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Map of the Murray-Darling Basin

The Murray-Darling Basin is 3,375 km long, drains one-seventh of the Australian land mass, and is currently by far the most significant agricultural area in Australia. The name of the basin is derived from its two major rivers, the Murray River and the Darling River.

Most of the 1,061,469 km² basin is flat, low-lying and far inland, and receives little rainfall. The many rivers it contains tend to be long and slow-flowing, and carry a volume of water that is large only by Australian standards.

Contents

Overview

Although the Murray-Darling Basin receives 6% of Australia's annual rainfall, a very large proportion of Australia's irrigation resources are concentrated there. It contains 42% of the nation's farmland and produces 25% of the nation's food.

The basin drains roughly three-quarters of New South Wales (including all of the A.C.T.), half of Victoria, a substantial portion of southern Queensland, and a small part of eastern South Australia. In the drought throughout much of Australia during the early years of the 2000s, many people complained that upriver dams were stopping too much of the water flow in the lower parts of the river system.

In general, the climate is hot and dry in summer and mild in winter. Much of the terrain is semi-arid and nearly all of it is only a few tens of metres above sea level. Typically, tree-lined watercourses meander slowly through mulga or mallee scrub, grasslands or chenopod shrublands. Rainfall is unpredictable and varies from place to place, as well as year to year, but is typically 250 to 300 mm a year (10 to 12 inches).

In the south-east, average temperatures are lower, it 500 mm (20 inches) a year is representative, most of it falling in winter and spring. Along the southern and eastern borders of the basin are the inland slopes of the Great Dividing Range. It is here that most of the water in the rivers of the basin originates either as rainfall or, in the case of the Australian Alps which straddle the New South Wales-Victoria border, as winter snowfall.

The confluence of the Darling and Murray Rivers at Wentworth, New South Wales

Native fauna

The Murray Darling Basin is home for many native animal species. The true numbers are not known, but a fairly accurate estimate has been made of these animals and the current status of their population. Among the indigenous fauna in the region, the study found that there were:

  • 367 species of birds, with 35 endangered.
  • 85 species of mammals, with 20 extinct and 16 endangered.
  • 53 species of frogs, with none endangered.
  • 46 species of snakes, with 5 endangered.
  • 5 tortoises, with none endangered
  • 34 species of fish, with none endangered.

Introduced species

The carp, introduced in 1850, is now a nuisance species in the Murray-Darling river system

The Basin has also played host to a variety of introduced species. One of the most well known is the Carp.

Introduced in around 1850, the four varieties of Carp were used to stock up fish dams. Since then they have made their way into the river systems, where they spread quite quickly. These fish are very mobile, as they can travel easily on flood waters and their eggs can be transported by birds.

These fish are a problem because they feed by sucking gravel from the river bed and taking all the edible material off it, before returning the rest to the water. This stirs up all the sediment, reducing the quality of the water.

Physiography

This area is one of the physiographic provinces of the larger East Australian Basins division, and encompasses the smaller Naracoorte Platform and Encounter Shelf physiographic sections.

Hydrology

Total water flow in the Murray-Darling basin in the period since 1885 has averaged around 24,000 gigalitres per year, although in most years only half of it reaches the sea and in dry years much less. Estimated total annual flows for the basin range from 5,000 gigalitres in 1902 to 57,000 gigalitres in 1956. The hydrology of the streams within the basin is quite varied even considering its size. There are three main types1:

