Canberra
30.3 Runoff harvesting into off-channel water store

This line item represents the runoff harvesting that occurred in the Canberra region. For 2013 the volume reported was 5,505 ML.

Bureau of Meteorology (The Bureau), National Climate Centre daily climate grids (rainfall, temperature and solar radiation); Geoscience Australia, mapping of human-made waterbodies spatial dataset, built-up areas spatial dataset and 9 arc-second digital elevation model (DEM).

Rainfall-runoff harvesting to private storages was estimated based on the AWRA-L version 3.0 streamflow, precipitation and potential evaporation from the AWRA-L version 3.0, and applying the farm dam algorithm written by the Bureau.

Using climate grid data for the Canberra region (including precipitation, temperature and solar radiation data), AWRA-L (Van Dijk, 2010) was used to estimate the runoff depth at each gridpoint within the region.

The Canberra region was divided up into five subregions for the purpose of estimating the water balance of the off-channel water store. Four of the subregions were the catchments of the major storages and the fifth was the remaining land.

The off–channel storages filled primarily by rainfall-runoff. These were determined from waterbody mapping provided by Geoscience Australia and were waterbodies that were greater than 50 m from built-up areas in the Canberra region.

The average runoff depth across the Canberra subregions was determined as the weighted mean of runoff occurring from the relevant grid points within the region boundary. Points were weighted upon the area they represented within the Canberra landscape 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. The average runoff depth was converted to a volume by multiplying depth by the total area and was used as an input into the farm dam algorithm.

• The gridded climate input data are subject to approximations associated with interpolating observation point data to a national grid detailed in Jones et al. (2007).
• The estimated volume of runoff harvested is subject to the assumptions associated with the farm dam algorithm and the parameters used.
• The spatial extent of waterbodies is subject to the assumptions and methods associated with the recent data (revised in May 2009) provided by Geoscience Australia.
• The use of a 9 arc-second DEM to determine catchment area may result in off-channel water stores being assigned a catchment much larger or smaller than the true catchment. In some cases an off-channel water store may be assigned the catchment of a stream line hundreds of metres away.

The uncertainty estimate was not quantified.