31.1 Evaporation from off-channel water storages
The volumetric value for the line item for the 2010–11 year was 1,696,154 ML. The line item represents the volume of water that passes into the atmosphere across a water/air interface from off-channel water storages within the Murray–Darling Basin (MDB) region. The following table presents breakdown information for the line item on a surface water resource plan area basis.
|Surface water resource plan area||
|SW11–12 and SW17–19||Warrego – Paroo – Nebine, Condamine–Balonne, Moonie, NSW Intersecting Streams and Barwon–Darling watercourse||Qld and NSW||348,394|
|SW15–16||Qld and NSW Border Rivers||Qld and NSW||185,058|
|Sub-total Northern Basin||1,040,673|
|SW1 and 8||Murrumbidgee NSW and ACT||NSW and ACT||175,414|
|SW2, 4, 5 and 7||NSW Murray and Lower Darling, Vic Murray, SA Murray and Wimmera–Mallee||NSW, Vic and SA||115,635|
|SW6||Eastern Mount Lofty Ranges||SA||14,016|
|Sub-total Southern Basin||655,482|
|Total for the region||1,696,154|
The Priestly and Taylor method to estimate potential evaporation (as calculated by the WaterDyn model [Raupach et al. 2008]) was used to estimate evaporation from the off-channel water store. The tool for estimating dam impacts (STEDI) model (Sinclair Knight Merz 2011) was used to determine the amount of water available for evaporation from individual off-channel water storages.
Using climate grid data for the MDB region (including precipitation, temperature and solar radiation data), monthly, open water evaporation data produced by the Bureau of Meteorology were calculated, based on daily gridded climate data that is available on a 0.05 degree (approximately 5 km) national grid.
The MDB was divided into 105 regions for the purpose of modelling the off-channel water store. The off-channel water store consisted of storages filled primarily by local catchment runoff. These were determined from waterbody mapping conducted by Geoscience Australia as those that:
are not named storages (assuming that any storage with a name is unlikely to be a off-channel water storage)
are above 600 m in elevation
are below 600 m in elevation in areas that receive greater than 400 mm per annum in precipitation and are not within 50 m of a major or perennial stream.
The above rules attempt to divide storages into those that are likely to be filled primarily by local catchment runoff and those that are filled by abstraction from surface water, groundwater or floodplain harvesting. The catchment of each individual storage was determined via analysis of the 9 arc-second DEM.The potential average evaporation depth across the MDB region was determined as the weighted mean of potential evaporation occurring from the relevant grid points within the region boundary. Points were weighted upon the area they represented within the MDB landscape to remove edge effects (where the area represented is not wholly within the MDB region) and the effect of changing area represented with changing latitude. The average potential evaporation was used as an input into the STEDI model. The STEDI model determines the water stored in each off-channel water storage at each time step and determines the volumetric potential evaporation by multiplying potential evaporation by storage surface area. STEDI assumes that actual evaporation will occur at the same rate as potential evaporation unless storage empties, at which time, evaporation will cease.
Assumptions, Limitations, Caveats and Approximations
The Priestly and Taylor 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 STEDI model and the parameters used.
The spatial extent of water bodies subject to the assumptions and methods associated with the data provided by Geoscience Australia.
The uncertainty estimate was not quantified.
In the 2011 Account, the following changes were made that caused the 2009–10 year value to be restated:
The scope of the line item was changed.
The methodology used to quantify the line item was improved and resulted in a material change in volume.
The value reported for the 2009–10 year in the 2010 Account was different to the restated value in the 2011 Account. This was primarily due to the exclusion of off-channel storages that are not filled by local catchment runoff and was also due to the use of Priestly and Taylor potential evaporation instead of Penman open water potential evaporation. The difference of 833,036 ML represents a change of approximately 36% of the volume provided for the 2010 Account. The changes and their respective values are detailed in the following table.
Volume for the 2009–10 year reported in the 2010 Account (ML)
Difference due to scope and calculation method change (ML)
Volume for the 2009–10 year reported in the 2011 Account (ML)