9.4 Runoff to surface water
This line item refers to the total runoff from the landscape into the surface water store. The surface water store includes drains, rivers and storages within the region.
The volume recognised in the water accounting statements (645,981 ML) represents the total volume of runoff into surface water during the 2011–12 year. This is more than five times larger than the comparison year. This large increase in runoff reflects the improved rainfall conditions observed throughout the region during the 2011–12 year (see Climate overview) compared to the record low rainfall recorded during the comparison year (see the 2011 Account).
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).
Assumptions, limitations, caveats and approximations
The estimated runoff was compared against historical flows in unimpaired catchments within the Perth region for the 2011–12 year and provided a suitable representation of the runoff for this year. The GR4J model was selected over the Australian Water Resource Assessment system Landscape (AWRA-L) version 2.0.0 model (used to estimate runoff for all other 2012 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).