Murray–Darling Basin
30.3 Runoff harvesting into off-channel water store

The volumetric value for the line item for the 2011–12 year was 1,320,947 ML. The line item includes collection of runoff into off-channel water storages including local catchment runoff into off-channel water storages 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.

Runoff harvesting into 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	268,185
	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	100,793
SW15 NSW Border Rivers	SS23	NSW Border Rivers	NSW
SW14 Gwydir	SS22	Gwydir	NSW	98,936
SW13 Namoi	SS21	Namoi	NSW	89,669
SW10 Macquarie–Castlereagh	SS20	Macquarie–Castlereagh	NSW	185,387
Northern Basin				742,969
SW9 Lachlan	SS16	Lachlan	NSW	164,636
SW8 Murrumbidgee	SS15	Murrumbidgee  NSW	NSW	158,636
SW1 ACT	SS1	ACT	ACT
SW7 NSW Murray and Lower Darling	SS18	Lower Darling	NSW	83,005
	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.	23,748
	SS5	Broken	Vic.	79,334
	SS6	Goulburn	Vic.
	SS7	Campaspe	Vic.	20,774
	SS8	Loddon	Vic.	34,351
SW6 Eastern Mount Lofty Ranges	SS13	Eastern Mount Lofty Ranges	SA	13,493
	SS12	Marne Saunders	SA
Southern Basin				577,978
Whole MDB region				1,320,947

 

(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).

The Bureau.

Rainfall runoff harvesting to off-channel water storages was estimated using the Australian water resources assessment system landscape (AWRA-L) version 2.0.0 model and a water balance tool based on a Fortran code.

Using climate grid data for the MDB region (including precipitation, temperature and solar radiation data), AWRA-L (Van Dijk 2010) was used to estimate the runoff depth at each grid-point within the region.

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 average runoff depth across the MDB sub-regions was determined as the weighted mean of runoff 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 runoff depth was converted to a volume by multiplying depth by the total area and was used as an input into the water balance tool based on the Fortran code.

• 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 spatial extent of water bodies subject to the assumptions and methods associated with the data provided by the Geoscience Australia.
• The use of a 9 arc-second DEM to determine catchment area may result in storages being assigned a catchment much larger or smaller than the true catchment. In some cases a storage may be assigned the catchment of a stream line hundreds of metres away.

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 30.3 Runoff harvesting into off-channel water store

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	773,931	778,663	-4,732
Southern Basin	741,628	727,822	13,806
Whole region	1,515,559	1,506,485	9,074

The volume estimated for the comparison year for the 2012 Account (1,515,559 ML) is higher than the volume reported for the 2011 Account (1,506,485 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 water balance tool. The AWRA-L v2.0.0 model is more reliable than previous models in estimating runoff. The difference of 9,074 ML represents a change of 6% of the previously reported value.