Murray-Darling Basin

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Semi-arid grazing country near Burra Creek, South Australia

The Murray-Darling Basin drains one-seventh of Australia and is by far the most significant agricultural area on that continent. 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. Although the Murray-Darling Basin receives only 6% of Australia's annual rainfall, over 70% of Australia's irrigation resources are concentrated there. It contains 42% of the nation's farmland and produces 40% 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 general, the climate is hot and dry in summer, 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 around 250 or 300 mm a year (10 to 12 inches) on average.

Image:Saltbush country.jpg

Chenopod shrubland in Northern Victoria. On the horizon are the River Red Gums on the banks of the Murray River

In the south-east, average temperatures are lower, elevations a little higher, and rainfall more frequent: 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.

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 types:

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 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 years. 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 southwestern 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 runoff ratios are exceedingly low (for comparison, around a tenth that of a European or North American catchment with a similar climate). 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.

The Murrumbidgee, Murray and Goulburn basins (except the Broken River which resembles the southwestern 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 snowmelt 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 tobacco, 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 runoff coefficients as to low rainfall).

[edit] Major streams

The Murray is the largest of the basin's many rivers, the Darling the longest. It is 3370 kilometres from the Queensland headwaters of the Darling to the Coorong where the Murray flows into the sea.