Murray-Darling Basin
17.10 River and floodplain leakage, evaporation and errors

Supporting Information

The volumetric value for the line item for the 2010–11 year was 42,314,414 ML. The line item represents river and floodplain losses including evaporation and seepage, and gauging errors in river inflow and outflow measurements in the Murray–Darling Basin (MDB) region. A breakdown of line item volume is provided in the following table.

 

River and floodplain leakage, evaporation and errors in the MDB region for the 2010–11 year
Surface water resource plan area State Volume (ML) for the 2010–11 year  
 Code Name
SW 11–12 and SW17–19 Warrego – Paroo – Nebine, Condamine–Balonne, Moonie, NSW Intersecting Streams and Barwon–Darling watercourse Qld and NSW 18,448,591
SW 15–16 Qld and NSW Border Rivers Qld and NSW 905,437
SW14 Gwydir NSW 315,729
SW13 Namoi NSW 2,027,855
SW10 Macquarie–Castlereagh NSW 2,070,290
Sub-total Northern Basin 23,767,902
SW9 Lachlan NSW 2,507,684
SW 1 and 8 Murrumbidgee NSW and ACT NSW and ACT 5,208,573
SW 2,4,5,6 and 7 NSW Murray and Lower Darling, Vic Murray, SA Murray, Wimmera–Mallee and Eastern Mount Lofty Ranges NSW, Vic and SA 8,090,191
SW3 Northern Victoria Vic 2,740,064
Sub-total Southern Basin 18,546,512
Total for the region 42,314,414

 

Volumes for the line item were based on hydrological boundaries of river catchments within the region.  These volumes were not available individually for some of water resources planning areas. Therefore, the volumes were provided in the previous table individually for some of the water resource plan areas and as clusters for the others.

Quantification Approach

Data Source

There was no separate data source for the line item. Values of other line items for the MDB region water account were used as the inputs for the calculations.

Provided by

Bureau of Meteorology (the Bureau).

Method

A water balance approach was adopted in calculating the volumes for the line item. The calculations were based on hydrological boundaries of river catchments within the region. Total inflows less total outflows and changes in surface water storage for surface water in a reporting unit (a water resource plan area) was considered as the river loss (river and floodplain leakage, evaporation and errors) for the same unit. Details of the calculation method applied in the water balance approach are as follows.

The line item value for the water resource plan area (stated in the table provided under Supporting information) for which necessary details are available, is equal to:

 

I– O – ΔS

where:

I is total inflow to the water resource plan area

O is total outflow from the water resource plan area

ΔS is change in total surface water storage during the 2010–11 year in the water resource plan area.

 

Total inflow to the water resource plan area was calculated using the following equation:

 

I =  R1+R2 + P +Qi

where:

R1 is rainfall runoff to surface water (line item 9.4)

R2 is return flows (line items 9.3, 9.5, 9.9 and 9.11)

P is precipitation on surface water (line item 9.1)

Qi is inflow from upstream water resource plan areas (estimated using relevant streamflow gauge data, not applicable to all water resource plan areas).

 

Total outflow (O) was calculated using the following equation:


O =  D+E +Qo

where:

D is diversions from surface water (line items 17.3, 17.5 to 17.8, 17.11 and 17.12)

E is evaporation from surface water (line item 17.1)

Qo is outflow to downstream water resource plan areas or sea (estimated using relevant streamflow gauge data, not applicable to all water resource plan areas).

 

Change in surface water storage (ΔS) in the water resource plan area is the difference between total storage volumes at the end and beginning of the 2010–11 year.  Storage information is available in line items 1.1, 1.3, 1.4 and 1.10.

Assumptions, Limitations, Caveats and Approximations

Assumptions made for the line items (see the above Method for input variables) from which values were used as input variables may affect accuracy of the calculations.

A water balance approach was adopted in calculations (total inflows less total outflows and changes in surface water storage was considered as the volumes for the line item).

At present, attempts to calculate the line item value on small spatial units are limited by the resolution of available data particularly diversion and return data at present. The Bureau is looking to implement the AWRA-R model over the next 2

–3 years, which will be able to better attribute some of the inputs for the calculation and attribute these inputs to individual river reaches.

Uncertainty Information

The uncertainty estimate was not quantified.

Comparative year

 This line item corresponds to line item 14.10 Riparian and floodplain use, reported in the 2010 Account. In the 2011 Account, following changes were made that caused the 2009–10 year value to be restated.

  • The same water balance approach applied for the 2010–11 year was used to calculate the 2009–10 year value for the 2011 Account. Therefore, restatements associated with all line items (related to inflows, outflows and storage volumes that are inputs in the water balance calculation) contributed to the difference between the  2009–10 year value calculated for the 2011 Account and reported for the 2010 Account.

The difference due to the changes is 7.6% of the value reported for the 2010 Account. The changes and their respective values are detailed in the following table.

 

Restatement of comparative year information made for the line item 17.10 River and floodplain leakage, evaporation and errors
2010 Account line items Segment Volume for the 2009–10 year reported in the 2010 Account (ML) Difference due to the changes (ML) Volume for the 2009–10 year reported in the 2011 Account (ML)
14.10 Riparian and floodplain use Northern Basin

Not available

Not available

21,559,916

Southern Basin

Not available

Not available

10,301,265

Whole region

34,479,014

–2,617,833

31,861,181