Murray–Darling Basin
Physical information

This section provides details about the Murray–Darling Basin (MDB) region, its water resources, land use, and water-related infrastructure.

Area: 1,055,600 km² (Australian Bureau of Agricultural and Resource Economics and Sciences 2010)

Population: 2,190,000 (Australian Bureau of Statistics 2012)

The MDB is defined by the catchment areas of the Murray and Darling rivers and their many tributaries. The MDB covers 75% of New South Wales, more than 50% of Victoria, significant portions of Queensland and South Australia, and all of the Australian Capital Territory (shown in figures P1 and P2). It contains:

• around 440,000 km of rivers, of which 40,000 km are major
• some 30,000 wetlands, covering an area of around 25,000 km²
• about 60,000 km² of floodplain area, which represents approximately 6% of the MDB.
  

Figure P1 Location map of the MDB within Australia

Source for the boundaries of the MDB: Australian Government Department of Sustainability, Environment, Water, Population and Communities.

As shown in Figure P2, major rivers in the MDB include:

• Murray River, which forms the border of Victoria and New South Wales, and flows east to west for 2,510 km
• Darling River, which begins in northern New South Wales and flows 2,740 km (including its major tributaries—the Culgoa, Balonne and Condamine rivers) southwest to its junction with the Murray River at Wentworth
• Murrumbidgee River, which flows 1,485 km from southern New South Wales, through the Australian Capital Territory and then westwards to the Murray River
• Lachlan River, which starts in the central highlands of New South Wales and flows 1,339 km southwest to the Murrumbidgee River
• Goulburn River, which flows 654 km from central northern Victoria northwards into the Murray River (Geoscience Australia 2010).

The MDB is a complex, interconnected river system rather than a series of separate catchments; however, its diverse climate and landscape and the presence of artificial structures means not all parts of the system are connected to the same extent. For example, rivers such as the Paroo, Lachlan, and Wimmera only rarely contribute flows further downstream.

Figure P2 Contextual map of the MDB showing the major rivers

Sources: Australian Government Department of Sustainability, Environment, Water, Population and Communities, and Geoscience Australia (2010).

The MDB is defined in Section 18A of the Australian Government Water Act 2007 (the Act). It includes all water resources within or beneath the MDB, but does not include groundwater that forms part of the Great Artesian Basin (GAB).

For the 2013 Account, the MDB region (see Figure P2) is defined as all the surface water connected to the channel network and all the groundwater (excluding any water in the GAB) located within the geographical boundaries of the MDB specified by the Act, except:

• the areas drained by the Snowy Mountains Hydroelectric Scheme storages that are located inside the legal MDB's boundaries
• the towns of Port Elliot and Middleton in South Australia.

Further, the following elements are not included in the MDB region water account:

• off-channel water storages, such as landscape catchment storages (also known as farm dams) used to harvest runoff and floodwaters (these constitute water abstracted before it reaches the rivers or water owned by the users)
• urban water systems at utility level, irrigation systems and private water supply systems.

The MDB region presents a varied landscape, from semi-arid ephemeral river systems in the north to highly regulated river systems in the south fed from the Australian Alps. To the east and south, the highlands of the Great Dividing Range form the limit of the MDB region, while in the north, west, and southwest the boundaries are much less distinct. The greater proportion of the MDB region is made up of extensive plains and low undulating areas; most of them are less than 200 m above sea level.

The 2013 Account information is provided for the whole MDB region and in two segments: Northern Basin and the Southern Basin (see Figure P3). Boundaries for the Northern Basin and Southern Basin are largely determined by the physical geography of the Murray–Darling Basin, previous water management boundaries, and the level of hydrological connectivity. The hydrological boundary between the Northern Basin and the Southern Basin is located where the Darling River meets the upstream end of the Menindee Lakes.

The Northern Basin typically has less rainfall, less regulation, less development, and less water use than the Southern Basin. Furthermore, water management is characterised in the Northern Basin by a different rules framework, fewer water storages, and highly variable hydrological connectivity when compared with water management in the Southern Basin.

