9.4 Runoff to surface water
The volumetric value for the line item for the 2011–12 year was 63,583,187 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.
|Water resource plan area||
Sustainable diversion limit area
Volume (ML) for the 2011–12 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||
|SW15 NSW Border Rivers||SS23||NSW Border Rivers||NSW|
|SW8 Murrumbidgee||SS15||Murrumbidgee NSW||NSW||
|SW7 NSW Murray and Lower Darling||SS18||Lower Darling||NSW||
|SW2 Vic. Murray||SS3||Kiewa||Vic.|
|SW5 SA Murray||SS11||SA Murray||SA|
|SS10||SA Non-prescribed areas||SA|
|SW3 Northern Victoria||SS4||Ovens||Vic.||
|SW6 Eastern Mount Lofty Ranges||SS13||Eastern Mount Lofty Ranges||SA||
|Whole MDB region||
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 2.0.0 (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 2011–12 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.
A change made to the calculation method resulted in the restatement of the 2010–11 year volume. 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 calculation method change (ML)|
The volume of runoff estimated for the comparison year for the 2012 Account (82,982,453 ML) is higher than the volume reported for the 2011 Account (74,210,965 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 v2.0.0 model instead of the previously used AWRA-L v1.0.0. The AWRA-L v2.0.0 model is more reliable than previous models in estimating runoff. The difference of 8,771,488 ML represents a change of approximately 12% of the previously reported value.