31.1 Evaporation from off-channel water storages
Potential evaporation is an estimate of the evaporative demand of the environment. The Priestly and Taylor method was used to estimate potential evaporation (as calculated by the WaterDyn model (Raupach 2008 et al.) from off-channel water storages. The Spatial Tool for Estimating Dam Impacts (STEDI) model (Sinclair Knight Merz 2010) was used to determining the amount of water available for evaporation from individual off-channel water 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 0.05 degree (5 km) national grid and included downward solar irradiance, and maximum and minimum air temperature. The methods used to generate these gridded datasets are outlined in Jones et al. (2007).
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 subregions. The region was divided using the boundaries of the AHGF contracted catchments between McLaren Vale and the Onkaparinga Valley. The northern region included the Barossa Valley, the Northern Adelaide Plains, the River Torrens Catchment, the Patawalonga Catchment and the Onkaparinga Catchment. The southern region included 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 the South Australian Department for Water, 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 nine arc-second DEM was used to determine the catchment area of each off-channel water storage in the Adelaide region.
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 STEDI model. The model determines the water stored in each off-channel water storage, at each monthly time step and determines the volumetric potential evaporation by multiplying potential evaporation by storage surface area. The STEDI model assumes that actual evaporation will occur at the same rate as potential evaporation unless the storage empties, at which time evaporation will cease.
Assumptions, Limitations, Caveats and Approximations
- The Priestly and Taylor potential evaporation estimates were 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 was 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 the South Australian Department for Water.