Ord
9.4 Runoff to surface water

The volume recognised in the water accounting statements (37,140,631 ML) represents runoff from rainfall that entered rivers in the Ord region during the 2010–11 year.

Relatively wet conditions in the Ord region during the 2010–11 year are reflected in relatively high runoff figures. A breakdown of runoff to rivers is provided in the table below.

Runoff to various rivers during the 2010–11 year

Component	Runoff (ML) 2010–11
Runoff to Ord River upstream of Lake Argyle	20,908,921
Runoff to Ord River downstream of Lake Argyle	10,259,733
Runoff to Keep River	5,971,977
Total	37,140,631

 

Bureau of Meteorology: National Climate Centre (NCC), daily climate grids (rainfall, temperature and solar radiation), Australian Hydrological Geospatial Fabric (AHGF) waterbody feature class. Commonwealth Scientific and Industrial Research Organisation (CSIRO): AWRA-L model parameters; monthly climatological average radiation grid data.

Bureau of Meteorology.

Rainfall runoff is calculated as runoff into rivers.

Rainfall runoff to surface water was based on streamflow estimates from the AWRA-L version 1.0.0 model outputs.

Using climate grid data for the Ord 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 was considered; therefore, the surface areas of the major storages were excluded from the analysis.

The average runoff depth from the landscape into the connected surface water store was determined as the weighted mean of the relevant grid points within the region boundary. Points were weighted based upon the area they represented within the reporting region to remove edge effects (where the area represented is not wholly within the reporting region) and the effect of changing area represented with changing latitude. 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 Ord region for the 2010–11 water 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 by Van Dijk (2010).

The uncertainty estimate was not quantified.

The following table compares the value for the 2009–10 year reported in this account with the corresponding value reported in the 2010 Account.

Comparative information from the 2010 Account for line item 9.4

2011 Account line item	2010 Account line item	Value reported in the 2010 Account (ML)	Value reported in the 2011 Account (ML)	Additional information
9.4 Runoff to surface water	13.4.1 Runoff to connected surface water	7,538,699	5,426,262	Change in method

The runoff to surface water during the 2009–10 year has been restated because of improvements in the methodology of calculating runoff into the river components of surface water.

The volume of runoff estimated for the comparison year (5,426,262 ML) is less than the volume reported for the 2010 Account (7,538,699 ML). This is due to a combination of erroneous data being used in previous modelling and a change in modelling methods. Prior to the publication of the 2010 Account it was realised that the bias correction applied to the satellite-derived solar radiation dataset was incorrect resulting in an overestimation of incoming solar radiation. When the bias correction was updated and rainfall runoff models were rerun, the estimate of runoff to surface water increased to 8,072,136 ML. The decrease between the prior 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 difference of 2,112,438 ML represents a change of approximately 28% of the originally published volume.