The region is divided into 20 surface water planning areas and 22 groundwater planning areas (both numbers include six combined surface water and groundwater water resource plan areas) to provide information in the 2013 Account. Detailed information has been provided where data are available at sustainable diversion limit areas. Details of water planning areas and sustainable diversion limit areas are included under Surface water and Groundwater.

The major population centres of the MDB region and their population (as at 28 March 2013) include:

• Canberra–Queanbeyan: 391,645

• Albury–Wodonga: 82,083

• Bathurst: 31,294

• Dubbo: 32,327

• Orange: 34,992

• Tamworth: 36,131

• Wagga Wagga: 46,913

• Bendigo: 82,794

• Mildura–Buronga: 33,432

• Shepparton: 42,741

• Toowoomba: 96,567 (Australian Bureau of Statistics 2013).

Note: The numbers are related to urban centre / locality.

There are also a number of smaller urban centres throughout the MDB region such as Griffith, Echuca, Moree, Horsham, Wangaratta, Warwick, and Murray Bridge.

The MDB is one of the most productive food and fibre regions in Australia. It contains approximately 40% of all Australian farms (by number), which produce wool, cotton, wheat, sheep, cattle, dairy products, rice, oilseeds, wine, fruit, and vegetables for domestic and overseas markets (Murray–Darling Basin Authority 2011).

The major land use activities in the MDB region are shown in Table P1. Figure P3 shows the distribution of these land uses within the region. Land use activities that are major water users in the MDB region are urban supply and irrigated agriculture.

Table P1  Major land use activities in the MDB region

Land use activity	Area (km2)	Total area of the region (%)
conservation and natural environments	107,600	10
dryland agriculture	133,300	13
forestry	34,000	3
grazing	727,800	69
irrigated agriculture	24,700	2
mining	300	<1
other intensive uses	1,200	<1
urban	14,300	1
water	12,400	1
Total	1,055,600

Source: Australian Bureau of Agricultural and Resource Economics and Sciences (2010)

 

Figure P3 Map of land use in the MDB region

Source: Australian Bureau of Agricultural and Resource Economics and Sciences (2010); source for boundaries of the MDB region: Australian Government Department of Sustainability, Environment, Water, Population and Communities.

 

The MDB region contains 16 Ramsar-listed wetlands (Department of Sustainability, Environment, Water, Population and Communities 2010) of international importance:

• Banrock Station wetland complex (South Australia)
• Barmah Forest (Victoria)
• Coorong and Lakes Alexandrina and Albert (South Australia)
• Currawinya Lakes National Park (Queensland)
• Fivebough and Tuckerbil swamps (New South Wales)
• Ginini Flats (Australian Capital Territory)
• Gunbower Forest (Victoria)
• Gwydir Wetlands (New South Wales)
• Hattah–Kulkyne Lakes (Victoria)
• Kerang Lakes (Victoria)
• Lake Albacutya (Victoria)
• Macquarie Marshes Nature Reserve (New South Wales)
• Narran Lake Nature Reserve (New South Wales)
• New South Wales Central Murray State forests
• Paroo River wetlands (New South Wales)
• Riverland (South Australia).

The MDB region also contains nationally important wetlands, listed in the Directory of Important Wetlands in Australia (Department of Sustainability, Environment, Water, Population and Communities 2011).

The Murray–Darling river system has been of extreme importance to Aboriginal people and their history in the Basin extends over at least 45,000 years to the present day. There are at least 10,000 known Aboriginal sites in the Basin, indicating the deep physical, economic, and spiritual dependence of Aboriginal people on the Basin's rivers, creeks, lakes, and wetlands and other resources. The rivers and flood plains are of particular importance to the traditional cultural beliefs and practices of Aboriginal Nations. Today there are over 40 Aboriginal Nations in the  Murray–Darling Basin (Murray–Darling Basin Authority 2014 c).

