9.4 Runoff to surface water
This line item represents the runoff to major storages, rivers and drains that make up the connected surface water system in the Canberra region. For the 2011 Account the volume was 1,179,859 ML. The volume reported in the 2011 Account is much greater than that reported in the 2010 Account. The large increase is due to both a change in methodology and increased rainfall in the region.
Rainfall-runoff to surface water was estimated based on the Australian Water Resources Assessment system Landscape model (AWRA-L) version 1.0.0 (Van Dijk 2010) streamflow model outputs.
Using climate grid data for the Canberra 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 reservoirs and the off–channel water storages were excluded from the analysis.
Runoff from the landscape is divided into two components: (i) runoff into the surface water store (major reservoirs, rivers and drains); and (ii) runoff into off–channel water storages. Only runoff into the surface water store is considered in this line item.
The average runoff depth from the landscape into the surface water 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. Mean runoff depth was converted to a runoff volume by multiplying runoff depth by the total area of the region (excluding water storages).
Assumptions, Limitations, Caveats and Approximations
The estimated runoff was compared against historical flows at unimpaired catchments within the Canberra region for the 2010-11 reporting period 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 off–channel water storages was estimated using an off–channel water storage water balance model (STEDI). Where this was applied, the runoff estimates inherited the approximations, assumptions and caveats of STEDI and the parameters used total grid point area.
The uncertainty estimate was not quantified.
This line item corresponds to Line item 13.4.1 Runoff to connected surface water reported in the 2010 Account. There was a method change in the 2011 Account. These changes are summarised in the following table.
|Corresponding 2010 Account line item||Value reported for the 2010 Account closing balance (ML)||Prior period error correction to the 2010 values (ML)||Value difference due to method change (ML)||Restated value for the 2010 Account closing balance (ML)|
|Line item 13.4.1 Runoff to connected surface water||280,116||0||59,433||339,549|
It was recognised that the volume of runoff estimated for the comparison year (339,549 ML) was greater than the volume reported for the 2010 Account (280,116 ML). This was due to a change in modelling methods. This AWRA-L model has been used to restate the 2010 value due to it being a more accurate estimate. The difference 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 59,433 ML represents a change of approximately 21% of the originally submitted value. This difference is material and requires restatement.