Chapter 1Chapter 2Chapter 3Chapter 4Chapter 5
1 Introduction
2 Rivers of the
Murray-Darling Basin
3 Conclusion
4 References

1. Introduction

The Murray-Darling is Australia's largest river system. It is also one of the world's major river systems, ranking fifteenth in terms of length and twenty frst in terms of area (Kurian 1989). Table 1 provides some comparisons with other river systems of similar length and catchment area. What immediately stands out is the Murray-Darling's very low mean annual discharge in comparison with the other river systems. Whilst the Murray-Darling is a major river system in terms of its length and catchment area, it is a small one in terms of water discharge or runoff (Figure 1). In fact, of the world's major river systems, its surface runoff is among the smallest.


Figure 1. Major river basins and average yearly discharges of rivers in the MDB. This information is the average of 109 years (1981-2000) of modelled natural conditions discharge. The information comes from the MDBC River Murray Model and the State’s tributary models. (source: MDBC 2005).

River system Length, in km Catchment (km2)+ Mean discharge, in ML/sec
Murray-Darling, Australia 3,780 1,060,000 0.4
Nelson, North America 2,575 1,072,000 2.0
Indus, Asia 2,900 1,166,000 5.0
Danube, Europe 2,850 816,000 7.0
Ganges-Brahmaputra, Asia 2,897 1,621,000 38.0
Zambesi, Africa 3,500 1,330,000 7.0
Tocantins, South America 2,699 906,000 10.0
Tigris-Euphrates, Middle East 2,800 1,114,000 1.0


* These are river systems with similar features to the Murray-Darling in terms of length, catchment area, and discharge.
+ These figures are rounded. The area used elsewhere in this document for the MDB is 1,061,469km2

Table 1. Some of the world’s major river systems* (source: Encyclopaedia Britannica, Volume 26)

Some definitions on Surface Water Resources

Water measurement
Water is measured in megalitres (ML), equivalent to one million litres, and gigalitres (GL), equivalent to 1,000 megalitres. Runoff and outflow were generally estimated using information from stream gauging stations. Where there was a lack of such information, they were estimated using rainfall and other data. The estimates were made for the runoff under natural conditions. In regulated systems, such as those in the Murray- Darling Drainage Division, simulation models were used to estimate the natural runoffs.

Mean annual runoff
For river basins with a runoff regime in which the flow increases downstream, the flow is greatest at the mouth of the river basin. In such cases, the mean annual runoff was defined as the outflow from the basin. In many river basins, however, particularly in the Murray-Darling Drainage Division, the flow in the rivers decreases downstream, often with little or no outflow from the basin. In these situations, mean annual runoff was defined as the combined mean annual runoff of each of the major catchments in the river basin, calculated at the point where the flow is greatest and excluding runoff from upstream basins.

Mean annual outflow
Because of the tendency for the outflow from some river basins to be less than the total runoff generated within them due to large channel losses,
the mean annual outflows are also evaluated. These are defined as the estimated outflow from a river basin under natural conditions. The basin outflow can be either to the sea or an adjacent basin but not to a sink or closed lake within the basin. This concept is of particular relevance to the MDB and its northern rivers. For the Division as a whole, the mean annual outflow at the mouth of the Murray is 12,890 GL under natural conditions. However, the mean annual runoff within the Division is 23,850 GL. This indicates that nearly 50 per cent of the water that originates in the Division is lost through natural processes before reaching the sea.

Definitions of terms used
(source: AWRC 1987, Volume 1, 16-17 and MDBC 2005)

The Murray-Darling Basin is the catchment for the Murray and Darling Rivers and their many tributaries. The Basin extends from north of Roma in Queensland to Goolwa in South Australia and includes three quarters of New South Wales and half of Victoria. Table 2 lists the key indicators of surface water resources for each of the Drainage Divisions (MDBC 2005, NLWRA 2001). Comparing the Murray-Darling Basin with the other Divisions, it ranks third in area, first in terms of developed yield, diversions and water use but third for sustainable yield.


Figure 2. Australian Drainage Divisions (source: Australian Water Resources Council, 1987)

Drainage Division Area (km2) Developed Yield (ML) Diversion (ML) Water Use (ML) Water Allocation (ML) Sustainable Yield (ML)
North-East Coast 451,000 2,981,738 2,190,627 2,144,427 2,595,656 n/a
South-East Coast 274,000 1,511,660 1,283,468 1,241,778 1,695,246 2,263,936
Tasmania 68,200 3,542,690 3,410,713 450,733 3,367,757 17,552,184
Murray-Darling 1,060,000 11,719,128 12,050,677 11,148,531 11,579,006 9,126,997
South
Australian Gulf		 82,300 149,108 143,681 272,384 4,550 164,000
South-West Coast 315,000 506,811 373,212 361,844 0 1,607,948
Indian Ocean 519,000 26,623 12,409 12,409 0 440,106
Timor Sea 547,000 370,568 316,754 316,754 46,888 9,983,401
Gulf of Carpentaria 641,000 58,664 51,559 97,759 69,508 3,824,000
Lake Eyre 1,170,000 375 7,332 7,332 6,957 153,000
Bulloo-Bancannia 101,000 0 135 135 135 0
Western Plateau 2,450,000 1,165 1,165 14,036 0 75,931
Total 7,680,000 20,868,530 19,864,602 16,068,122 19,365,703 45,191,203

Table 2. Australia's surface water resources, by Drainage Divisions (source: NLWRA 2001) (To be revised)