Archaeological evidence can be found including such things as; shell middens, quarries, rock shelters with archaeological deposits (e.g., stone artefacts), open camp sites, rock paintings and engravings, axe grinding rocks with their grooves, burial grounds, and the sacred and ceremonial sites. The sites are dated from a range of ages:

• 9,000–13,000 years old: The Kow Swamp burial site in northern Victoria
• At least 20,000 years old: Excavations indicating Aboriginal presence at Keniff Cave, at the headwaters of the Darling
• Around 40,000 years old: Evidence of Aboriginal populations from Lake Mungo and other lakes of the Willandra system
• Of more recent origin are the many carved and scarred trees from which bark canoes, shields, boomerangs and carrying dishes were cut. These trees can be found along most of the Basin's waterways

Aboriginal people view themselves as an integral part of the land and river systems and feel a strong responsibility for the health of rivers and wetlands.The rivers have always been of great importance to the Aboriginal people, especially as sources of food. As an example, the fish traps of the Ngemba in the Barwon River at Brewarrina illustrate how the river serves as a critical source of food. Brewarrina is well known for the 40,000 year old aboriginal fishtraps which are located just below the weir in the Brewarrina Township. These traps are constructed from carefully placed rocks in a circular arrangement and are still used today.

The water resources in the MDB region are highly developed, as illustrated by the following statistics:

• As a long-term average 42% of the total surface water runoff to the MDB region is diverted for consumption or environmental management, while 58% currently remains in the environment (Murray–Darling Basin Authority 2013b).
• The majority of the surface water consumed in the MDB region, 10.9 million ML/year, is diverted from the watercourse and used for irrigation and urban supply (the 2013 Account reports diversion of 10.7 million ML). In addition 2.7 million ML/year is intercepted by local catchment storages and forestry plantations that intercept runoff before it reaches the watercourse (Murray–Darling Basin Authority 2011).

Around 1.8 million ML of groundwater is consumed each year from the MDB region's water resources (Murray–Darling Basin Authority 2014d); however, due to abundance of surface water resources, the consumption has been below this volume during recent years. The majority of groundwater use is centred on a small number of large alluvial aquifers in New South Wales, Queensland, Victoria, and South Australia where the groundwater is used for agricultural purposes. Elsewhere there are areas in the Basin where groundwater is the only reliable source of water for stock and domestic uses.

A Murray–Darling Basin Cap (the Cap) was established in 1995 to limit the volume of surface water that could be diverted in the MDB region in any year to that based on the 1993–1994 levels of development (Murray–Darling Basin Authority 2012h). The Cap does not place a restriction on groundwater extraction; however, the Basin Plan 2012 (Murray–Darling Basin Authority 2013b) assessed sustainable diversion limits for both groundwater and surface water within the MDB region. See Operating rules and constraints within the 'Water rights' section for further information on the Cap.

The southern part of the MDB region is mostly a regulated system with major storages in many rivers. The storages in the three major southern rivers—the Murrumbidgee, Murray, and Goulburn—are used to provide regulated flows downstream as far as the lower lakes in South Australia.

Although there are major storages in the rivers including Border, Gwydir, Namoi and Macquarie, the northern part of the MDB region consists mostly of unregulated systems. Many of the rivers and streams in the northern MDB region are ephemeral and fed by seasonal rainfall. Generally, water users in much of the northern MDB region rely on collecting water during floods and storing it on-farm for later use.

The 2013 Account presents information on the surface water volumes for the 20 water resource planning areas for the MDB region as shown in Figure P4 (Murray–Darling Basin Authority 2011). Table 2 lists both water resource planning areas and sustainable diversion limit areas.

 

P4 Map of Murray–Darling Basin surface water resource plan areas

Source: Murray–Darling Basin Authority

 

Table P2  Surface water resource planning areas and sustainable diversion limit areas in the 2013 Account for the MDB region

