Perth

This line item is comprised of two main components:

1. rainfall-runoff into the major reservoirs
2. rainfall-runoff into rivers and drains

The volume of rainfall-runoff into each major reservoir within the Perth region during 2009-10 is given below.

(1) Rainfall-runoff was not calculated for Drakes Brook due to a lack of available data.

The volume of rainfall-runoff into rivers and drains within the Perth region during 2009-10 is calculated as shown in the table below.

This table shows that total rainfall-runoff into the surface water store (major reservoirs and rivers) within the region during 2009-10 has been modelled. However, given that rainfall-runoff into reservoirs during 2009-10 has also been derived by the Water Corporation using measured data, the rainfall-runoff into reservoirs and rivers has been presented here separately.

Data source

Water Corporation: Operational Data Storage System database.

Bureau of Meteorology: National Climate Centre daily climate grids (rainfall, temperature and solar radiation); and Australian Hydrological Geospatial Fabric waterbody feature class.

Commonwealth Scientific and Industrial Research Organisation (CSIRO): WaterDyn and AWRA-L model parameters; and monthly climatological average radiation grid data.

Geoscience Australia: southwest Western Australia man-made water body feature class.

Data provider

Water Corporation and Bureau of Meteorology.

Method

The runoff to connected surface water is comprised of two components: rainfall-runoff into the major reservoirs and rainfall-runoff into rivers and drains.

Rainfall-runoff into reservoirs

Rainfall-runoff into major reservoirs was calculated based on a water balance calculation using measured data collected at each major reservoir. The inflow into each reservoir is calculated using the following water balance equation:

Inflow =  ΔS + O + E - Pb - T

where:

ΔS is the change in storage

O is total outflow from the reservoir

E is evaporation from the reservoir

Pb is the volume of water pumped back into the reservoir, and

T is the volume of water transferred into the reservoir from the water distribution system

Total outflow (O) is calculated using the following equation:

O = Sp + Sc + D + R

where:

Sp is spillage

Sc is scour

D is volume of water abstracted from the reservoir

R is volume of riparian releases from the reservoir

Rainfall-runoff into rivers and drains

Rainfall-runoff into rivers and drains was estimated based on the average of the WaterDyn discharge and the AWRA-L streamflow model outputs.

Using climate grid data for the Perth region (including precipitation, temperature and solar radiation data), WaterDyn and AWRA-L were 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 reservoirs and the local catchment reservoirs were excluded from the analysis.

Runoff from the landscape is divided into two components: (i) runoff into the connected surface water store (major reservoirs, rivers and drains); and (ii) runoff into local catchment reservoirs. Only runoff into the connected surface water store is considered here.

The average runoff depth from the landscape into the connected surface water store was determined as the unweighted arithmetic mean of the relevant AWAP 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 reservoirs). Finally, the runoff into reservoirs (calculated above) is subtracted from the total modelled runoff into the connected surface water store to establish the rainfall-runoff into rivers and drains only.

Uncertainty

Ungraded.

Approximations, assumptions, caveats/limitations

• Part of the rainfall-runoff into the reservoirs has been based on estimated precipitation and evaporation data. The volume of runoff estimated was approximately 2% of the total runoff into reservoirs.
• Rainfall-runoff was not calculated for Drakes Brook Reservoir due to a lack of available precipitation and evaporation data collected at the site. However, it is estimated that inflows into this reservoir are relatively minor and the omission of these inflows do not significantly affect total rainfall-runoff into reservoirs.
• The estimated runoff into rivers was not verified by real-time analysis of streamflow records. The reported value was an estimate of water that was likely to have entered the connected water store from the landscape.
• The runoff estimates were subject to the assumptions of the WaterDyn model detailed in Raupach et al. (2008) and 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 reservoirs is estimated using a local catchment reservoir water balance model (STEDI). Where this is applied, the runoff estimates inherit the approximations, assumptions and caveats of the water balance model (STEDI) and parameters used.
• Landscape runoff attributed to grid cells (5 x 5 km) that intersected the catchment boundary (i.e. that had some part of the grid cell falling outside the catchment boundary) was included fully in runoff calculations for the catchment. This had a limited influence because the average of runoff from all grid cells was multiplied by the catchment area.