Murray-Darling Basin
9.4 Runoff to surface water

The volumetric value for the line item for the 2010–11 year was 74,210,965 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.

 

Summary of rainfall runoff to surface water 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	17,190,577
SW 15–16	Qld and NSW Border Rivers	Qld and NSW	3,421,824
SW14	Gwydir	NSW	1,223,188
SW13	Namoi	NSW	2,734,354
SW10	Macquarie–Castlereagh	NSW	6,439,842
Sub-total Northern Basin			31,009,785
SW9	Lachlan	NSW	4,002,049
SW 1 and 8	Murrumbidgee NSW and ACT	NSW and ACT	9,768,019
SW 2,4,5 and 7	NSW Murray and Lower Darling, Vic Murray, SA Murray and Wimmera–Mallee	NSW, Vic and SA	15,928,131
SW3	Northern Victoria	VIC	13,236,570
SW6	Eastern Mount Lofty Ranges	SA	266,410
Sub-total Southern Basin			43,201,180
Total for the region			74,210,965

 

Rainfall runoff volumes 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 above table individually for some of the water resource plan areas and as clusters for the others.

Data sources were (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) 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 1.0.0 (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 2010–11 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 (STEDI). Where this is applied, the runoff estimates inherit the approximations, assumptions and caveats of STEDI and the parameters used.

The uncertainty estimate was not quantified.

This line item corresponds to line item 13.4.1 Rainfall runoff to connected surface water, reported in the 2010 Account. In the 2011 Account, the following change was made that caused the 2009–10 year value to be restated:

• The methodology used to quantify the line item was improved and resulted in a material change in volume.

The changes and their respective values are detailed in the following table.

 

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

2010 Account line items	Segment	Volume for the 2009–10 year reported in the 2010 Account (ML)	Difference due to method change (ML)	Volume for the 2009–10 year reported in the 2011 Account (ML)
13.4.1 Rainfall runoff to connected surface water	Northern Basin	30,056,630	–4,278,586	25,778,044
	Southern Basin	14,214,073	1,301,260	15,515,333
	Whole region	44,270,703	–2,977,326	41,293,377

 

The volume of runoff estimated for the comparison year for the 2011 Account (41,293,377 ML) is less than the volume reported for the 2010 Account (44,270,703 ML). This was due to a change in modelling methods. The difference between the previously reported estimate of runoff and the estimate produced for the comparison year can be attributed to the choice of the AWRA-L v1.0.0 model instead of the previously used average of WaterDyn v26 and AWRA-L v0.5. The AWRA-L v1.0.0 model is more reliable than previous models in estimating runoff. The difference of 2,977,326 ML represents a change of approximately 6.7% of the previously reported value.