Segment	Jurisdiction	Water resource plan area		Sustainable diversion limit area		River system
		Code	Name	Code	Name
Southern Basin	ACT	SW1	ACT (surface water)	SS1	ACT (surface water)	Murrumbidgee ACT
	Vic.	SW2	Victorian Murray	SS2	Victorian Murray	Victorian Murray
				SS3	Kiewa	Kiewa
	Vic.	SW3	Northern Victoria	SS4	Ovens	Ovens
				SS5	Broken	Broken
				SS6	Goulburn	Goulburn
				SS7	Campaspe	Campaspe
				SS8	Loddon	Loddon
	Vic.	SW4	Wimmera–Mallee (surface water)	SS9	Wimmera–Mallee (surface water)	Wimmera–Mallee
	SA	SW5	South Australian Murray Region*	SS10	SA Non-Prescribed Areas	SA Non-Prescribed Areas
	SA	SW6	South Australian River Murray	SS11	SA Murray	SA Murray
	SA	SW7	Eastern Mount Lofty Ranges*	SS12	Marne–Saunders	Marne–Saunders
				SS13	Eastern Mount Lofty Ranges	Eastern Mount Lofty Ranges
	NSW	SW 8	NSW Murray and Lower Darling	SS14	NSW Murray	NSW Murray
				SS18	Lower Darling	Lower Darling
	NSW	SW9	Murrumbidgee	SS15	Murrumbidgee  NSW	Murrumbidgee  NSW
	NSW	SW10	Lachlan	SS16	Lachlan	Lachlan
Northern Basin	NSW	SW11	Macquarie–Castlereagh	SS20	Macquarie–Castlereagh	Macquarie–Castlereagh
	NSW	SW12	Barwon–Darling Watercourse	SS19	Barwon–Darling Watercourse	Barwon–Darling
	NSW	SW13	NSW Intersecting Streams	SS17	NSW Intersecting Streams	Paroo, Warrego, Culgoa, Bokhara, Narran, and Moonie
	NSW	SW14	Namoi	SS21	Namoi	Namoi
	NSW	SW15	Gwydir	SS22	Gwydir	Gwydir
	NSW	SW16	NSW Border Rivers	SS23	NSW Border Rivers	NSW Border Rivers
	Qld	SW17	Queensland Border Rivers*	SS24	Queensland Border Rivers	Queensland Border Rivers
	Qld	SW18	Moonie*	SS25	Moonie	Moonie
	Qld	SW19	Condamine–Balonne*	SS26	Condamine–Balonne	Condamine–Balonne
	Qld	SW20	Warrego–Paroo–Nebine*	SS27	Nebine	Nebine
				SS28	Warrego	Warrego
				SS29	Paroo	Paroo

 * Combined surface water and groundwater resource plan areas

The total storage capacity (including dead storage) for the 54 major storages (excluding weirs not considered as storages) within the MDB region for which data are available for the 2013 Account is 23,193,359 ML. A full list of these storages and their individual total storage capacities can be found in line item 1.1 Storages.

The 2013 Account includes streamflow summary at the following gauging stations within the region:

• River Murray at Doctors Point (Albury) (Station 409017)
• Darling River at Bourke (Station 425003)
• Ovens River at Peechelba (Station 403241).

The River Murray at Doctors Point (Albury) represents a regulated reach of the Murray downstream of major regulating storages (within the Southern Basin).

The Darling River at Bourke represents the unregulated Northern Basin.

The Ovens River at Peechelba represents a largely unregulated major river in the Southern Basin.

Figure P5 shows mean monthly rainfall and flow volumes for these three gauging stations. The locations of the gauging stations are shown in Figure P6.

 

Figure P5 Graph of mean monthly flows at selected gauging stations in the Murray, Darling and Ovens rivers; mean monthly rainfall for the region is also shown

Figure P6 Map of the streamflow gauging station locations used to develop flow charts

 Sources: Bureau of Meteorology and Murray–Darling Basin Authority

 

Information on streamflow patterns, monthly flow volumes and variations for the flows at the stations are available in Streamflow in the 2013 Account 'Water overview' for MDB.

 

In a number of locations, water is transferred into or out of the region, as well as from one catchment to another within the region (see Figure P7). 

