the long term annual extractable limits plus basic landowner water rights defined in water sharing plans. The following table shows that the change in groundwater asset during the 2010–11 year is explained solely by the inclusion of additional sustainable diversion limit (SDL) areas in the scope of the groundwater asset (see line item 10.13 Other groundwater increases) and the reduction of supplementary groundwater entitlements on issue in the New South Wales water sharing plans (see line item 18.18 Other groundwater decreases). The table also shows that the balancing item – groundwater store is due to the difference between physical water inflows and outflows that occurred during the 2010–11 year.

¹ Physical water flows include the volumes for all groundwater inflows and outflows line items provided in Table 6 of the Groundwater note, except line items 10.13 Other groundwater increases and 18.18 Other groundwater decreases.

Explaining the balancing item – groundwater store

 Expressed in physical terms, balancing item is based on two mass balance equations:

 Equation (1)   change in storage = storage at end of year – storage at start of year                                           

Change in storage can also be separately estimated by measuring inflows and outflows, and subtracting these two variables:

 Equation (2)   change in storage = annual inflows – annual outflows                   

In general, it is easier to calculate storage at a point in time more accurately than inflows. Because of this difference in estimation accuracy, some difference between Equations (1) and (2) is usually expected due to sampling error.

 The volume for the balancing item is the difference between the numbers calculated for Equation (1) and Equation (2), when change in storage is taken to represent change in asset.

 In the case of surface water, the surface water asset is defined as the physical storage. So the storage at end of year variable in Equation (1) is the surface water asset reported at the end of the year.

 However, for the MDB region, the groundwater asset is not defined in this physical way. For the MDB region, groundwater asset volume was estimated using the long term annual extractable limits (include volume of supplementary access licence that was available for use in New South Wales in the 2010–11 year) and basic landholder rights set in the jurisdictional groundwater sharing plans. These non-physical groundwater assets are based on long term averages. Unless there are changes to the groundwater sharing plans in the region (approval of new plans, cancellation of existing plans and changes to asset volumes stated in existing plans), total groundwater asset volume does not change year-to-year. For the MDB region, there was increase (which is relatively a small change compare to inflow-outflow difference) in 131,152 ML asset volume for the 2010–11 year due to approval and cancellation of water sharing plans (see the previous table).

 The consequence of defining the groundwater asset in this non-physical way is that the balancing item is forced to absorb all of the physical change in storage, because that physical change is not taken into account in the change in groundwater asset. This gives a balancing item volume which is a deliberate over-estimate of the actual physical difference between Equations (1) and (2).

 For the balancing item, Equation (1) is 5,233,198 ML in physical terms (i.e. physical change in actual groundwater storage, measured as the difference in groundwater levels, converted to volume). So, with a physical interpretation of the attributes, the balancing item volume is actually much smaller:

 Equation (1) = 5,233,198 ML (from Table 7 of the Groundwater note for SDL areas for which asset volumes are available

 Equation (2) = 9,423,487 ML (see the previous table)

Therefore:

 Equation (3)    Balancing item = Equation (2) – Equation (1)

                                               = 9,423,487 – 5,233,198

                                               = 4,190,289 ML.

This residual volume can be explained in large measure as systematic sampling error resulting in underestimation. The 5,233,198 ML value for change in groundwater storage was calculated only for a portion of the included SDL areas: specifically, for areas within a 5 km radius of a groundwater bore. If the whole SDL areas were included, the 5,233,198 ML value would be considerably higher, yielding a considerably smaller overall number for physical balancing item volume in Equation 3.

 The magnitude and direction of the balancing item volume is informative; in wet years with large recharge volumes, the balancing item volume is large and positive; conversely in very dry years, the balancing item volume is expected to be large and negative. As the 2010–11 year was a relatively wet year, this accounts for the large positive value reported in the 2011 Account.

 Another key issue for physical change in groundwater storage is the lag effect: groundwater recharge generally occurs over long time horizons, with change to physical groundwater storage often not manifested in the same year as the recharge rainfall. This is an example of a physical factor that can also contribute to large balancing item volume for groundwater; as Equation (2) may be large but Equation (1) is smaller because of the delayed response in groundwater storage.

 The design of the balancing item requires special consideration to be meaningful when applied to groundwater. It is acknowledged that the number reported for groundwater balancing item for the MDB region for the 2011 Account is a result of the difference between climatically sensitive, physically quantified inflows and outflows with no inputs from physically quantified groundwater assets. The current application of balancing item to groundwater is being reviewed, and the current treatment is included as an interim, transitional measure.

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 sole contributor to the unaccounted-for difference reported for the 2009–10 year because surface water balancing item volume was included in line item 14.10 Riparian and flood plain use.

The methodology change adopted for line item 10.3 Recharge from landscape is the main contributor to the difference between the restated value in the 2011 Account and the reported value in the 2010 Account.