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.
|Surface water resource plan area||
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||17,190,577|
|SW 15–16||Qld and NSW Border Rivers||Qld and NSW||3,421,824|
|Sub-total Northern Basin||31,009,785|
|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|
|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.
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).
Assumptions, Limitations, Caveats and Approximations
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.
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.
|2010 Account line items||
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|
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.