Murray–Darling Basin
27.1 Off-channel water storages

The volumetric value for the line item for the 2011–12 year was 1,152,737 ML. The line item represents the volume of water held in constructed storages that are not connected either seasonally or perennially to rivers, filled predominantly by local catchment runoff within the Murray–Darling Basin (MDB) region. The following table presents breakdown information for the volumetric value on a surface water resource plan area basis.

Details of volume of water stored in off-channel water storages in the MDB region at the start and end of the 2011–12 year

Water resource plan area	Sustainable diversion limit area		State/Territory	Volume on 30 June 2012 (ML)	Volume on 30 June 2011 (ML)
	Code	Name
SW19 Warrego–Paroo–Nebine	SS29	Paroo	Qld	191,684	206,181
	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	57,253	56,857
SW15 NSW Border Rivers	SS23	NSW Border Rivers	NSW
SW14 Gwydir	SS22	Gwydir	NSW	52,677	34,992
SW13 Namoi	SS21	Namoi	NSW	64,007	57,108
SW10 Macquarie–Castlereagh	SS20	Macquarie–Castlereagh	NSW	168,615	158,212
Northern Basin				534,236	513,351
SW9 Lachlan	SS16	Lachlan	NSW	168,115	149,845
SW8 Murrumbidgee	SS15	Murrumbidgee  NSW	NSW	168,527	158,443
SW1 ACT	SS1	ACT	ACT
SW7 NSW Murray and Lower Darling	SS18	Lower Darling	NSW	87,496	100,354
	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.	32,570	33,315
	SS5	Broken	Vic.	96,181	99,205
	SS6	Goulburn	Vic.
	SS7	Campaspe	Vic.	20,990	24,906
	SS8	Loddon	Vic.	32,743	41,215
SW6 Eastern Mount Lofty Ranges	SS13	Eastern Mount Lofty Ranges	SA	11,879	11,304
	SS12	Marne Saunders	SA
Southern Basin				618,502	618,587
Whole MDB region				1,152,737	1,131,938

(1) Bureau of Meteorology (the Bureau): National Climate Centre daily climate grids (rainfall, temperature and solar radiation); and (2) Geoscience Australia: mapping of man-made waterbodies in the MDB spatial dataset, and 9 arc-second digital elevation model (DEM).

The Bureau.

A Fortran code water balance routine was used to determine the volume of water stored in off-channel water storages. The calculation routine was provided inputs: climate data from gridded climate datasets, runoff from the Australian water resources assessment system landscape (AWRA-L) model version 2.0.0 (Van Dijk 2010) and storage details derived from spatial data.

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 calculation routine 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 storage is an output of this water balance.

 

• The gridded climate input data are 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 Fortran code and the parameters used.
• The spatial extent of water bodies are subject to the assumptions and methods associated with the data provided by the Geoscience Australia.

The uncertainty estimate was not quantified.

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 27.1 Off-channel water storages

Segment	2012 Account volume at 30 June 2011 (ML)	2011 Account volume at 30 June 2011 (ML)	Difference due to calculation method change (ML)
Northern Basin	513,351	456,528	56,823
Southern Basin	618,587	580,559	38,028
Whole region	1,131,938	1,037,087	94,851

The volume estimated for the comparison year for the 2012 Account (1,131,938 ML) is higher than the volume reported for the 2011 Account (1,037,087 ML). This was due to a change in modelling methods. 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 calculation routine. The AWRA-L v2.0.0 model is more reliable than previous models in estimating runoff. The difference of 94,851 ML represents a change of 9% of the previously reported value.