  • The Darling and Lachlan basins. These have extremely variable flows from year to year, with the smallest annual flow being typically as little as 1 percent of the long-term mean and the largest often more than ten times the mean. Periods of zero flow in most rivers can extend to months and in the drier parts (Warrego, Paroo and Lower Darling basins) to years2. Flows in these rivers are not strongly seasonal. Though in the north most floods occur in the summer from monsoonal penetration, in most of the Darling and Lachlan catchments it is typical to see high or low flows begin in winter and extend to the following autumn (see El Niño).
  • The south-western basins (Campaspe, Loddon, Avoca, Wimmera). These have a marked winter rainfall maximum and relatively lower precipitation variability than the Lachlan or Darling. However, the age and infertility of the soils means that run-off ratios are exceedingly low (for comparison, around a tenth that of a European or North American catchment with a similar climate3). Thus, variability of runoff is very high and most of the terminal lakes found in these basins very frequently dry up. Almost all runoff occurs in the winter and spring and, in the absence of large dams for regulation, these rivers are often or usually seasonally dry during summer and autumn.
    • A number of small catchments in South Australia, of which the largest are the Angas River flowing through Strathalbyn and the Finniss River further west, are considered part of the Murray-Darling Basin because they flow into Lake Alexandrina, even though they have no actual connection with the Murray. These rivers are seasonal, being usually dry in the summer, but their winter rainfall and stream flow is more reliable than those of the south-western rivers of the "proper" Murray-Darling Basin.
  • The Murrumbidgee, Murray and Goulburn basins (except the Broken River which resembles the south-western basins). Because these catchments have headwaters in alpine country with relatively young peaty soils, the runoff ratios are much higher than in other parts of the basin. Consequently, although gross precipitation variability is no lower than in the Lachlan or Darling basins, runoff variability is markedly lower than in other parts of the basin. Typically these rivers never cease to flow and the smallest annual flow is typically around 30 percent of the long-term mean and the largest around three times the mean. In most cases the flow peaks very strongly with the spring snow melt and troughs in mid-autumn.

Of the approximately 13,000 gigalitres of flow in the basin which studies have shown to be divertible, 11,500 gigalitres is removed for irrigation, industrial use, and domestic supply. Agricultural irrigation accounts for about 95% of the water removed, with the growing of rice and cotton being highly controversial among scientists in Australia, owing to their high water use in a region extremely short of water (as much due to exceptionally low run-off coefficients as to low rainfall).

Major streams

Confluence of the Murray and Darling Rivers ar Wentworth, New South Wales.

The Murray River is the largest of the basin's many rivers, the Darling River the longest. It is 3370 kilometres from the Queensland headwaters of the Darling to the Coorong in South Australia, where the Murray flows into the sea. The major rivers of the basin are:

2007 drought and Commonwealth Government take-over

Main article: Drought in Australia

In April 2007 the former Australian prime minister John Howard announced that the region was facing an "unprecedentedly dangerous" water shortage and that water might have to be reserved for "critical urban" water supplies.4 He commented that "We should all pray for rain because the situation for the farmers of Australia in the irrigation area, the Murray-Darling Basin, is critical" amid concerns about the impact of the drought on Australian agriculture.5

The Federal Government proposed a $10 billion Commonwealth take-over of the Murray-Darling Basin, arguing that effective management could not be undertaken by competing state governments. While the states of New South Wales, Queensland and South Australia as well as the Australian Capital Territory accepted the proposal, the state of Victoria refused to co-operate, arguing that its irrigators would be disadvantaged. The Victorian Farmers Federation, however, has since backed the Federal Government's plan.6

In July the Federal Government said that in the absence of co-operation from all states it would legislate to achieve a federal take-over7, but the Victorian Government has declared that it would challenge this in the High Court.

Legislation to create the Murray-Darling Basin Commission was introduced into Federal Parliament and was passed in both the House of Representatives and the Senate in August 2007.8

On 26 March 2008, at the Council of Australian Governments meeting, Premier John Brumby indicated that the Victorian government would participate in the program, in return for $1 billion to upgrade irrigation and continue water security for farmers.9

See also

References

  1. ^ Brown, J. A. H. (John Alexander Henstridge); Australia's Surface Water Resources. ISBN 0644026170
  2. ^ Ibid
  3. ^ McMahon, T.A. and Finlayson, B.L.; Global Runoff: Continental Comparisons of Annual Flows and Peak Discharges; pp. 86-98 ISBN 3-923381-27-1.
  4. ^ Murray-Darling farmers face water shut-off
  5. ^ Murray-Darling situation likely to raise food prices
  6. ^ VFF decides to back national water plan
  7. ^ PM to take over Murray-Darling - Drought - Specials - smh.com.au
  8. ^ Turnbull takes aim at Vic over Murray-Darling - ABC News (Australian Broadcasting Corporation)
  9. ^ $1bn Murray breakthrough, The Age, 27 March 2008.

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