30.3 Runoff harvesting into off-channel water store
The volume reported (12,286 ML) represents the volume of runoff harvested into off-channel water storages during the 2012–13 year.
Bureau of Meteorology: National Climate Centre daily climate grids (rainfall, temperature and solar radiation), Australian Hydrological Geospatial Fabric (AHGF) waterbody feature class; South Australian Department of Environment, Water and Natural Resources (DEWNR): Geographical Information System layers; Geoscience Australia: 9 arc-second digital elevation model (DEM).
Bureau of Meteorology.
Rainfall runoff harvesting to off-channel water storages was estimated based on the AWRA-L version 3.0 streamflow, precipitation and potential evaporation from the AWRA-L version 3.0 (Van Dijk 2010), and applying the farm dam algorithm written by the Bureau of Meteorology.
Using climate grid data for the Adelaide 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 Adelaide region was split into two sub-regions for the purpose of estimating the water balance of the off-channel water store. The region was divided using the boundaries of the AHGF contracted catchments between McLaren Vale and the Onkaparinga Valley. The northern region includes the Barossa Valley, the Northern Adelaide Plains, and the River Torrens, Patawalonga and the Onkaparinga catchments. The southern region includes catchments throughout McLaren Vale and the Fleurieu Peninsula.
Only off-channel water storages filled primarily by rainfall-runoff were considered. These were determined from waterbody mapping provided by DEWNR, and excluded waterbodies that were within the Virginia Pipeline Scheme service area and waterbodies that were less than 20 metres away from a channel of second order or higher, or an active bore. The catchment of each individual storage was determined via analysis of the 9 arc-second DEM.
The average runoff depth across the Adelaide 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 Adelaide 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.
Assumptions, limitations, caveats and approximations
- The gridded climate input data were 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 was subject to the assumptions associated with the farm dam algorithm and the parameters used.
- The spatial extent of waterbodies was subject to the assumptions and methods associated with the spatial data provided by DEWNR.
- The use of a 9 arc-second DEM to determine catchment area may result in off-channel water storages being assigned a catchment much larger or smaller than the true catchment. In some cases, an off-channel water storage may be assigned to the catchment of a stream line hundreds of metres away.