Murray–Darling Basin
9.4 Runoff to surface water

The volumetric value for the line item for the 2012–13 year was 25,143,874 ML.

The line item represents runoff volumes to surface water within the Murray–Darling Basin (MDB) region.  River losses have not been included in this line item.  They are included in line item 17.10 River and floodplain leakage, evaporation and errors.

Rainfall runoff volumes to surface water within the MDB region are summarised in the following table.

 

Details of rainfall runoff to surface water in the MDB region for the 2012–13 year

Water resource plan area	Sustainable diversion limit area		State/Territory	Volume (ML) for the 2012–13 year
	Code	Name
SW20 Warrego–Paroo–Nebine	SS29	Paroo	Qld	4,358,448
	SS28	Warrego	Qld
	SS27	Nebine	Qld
SW19 Condamine–Balonne	SS26	Condamine–Balonne	Qld
SW18 Moonie	SS25	Moonie	Qld
SW12 Barwon–Darling Watercourse	SS19	Barwon–Darling Watercourse	NSW
SW12 NSW Intersecting Streams	SS17	NSW Intersecting Streams	NSW
SW17 Qld Border Rivers	SS24	Qld Border Rivers	Qld	2,208,273
SW16 NSW Border Rivers	SS23	NSW Border Rivers	NSW
SW15 Gwydir	SS22	Gwydir	NSW	1,061,710
SW14 Namoi	SS21	Namoi	NSW	1,262,186
SW11 Macquarie–Castlereagh	SS20	Macquarie–Castlereagh	NSW	1,788,890
Northern Basin				10,679,507
SW10 Lachlan	SS16	Lachlan	NSW	1,863,603
SW9 Murrumbidgee	SS15	Murrumbidgee  NSW	NSW	2,590,354
SW1 ACT	SS1	ACT	ACT
SW8 NSW Murray and Lower Darling	SS18	Lower Darling	NSW	6,055,919
	SS14	NSW Murray	NSW
SW2 Vic. Murray	SS3	Kiewa	Vic.
	SS2	Vic. Murray	Vic.
SW4 Wimmera–Mallee	SS9	Wimmera–Mallee	Vic.
SW5 SA Murray Region	SS10	SA Non-prescribed areas	SA
SW6 SA River Murray	SS11	SA Murray	SA
SW3 Northern Victoria	SS4	Ovens	Vic.	1530877
	SS5	Broken	Vic.	1,975,874
	SS6	Goulburn	Vic.
	SS7	Campaspe	Vic.	136898
	SS8	Loddon	Vic.	161154
SW7 Eastern Mount Lofty Ranges	SS13	Eastern Mount Lofty Ranges	SA	149,688
	SS12	Marne Saunders	SA
Southern Basin				14,464,367
Whole MDB region				25,143,874

(1) Bureau of Meteorology (the Bureau): (a) National Climate Centre (NCC) daily climate grids (rainfall, temperature and solar radiation) and (b) Australian Hydrological Geospatial Fabric (AHGF) waterbody feature class; (2) Commonwealth Scientific and Industrial Research Organisation (CSIRO): (a) Australian water resources assessment system landscape model (AWRA-L) model parameters and (b) CSIRO monthly climatological average radiation grids data; and (3) Geoscience Australia: MDB human-made waterbody feature class.

The Bureau.

Rainfall runoff to surface water was estimated based on the AWRA-L version 3.0 (Van Dijk 2010) model outputs.

• Using climate grid data for the MDB region (including precipitation, temperature and solar radiation data), AWRA-L was used to estimate the runoff depth at each grid point within the region. Only runoff from the landscape is considered; therefore, the surface areas of the major storages, local catchment storages and other mapped waterbodies were excluded from the analysis.

Runoff from the landscape is divided into two components: runoff into the surface water store (major storages, rivers and drains) and runoff into local catchment storages. Only runoff into the surface water store is considered here.

The average runoff depth from the landscape into the surface water was determined as the weighted mean of the relevant grid-points within the MDB region boundary. Points were weighted based upon the area they represented within the region to remove edge effects (where the area represented is not wholly within the MDB region) and the effect of changing area represented with changing latitude.  Mean runoff depth was converted to a runoff volume by multiplying runoff depth by the total area of the region (excluding storages). 

• The estimated runoff was compared against historical flows at unimpaired catchments within the MDB region for the 2012–13 year and provided a suitable representation of the runoff for this year.
• The runoff estimates were subject to the assumptions of the AWRA-L model detailed in Van Dijk (2010).
• The estimated runoff corresponds to the runoff expected from an unimpaired catchment. The impairment on runoff from local catchment storages is estimated using a local catchment storage water balance model. Where this is applied, the runoff estimates inherit the approximations, assumptions and caveats of the local catchment storage water balance model and the parameters used.

The uncertainty estimate was not quantified.

A prior period error and a change made to the calculation method resulted in the restatement of the 2011–12 year volume. The respective volumes associated with the change are detailed in the following table.

Restatement of comparative year information made for the line item 9.4 Runoff to surface water

Segment	2013 Account volume for the 2011–12 year (ML)	2012 Account volume for the 2011–12 year (ML)	Difference due to  a prior period error and calculation method change (ML)
Northern Basin	27,429,924	32,492,437	(5,062,513)
Southern Basin	30,168,072	31,090,750	(922,678)
Whole region	57,597,996	63,583,187	(5,985,191)

The volume of runoff estimated for the comparison year for the 2013 Account is lower than the volume reported for the 2012 Account. This was due to the following reasons:

• There was a prior period error in the line item volume reported in the 2012 Account. The prior period error was a result of using solar radiation data with bias correction errors. Solar radiation is an input for line item volume calculation.
• The difference between the previously reported estimate of runoff and the estimate produced for the comparison year can be partly attributed to the choice of the AWRA-L v3.0 model instead of the previously used AWRA-L v2.0. The AWRA-L v3.0 model is more reliable than previous models in estimating runoff.

The difference between the previously reported estimate of runoff and the estimate produced for the comparison year represents a change of approximately 9% of the previously reported value. It is not possible to estimate the difference separately for each reason.