Perth
9.4 Runoff to surface water

This volume represents the total runoff from the landscape into the drains, rivers and storages within the Perth region.

Bureau of Meteorology, National Climate Centre: daily climate grids (rainfall, temperature and solar radiation), Commonwealth Scientific and Industrial Research Organisation: monthly climatological average radiation grid data, Bureau of Meteorology: Australian Hydrological Geospatial Fabric – waterbody feature class.

Bureau of Meteorology.

Runoff in the Perth region was estimated based on the GR4J rainfall–runoff model running within the Catchment Water Yield Estimation Tool. The GR4J model has been used to estimate runoff from areas not modelled by the Department of Water's groundwater models (see figure below), which already incorporates landscape water movement. Therefore, runoff was only modelled on the Darling Range in the eastern part of the region.

Map showing the runoff model area within the Perth region

Using climate grid data for the Perth region (including precipitation, temperature and solar radiation data), the GR4J model was calibrated to a number of unimpaired catchments in and around the Perth region. The calibrated parameters were then regionalised using nearest-neighbour regionalisation to provide a parameter set for each 0.05º (approximately 5 km) gridpoint across the region. The GR4J model was then used to estimate the runoff depth at each gridpoint within the region. Only runoff from the landscape is considered; therefore, the surface areas of the major surface water storages were excluded from the analysis.

The average runoff depth from the landscape into the surface water store was determined as the weighted mean of the relevant grid points within the region boundary. Mean runoff depth was converted to a runoff volume by multiplying runoff depth by the total area of the region (excluding surface water storages).

The estimated runoff was compared against historical flows in unimpaired catchments within the Perth region for the 2010–11 year and provided a suitable representation of the runoff for this year. The GR4J model was selected over the AWRA-L v1.0.0 model (used to estimate runoff for all other 2011 Account regions) because the AWRA-L model did not provide a suitable representation of runoff in this region.

The runoff estimates were subject to the assumptions of the GR4J model detailed in Perrin et al. (2003).

The uncertainty estimate was not quantified.

The following table presents substantive changes in the line item reported in this 2011 Account with the corresponding line item in the 2010 Account.

Comparative year information for line item 9.4

2011 Account line item	2010 Account line item	2011 Account volume for the 2009–10 year (ML)	2010 Account volume for the 2009–10 year (ML)	Change from the 2010 Account to the 2011 Account
9.4 Runoff to surface water	13.4.1 Runoff to connected surface water	745,769	942,097	Methodology change: new model used with a revised model area Prior period error: solar radiation data were incorrect Change in line item name and number

The solar radiation data used in the 2010 Account were overestimated. The bias correction applied to the satellite-derived solar radiation dataset was incorrect, which resulted in an overestimation of incoming solar radiation.

A different rainfall–runoff model was used to estimate runoff in the Perth region for this account to that used in the 2010 Account. Using this new model, estimated runoff was more similar to observed data for the Perth region compared with the previous model.

The use of the new runoff model, combined with revised solar radiation data, resulted in a 26% decrease in the line item volume. Because this change is material, the 2009–10 year values were restated in the water accounting statements.