 

Figure P7 Map of water transfers into, out of, or between catchments within the MDB region

 Sources: Bureau of Meteorology and Murray–Darling Basin Authority

 

Major transfers into the MDB region are from:

• the Snowy Mountains Hydro-Electric Scheme into the Murrumbidgee and Murray rivers (see line item 1.5 Inter-region claim on water for more details)

• the Glenelg catchment to the Wimmera system

• Perseverance, Cressbrook, and Wivenhoe reservoirs (through Cressbrook reservoir, which started in 2010) in the Brisbane valley to Toowoomba in the Condamine–Balonne region.

Major transfers out of the MDB region include diversion of water from:

• the South Australian River Murray to supply metropolitan Adelaide and country towns

• the Macquarie River water sources (Fish River scheme) to the Blue Mountains area in Sydney

• the Goulburn River system via the north–south pipeline for distribution to Melbourne

• water transfers from the Goulburn–Broken region to Ballarat (in the Barwon River basin) via the Goldfields superpipe since May 2008

• water transfers from the Goulburn–Broken region (from Silver and Wallaby creeks, which are tributaries of the Goulburn River, not shown in Figure P7) to the Melbourne urban supply system.

Channels and pipelines in the river system of the Southern Basin allow water to be moved and traded from one catchment to another. For example, the Waranga Western Channel delivers water from the Goulburn River to the Campaspe, Loddon, and Wimmera–Avoca catchments.

The groundwater systems of the MDB region can be categorised into four distinct hydrogeological types:

• the aquifers in sedimentary deposits of the Murray basin and the Great Artesian Basin (GAB) within the low topography landscapes (including the Lower Murrumbidgee and the South Australian Murray)
• the shallow aquifers of the Darling River basin overlying the GAB ( including the Lower Namoi and Gwydir)
• valley-fill alluvium (including the Mid-Murrumbidgee, Upper Namoi and Upper Lachlan) in the highlands bordering the region
• the local fractured rock aquifers of the Great Dividing Range and other areas where the basement rock outcrops.

While the Great Artesian Basin is a major groundwater resource under the Basin, its management is not included in the Basin Plan 2012.

The boundaries of the groundwater systems do not coincide with those of the Murray–Darling Basin, which is defined on the basis of its surface water resources. The different groundwater systems behave somewhat independently of each other, with only relatively small volumes of groundwater directly flowing from one system to another; however, water from different aquifer systems is transferred across boundaries as surface water base flow.Through this process, a substantial volume of groundwater enters the surface streams in the upper and middle catchments as base flow and then re-enters the groundwater systems further down through seepage from stream beds (Murray–Darling Basin Commission 1998).

In the 2013 Account, groundwater information has been reported where data is available at the data collection stage for the 22 groundwater resource planning areas (Murray–Darling Basin Authority 2013b). Detailed information has been provided where data is available at the level of sustainable diversion limit areas. It is likely that any subsequent changes to groundwater information in the water resource planning areas after data collection occurred are not included in the 2013 Account. Figure P8 and tables P3 and P4 include details of groundwater water resource planning areas and sustainable diversion limit areas.

  

Figure P8 Map of groundwater resource plan areas within the Murray–Darling Basin

Source: Murray–Darling Basin Authority

Table P3  Groundwater resource planning areas and sustainable diversion limit areas in the Northern Basin of the MDB region for the 2013 Account

