Murray–Darling Basin
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.

River and floodplain leakage, evaporation and errors in the MDB region for the 2011–12 year

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
	Code	Name
SW19 Warrego–Paroo–Nebine	SS29	Paroo	Qld	17,649,763	18,448,591
	SS28	Warrego	Qld
	SS27	Nebine	Qld
SW18 Condamine–Balonne	SS26	Condamine–Balonne	Qld
SW17 Moonie	SS25	Moonie	Qld
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
SW14 Gwydir	SS22	Gwydir	NSW	1,811,336	315,729
SW13 Namoi	SS21	Namoi	NSW	974,052	2,027,855
SW10 Macquarie–Castlereagh	SS20	Macquarie–Castlereagh	NSW	3,247,055	2,070,290
Northern Basin				24,648,167	23,767,902
SW9 Lachlan	SS16	Lachlan	NSW	3,927,170	2,507,684
SW8 Murrumbidgee	SS15	Murrumbidgee  NSW	NSW	3,441,863	5,208,573
SW1 ACT	SS1	ACT	ACT
SW7 NSW Murray and Lower Darling	SS18	Lower Darling	NSW	8,199,338	8,090,191
	SS14	NSW Murray	NSW
SW2 Vic. Murray	SS3	Kiewa	Vic.
	SS2	Vic. Murray	Vic.
SW4 Wimmera–Mallee	SS9	Wimmera–Mallee	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
	SS12	Marne Saunders	SA
SW3 Northern Victoria	SS4	Ovens	Vic.	2,740,601	2,740,064
	SS5	Broken	Vic.
	SS6	Goulburn	Vic.
	SS7	Campaspe	Vic.
	SS8	Loddon	Vic.
Southern Basin				18,308,972	18,546,512
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.

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.

Bureau of Meteorology (the Bureau).

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 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 +Q_i

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)

Q_i 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 +Q_o

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)

Q_o 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 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.

The uncertainty estimate was not quantified.

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.

 

Restatement of comparative year information made for the line item 17.10 River and floodplain leakage, evaporation and errors

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)
Northern Basin	26,656,458	23,767,902	2,888,556
Southern Basin	24,978,289	18,546,512	6,431,777
Whole region	51,634,747	42,314,414	9,320,333