17.10 River and floodplain leakage, evaporation and errors
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.
|Surface water resource plan area||State||Volume (ML) for the 2010–11 year|
|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|
|Sub-total Northern Basin||23,767,902|
|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|
|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.
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
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
R1 is rainfall runoff to surface water (line item 9.4)
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
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.
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.
|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||