Jurisdiction	Water resource plan area		Segment of the MDB region	Sustainable diversion limit area
	Code	Name		Code	Name
NSW	GW7	Darling Alluvium1	Northern Basin	GS42	Upper Darling Alluvium
NSW	GW11	Lachlan and South Western Fractured Rock2	Northern Basin	GS19	Kanmantoo Fold Belt
				GS20	Lachlan Fold Belt
NSW	GW12	Macquarie–Castlereagh Alluvium	Northern Basin	GS11	Bell Valley Alluvium
				GS14	Castlereagh Alluvium
				GS15	Collaburragundry–Talbragar Alluvium
				GS16	Cudgegong Alluvium
				GS26	Lower Macquarie Alluvium
				GS45	Upper Macquarie Alluvium
NSW	GW13	New South Wales Great Artesian Basin Shallow	Northern Basin	GS34	NSW Great Artesian Basin Surat Shallow
				GS35	NSW Great Artesian Basin Warrego Shallow
				GS36	NSW Great Artesian Basin Central Shallow
NSW	GW14	Namoi Alluvium	Northern Basin	GS29	Lower Namoi Alluvium
				GS30	Manilla Alluvium
				GS40	Peel Valley Alluvium
				GS47	Upper Namoi Alluvium
				GS48	Upper Namoi Tributary Alluvium
NSW	GW15	Gwydir Alluvium	Northern Basin	GS24	Lower Gwydir Alluvium
				GS43	Upper Gwydir Alluvium
NSW	GW16	Eastern Porous Rock	Northern Basin	GS17	Gunnedah-Oxley Basin
				GS41	Sydney Basin
NSW	GW17	New England Fractured Rock and Northern Basalts	Northern Basin	GS18	Inverell Basalt
				GS22	Liverpool Ranges Basalt
				GS37	New England Fold Belt
				GS49	Warrumbungle Basalt
NSW	GW18	New South Wales Border Rivers Alluvium	Northern Basin	GS32	NSW Border Rivers Alluvium
				GS33	NSW Border Rivers Tributary Alluvium
Qld	GW19	Queensland Border Rivers3	Northern Basin	GS54	Queensland Border Rivers Alluvium
				GS55	Queensland Border Rivers Fractured Rock
				GS57	Sediments above the Great Artesian Basin: Border Rivers
Qld	GW20	Moonie4	Northern Basin	GS59	Sediments above the Great Artesian Basin: Moonie
				GS62	St George Alluvium: Moonie
Qld	GW21	Condamine–Balonne5	Northern Basin	GS53	Condamine Fractured Rock
				GS56	Queensland MDB: deep
				GS58	Sediments above the Great Artesian Basin: Condamine–Balonne
				GS61	St George Alluvium (deep and shallow)
				GS64	Upper Condamine Alluvium (Central Condamine Alluvium and  tributaries)
				GS65	Upper Condamine Basalts
Qld	GW22	Warrego–Paroo–Nebine6	Northern Basin	GS60	Sediments above the Great Artesian Basin: Warrego–Paroo–Nebine
				GS63	St George Alluvium: Warrego–Paroo–Nebine
				GS66	Warrego Alluvium

^1-2 Straddles in both the Northern Basin and the Southern Basin

^3-6 Combined surface water and groundwater resource plan areas

 

Table P4  Groundwater resource planning areas and sustainable diversion limit areas in the Southern Basin of the MDB region for the 2013 Account

Jurisdiction	Water resource plan area		Segment of the MDB region	Sustainable diversion limit area
	Code	Name		Code	Name
ACT	GW1	Australian Capital Territory (groundwater)	Southern Basin	GS52	Australian Capital Territory (groundwater)
Vic.	GW2	Goulburn–Murray	Southern Basin	GS8	Goulburn–Murray (Shepparton Irrigation Region, sedimentary plain, highlands and deep)
Vic.	GW3	Wimmera–Mallee (groundwater)	Southern Basin	GS9	Wimmera–Mallee (sedimentary plain, highlands and deep)
SA	GW4	South Australian Murray Region1	Southern Basin	GS3	Mallee (pliocene sands, Murray limestone group and Renmark group)
				GS5	Peake–Roby–Sherlock (confined and unconfined)
				GS6	SA Murray
				GS7	SA Murray salt interception schemes
SA	GW5	Eastern Mount Lofty Ranges2	Southern Basin	GS1	Angas Bremer (Quaternary sediments,and Murray limestone group)
				GS2	Eastern Mount Lofty Ranges
				GS4	Marne Saunders (Murray limestone group and Renmark group)
NSW	GW6	Western Porous Rock	Southern Basin	GS50	Western Porous Rock
NSW	GW7	Darling Alluvium3	Southern Basin	GS23	Lower Darling Alluvium
NSW	GW8	Murray Alluvium	Southern Basin	GS13	Billabong Creek Alluvium
				GS27	Lower Murray Alluvium (deep[Renmark Group and Calivil formation]; and shallow[Shepparton formation])
				GS46	Upper Murray Alluvium
				GS38	Oaklands Basin
NSW	GW9	Murrumbidgee Alluvium	Southern Basin	GS21	Lake George Alluvium
				GS28	Lower Murrumbidgee Alluvium (deep[Renmark Group and Calivil formation]; and shallow[Shepparton formation])
				GS31	Mid-Murrumbidgee Alluvium
NSW	GW10	Lachlan Alluvium	Southern Basin	GS12	Belubula Alluvium
				GS25	Lower Lachlan Alluvium
				GS44	Upper Lachlan Alluvium
NSW	GW11	Lachlan and South Western Fractured Rock4	Southern Basin	GS10	Adelaide Fold Belt
				GS20	Lachlan Fold Belt
				GS39	Orange Basalt
				GS51	Young Granite

