29.2 Runoff harvesting
Rainfall-runoff harvesting to private storages was estimated based on the AWRA-L version 1.0.0 streamflow and STEDI (Sinclair Knight Merz 2011) farm dam water balance model outputs.
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 private store. The four of the subregions were the catchments of the major storages and the fifth was the remaining land.
The private store consisted of 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 STEDI model. The STEDI model determines the available capacity of each private storage at each time point; runoff that does not exceed the available capacity is considered harvested, while runoff that spills from the reservoir is considered to have contributed to the surface water store.
Assumptions, Limitations, Caveats and Approximations
- 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 STEDI model and the parameters used.
- The spatial extent of waterbodies subject to the assumptions and methods associated with the data provided by Geoscience Australia.
- The use of a 9 arc-second digital elevation model to determine catchment area may result in reservoirs being assigned a catchment much larger or smaller than the true catchment. In some cases a reservoir may be assigned the catchment of a stream line hundreds of metres away.
The uncertainty estimate was not quantified.