31.1 Evaporation from off-channel water storages
The volume reported (15,952 ML) represents the volume of water that evaporated from 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).
The potential evaporation estimate produced by the Australian Water Resources Assessment system landscape model (AWRA-L) version 3.0 (Van Dijk 2010) was used to calculate evaporation from off-channel water storages. The AWRA-L model uses a modified version of the Penman-Monteith method to produce the potential evaporation. The farm dam algorithm written by the Bureau of Meteorology was used to determine the amount of water available for evaporation from individual private storages.
This method used monthly open water evaporation data produced by the Bureau of Meteorology. These data are based on daily gridded climate data that are available on a 0.050 (5 km) national grid and included precipitation, temperature, and solar radiation data. The methods used to generate these gridded datasets are outlined in Jones et al. (2007).
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 potential average evaporation depth across the Adelaide subregions was determined as the weighted mean of potential evaporation occurring from the relevant grid points within the Adelaide region boundary. Points were weighted based on 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 with changing latitude. The average potential evaporation was used as an input into the farm dam algorithm written by the Bureau. It determines the water stored in each off-channel water store at each time point and determines the volumetric potential evaporation by multiplying potential evaporation by reservoir surface area. It assumes that actual evaporation will occur at the same rate as potential evaporation unless reservoir empties, at which time evaporation will cease.
Assumptions, limitations, caveats and approximations
- The AWRA_L potential evaporation estimates are subject to approximations associated with interpolating the observation point input data to a national grid as described in Jones et al. (2007).
- The estimated volume available in storage for evaporation is subject to the assumptions associated with the farm dam algorithm written by the Bureau and the parameters used.
- The spatial extent of water bodies was subject to the assumptions and methods associated with the spatial data provided by DEWNR.