17.10 River and floodplain leakage, evaporation and errors
The volumetric value for the line item for the 2011–12 year was 42,957,139 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.
|Water resource plan (WRP) area||Sustainable diversion limit (SDL) area||State/Territory||Volume (ML) for the 2011–12 year||Volume (ML) reported for the 2010–11 year|
|SW11 Barwon–Darling Watercourse||SS19||Barwon–Darling Watercourse||NSW|
|SW12 NSW Intersecting Streams||SS17||NSW Intersecting Streams||NSW|
|SW16 Qld Border Rivers||SS24||Qld Border Rivers||Qld||965,961||905,437|
|SW15 NSW Border Rivers||SS23||NSW Border Rivers||NSW|
|SW8 Murrumbidgee||SS15||Murrumbidgee NSW||NSW||3,441,863||5,208,573|
|SW7 NSW Murray and Lower Darling||SS18||Lower Darling||NSW||8,199,338||8,090,191|
|SW2 Vic. Murray||SS3||Kiewa||Vic.|
|SW5 SA Murray||SS11||SA Murray||SA|
|SS10||SA Non-prescribed areas||SA|
|SW6 Eastern Mount Lofty Ranges||SS13||Eastern Mount Lofty Ranges||SA|
|SW3 Northern Victoria||SS4||Ovens||Vic.||2,740,601||2,740,064|
|Whole MDB region||42,957,139||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 2011–12 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 2011–12 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.
Restatements made for the 2010–11 year in a number of line items, which are inputs to this line item, resulted in the restatement of the 2010–11 year volume.
The same water balance approach applied for the 2011–12 year was used to calculate the 2010–11 year value for the 2012 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 2010–11 year value calculated for the 2012 Account and reported for the 2011 Account. The major contributor for this difference was the change made to reported (for the 2011 Account) and recalculated value for line item 9.4 'Runoff to surface water' for the 2010–11 year. The change for that line item was 8,771,489 ML (12% increase) due to the choice of the AWRA-L v2.0.0 model instead of the previously used AWRA-L v1.0.0 for the calculations.
The line item shows an increase of the recalculated volume by 9,320,333 ML (22% change) for the 2010–11 year. The respective volumes associated with the change are detailed in the following table.
|Segment||2012 Account volume for the 2010–11 year (ML)||2011 Account volume for the 2010–11 year (ML)||Difference due to updated information (ML)|