27.1 Off-channel water storages
Off-channel water storages represent an important water resource in the Adelaide region. The volumes reported are the opening (15,158 ML) and closing (14,798 ML) storage volumes for the 2011–12 year and were derived from modelled data.
Bureau of Meteorology (the Bureau): 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 spatial tool for estimating dam impacts (STEDI) model (Sinclair Knight Merz 2010) was used to estimate the volume of water stored in off-channel water storages. Model inputs included daily gridded climate datasets, runoff from the AWRA-L version 2.0.0 model (Van Dijk 2010) and dam details derived from spatial data (surface area, volume, location and catchment area).
The Adelaide region has more than 10,000 off-channel water storages. This is the maximum number of off-channel water storages that the STEDI model can handle for a single region. Therefore, the Adelaide region was split into two smaller sub-regions. 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. A 9//arc-second DEM was used to determine the catchment area of each off-channel water storage in the Adelaide region.
The STEDI model performs a water balance on each individual storage at each time step using runoff and precipitation as inflows and spills, evaporation and usage as outflows. The volume of water held in off-channel water storages was an output of this water balance.
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 available in storage for evaporation is subject to the assumptions associated with the STEDI model and the parameters used.
- The spatial extent of waterbodies was subject to the assumptions and methods associated with the spatial data provided by DEWNR.