31.2 Leakage from off-channel water storages
The volume reported (999 ML) represents leakage from off-channel water storages to groundwater and the landscape during the 2012–13 year. It was not possible to separately quantify each of these items.
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).
The farm dam algorithm written by the Bureau of Meteorology was used to determine the volume of water leaking from off-channel water storages. Data input to the model included gridded climate datasets; runoff from the AWRA-L version 3.0 (Van Dijk 2010) model; and dam details derived from spatial data (surface area, volume, location and catchment area). Model parameters included a leakage factor of 5% per year.
The Adelaide region was split into two subregions 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 farm dam algorithm determines the water stored in each off-channel water storage at each time step and assumes that water will leak from the storage unless it empties, at which time leakage will cease.
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 spatial extent of water bodies was subject to the assumptions and methods associated with the spatial data provided by the DEWNR.
The uncertainty estimate was not quantified.