Murray–Darling Basin
31.1 Evaporation from off-channel water storages

Supporting information

The volumetric value for the line item for the 2011–12 year was 1,611,332 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.

 

Evaporation from off-channel water storages in the MDB region for the 2011–12 year
Water resource plan area Sustainable diversion limit area State/Territory   Volume (ML) for the 2011–12 year 
Code Name
SW19 Warrego–Paroo–Nebine SS29 Paroo  Qld 328,200
SS28 Warrego  Qld
SS27 Nebine  Qld
SW18 Condamine–Balonne SS26 Condamine–Balonne  Qld
SW17 Moonie SS25 Moonie  Qld
SW11 Barwon–Darling Watercourse SS19 Barwon–Darling Watercourse  NSW
SW12 NSW Intersecting Streams SS17 NSW Intersecting Streams NSW
SW16 Qld Border Rivers SS24 Qld Border Rivers  Qld 175,878
SW15 NSW Border Rivers SS23 NSW Border Rivers  NSW
SW14 Gwydir SS22 Gwydir NSW 187,311
SW13 Namoi SS21 Namoi  NSW 125,656
SW10 Macquarie–Castlereagh SS20 Macquarie–Castlereagh  NSW 222,810
Northern Basin 1,039,856
SW9 Lachlan SS16 Lachlan  NSW 150,209
SW8 Murrumbidgee  SS15 Murrumbidgee  NSW NSW 147,228
SW1 ACT SS1 ACT ACT
SW7 NSW Murray and Lower Darling SS18 Lower Darling  NSW 97,555
SS14 NSW Murray NSW
SW2 Vic. Murray SS3 Kiewa Vic.
SS2 Vic. Murray  Vic.
SW4 Wimmera–Mallee  SS9 Wimmera–Mallee  Vic.
SW5 SA Murray SS11 SA Murray SA
SS10 SA Non-prescribed areas  SA
SW3 Northern Victoria SS4 Ovens  Vic. 22,298
SS5 Broken  Vic. 82,306
SS6 Goulburn Vic.
SS7 Campaspe  Vic. 21,973
SS8 Loddon Vic. 37,553
SW6 Eastern Mount Lofty Ranges  SS13 Eastern Mount Lofty Ranges  SA 12,354
SS12 Marne Saunders  SA
Southern Basin 571,476
Whole MDB region 1,611,332

Quantification approach

Data source

(1) Bureau of Meteorology (the Bureau): National Climate Centre daily climate grids (rainfall, temperature and solar radiation) and (2) Geoscience Australia: MDB human-made waterbody feature class and 9 arc-second digital elevation model (DEM).

 

Provided by

The Bureau.

Method

The potential evaporation estimate produced by the Australian water resources assessment system landscape (AWRA-L) model version 2.0.0 (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. A water balance tool based on a Fortran code was used to determine the amount of water available for evaporation from individual off-channel water storages.

Using daily gridded climate data for the MDB region (including precipitation, temperature and solar radiation data), open water evaporation data on monthly basis were calculated. Daily gridded climate data were 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 which:

  • 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 which 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 water balance tool. The tool 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. The tool 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

  • 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 spatial extent of water bodies subject to the assumptions and methods associated with the data provided by Geoscience Australia.

 

Uncertainty information

The uncertainty estimate was not quantified.

 

Comparative year

A change made to the calculation method resulted in the restatement of the 2010–11 year volume. The method used to quantify the line item was improved and resulted in a material change in volume.

The respective volumes associated with the change are detailed in the following table.



Restatement of comparative year information made for the line item 31.1 Evaporation from off-channel water stogares
Segment 2012 Account volume for the 2010–11 year (ML) 2011 Account volume for the 2010–11 year (ML) Difference due to calculation method change (ML)
Northern Basin 976,387 1,040,673 -64,285
Southern Basin 596,232 655,482 -59,249
Whole region 1,572,620 1,696,154 -123,535


The volume of evaporation estimated for the comparison year for the 2012 Account (1,572,620 ML) is less than the volume reported for the 2011 Account (1,696,154 ML). 

The difference between the previously reported volume and the estimate produced for the comparison year can be attributed to the choice of the AWRA-L v2.0.0 model (instead of the previously used AWRA-L v1.0.0) to provide inputs to the water balance tool. The AWRA-L v2.0.0 model is more reliable than previous models in estimating runoff.

The difference of 123,535 ML represents a change 7% of the previously reported volume.