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 2012 Account the volume was 1,075,728 ML.
The Bureau of Meteorology (The Bureau): 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): Australian Water Resources Assessment system Landscape model (AWRA-L) parameters.
Runoff to surface water was estimated based on AWRA-L version 2.0.0 (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 2011–12 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.
The Bureau used AWRA-L v1.0.0 model to calculate the runoff volume published in the 2011 Account. The Bureau is currently using the AWARA-L v2.0.0 model (updated version of AWRA-L v1.0.0) and recalculated the 2010-11 year. Consequently, the volume has been restated from 1,179,859 ML (as published in the 2011 Account) to 1,240,466 ML.