^1-2 Combined surface water and groundwater resource plan areas

^3-4 Straddles in both the Northern Basin and the Southern Basin

 

The sedimentary aquifers within the region cover the main depositional areas of the Murray geological basin and the Darling River basin including upstream reaches of other major rivers such as the Murrumbidgee and Namoi rivers. The major aquifers within or at the margins of the Murray geological basin include the Shepparton, Calivil, Parilla-Loxton Sands, Murray limestone, and Renmark group aquifers, and the upland alluvium of the Cowra and Lachlan formations. The important alluvial sediments of the Darling River basin include the Gunnedah and Narrabri aquifers.

Sedimentary aquifers are the most important for groundwater extraction, with nearly all of the extracted groundwater (more than 95%) coming from these aquifers.  These groundwater aquifers have relatively high water holding capacity and hydraulic conductivity, and therefore are suited to extraction for use in irrigation and urban water supply. Most of the groundwater data used in the 2013 Account were for these aquifers. Groundwater quality is another significant consideration—water quality typically deteriorates down the groundwater flow path, constraining groundwater use to areas with acceptable water quality (primarily determined by low salinity).

The GAB is the largest groundwater basin in Australia. It underlies about one-third of the MDB (Figure P2) and extends beyond the MDB drainage basin boundary to the north and west. The GAB contains confined aquifers at depth, with confining layers that can be at or near the land surface.  The GAB provides vital water resources for domestic and town water supply, for stock use by the pastoral industry and water supplies for the operations of mining and petroleum industries and associated communities.

The GAB is recharged in small areas of the MDB (along the south-eastern edge of the GAB) where the aquifer layers are exposed at the land surface. The GAB is not considered to be part of the MDB water balance because the GAB aquifers are effectively hydraulically disconnected from near-surface systems in the MDB. Groundwater extraction and leakage from the GAB to the MDB are not represented in the 2013 Account, but may be represented in the future as an inter-basin transfer.

In the areas where the GAB confining layer is near or at the land surface, storage is limited to the near-surface soil zone and is lost from this zone almost entirely as evapotranspiration. Thus there is no need to evaluate change in groundwater storage in these areas to establish an annual regional water balance in the MDB. In the areas overlaying the GAB—where the confining layer does not outcrop—there are some shallow alluvial areas associated with drainage lines.

Fractured rock aquifers occur in the highland areas around the margins of the region (see Figure P8). In fractured rock aquifers, groundwater is stored in the fractures, joints, bedding planes and cavities of the rock mass and due to the difficulty of obtaining high yields from most fractured rocks, the volume of groundwater extracted from any one bore and in a given area is relatively low. In most fractured rock areas of the MDB, groundwater use is limited to stock and domestic supply. It is considered that the annual change in groundwater storage and annual groundwater flow out of fractured rock areas is much less significant to regional water balance. A groundwater balance in these areas is not explicitly estimated in the 2013 Account.

More details about the groundwater resources within the MDB region are available in the Groundwater note.

Across the MDB region, many towns and urban centres abstract water and then return treated urban wastewater to the river for subsequent use downstream. For example, on the Murrumbidgee, average returns to surface water from Canberra are more than half of the Australian Capital Territory diversions from the river. Returns may also flow from irrigation channels and infrastructure back to the rivers. Line item 9.9 Discharge from urban water system includes information on urban return flows.