Surface Water Resources

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 first 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 discharge or runoff (Figure 1). In fact, of the world's major river systems, its surface runoff is among the smallest. (See 'Definitions of terms used' at the end of this page.)

Figure 1 Major river basins and average yearly discharges of rivers in the MDB (source: MDBMC 1987, 76-77)

For the purposes of compiling water resources data, Australia is divided into twelve Drainage Divisions, one of which is the Murray-Darling Basin (Figure 2). It is the only one that consists of a single river system. Table 2 lists the key indicators of surface water resources for each of the Drainage Divisions. Comparing the Murray-Darling Basin with the other Divisions, it ranks third in area, sixth in mean annual runoff and mean annual outflow, fifth in divertible resources, but first in terms of developed resources. The significance of this last point is discussed further in the pages on Water Use and The Impacts of Water Regulation and Storage on the Basin's Rivers.

Figure 2 Australian Drainage Divisions (source: AWRC 1987)

Rivers of the Murray-Darling Basin

When all the rivers, creeks and water courses are plotted on a map, the Basin appears to have a mass of waterways. However, many of these only carry water at times of flood; for the rest of the time, they are dry. The nature of the Murray-Darling Basin means that, for most of their lengths, most of the rivers flow over plains. One consequence of this is that their individual courses are far from simple, as they meander across their floodplains. The actual course of the Darling is about three times as long as the direct distance that is involved.

Another consequence of the nature of the Murray-Darling Basin is that the rivers generally have extremely low gradients. As a result, under normal conditions, changes in flow propagate down them relatively slowly. Table 3 provides approximate travel times for changes in flow in sections of the River Murray and the Darling River, when flows are contained within the river banks. During floods, the travel times of the flood peaks are slower due to the effects of the floodwaters spreading out over the floodplains.

The Darling is the longest river in Australia, measuring 2,740 km from its source to its confluence with the Murray at Wentworth. The Murray is 2,530 km long from its source in the Australian Alps to its mouth on Encounter Bay in South Australia. For some 1,880 km of its length, the river marks the border between New South Wales and Victoria (it is actually on the top of the bank on the Victorian side of the river). The Murrumbidgee is 1,690 km long. The Darling, Murray and Murrumbidgee are among the world's longest rivers. The longest continuous stretch of river is from the source of the Condamine, about 100 km south-west of Brisbane, to the mouth of the Murray, less than 100 kilometres south-east of Adelaide, a distance of approximately 3,750 km.

While the Darling, Murray and Murrumbidgee are the longest and most important rivers, they are but three of the twenty major rivers in the Basin (Table 4). Between them, these major rivers have hundreds of tributaries.

Surface water resources in detail

Detailed information on the water resources of the Basin's 26 major catchments is shown in Table 5, with the catchments shown in Figure 1.

There is considerable variation in runoff from one part of the Basin to another. Further, runoff bears little relationship to catchment size (Figure 1). The catchments draining the Great Dividing Range on the south-east and southern margins of the Basin make the largest contributions to total runoff. For example, the Upper Murray, Murrumbidgee and Goulburn river catchments account for 45.4 per cent of the Basin's total runoff from 11 per cent of its area. The Upper Murray catchment alone accounts for 17.3 per cent of runoff from 1.4 per cent of the Basin. By contrast, the Darling group of rivers contribute 31.7 per cent of the Basin's mean annual runoff from 60.4 per cent of its area. The Darling catchment itself accounts for 10.9 per cent of the Basin's area but only 0.4 per cent of mean annual runoff.

Overall, some 86 per cent of the Basin contributes virtually no runoff to the river systems, except during floods (Figure 3). It is perhaps not surprising that Warrego means "river of sand".

Figure 3 Median annual runoff in the MDB

Variability of river flow

Australia's climate, compounded by the variability of its rainfall, means that virtually all of Australia's river systems are subject to considerable variability of flows from one year to another. In fact, on a global scale, Australia (together with Southern Africa) experiences higher runoff variability than any other continental area (McMahon et al. 1992) The Murray-Darling Basin is no exception to this, in spite of the fact that much of the river system is now highly regulated. By way of illustration, Figure 4 shows natural inflows into the Hume Reservoir over a period of one hundred years, showing not only the normal seasonal variations in any one year, but also the very great variations from one year to another. Over the period 1894-1993, the annual discharge at the mouth of the Murray-Darling system has ranged from 1,626 GL to 54,168 GL, with a mean of 10,090 GL and a median of 8,489 GL (Maheshwari et al. 1995).

Figure 4 Natural inflows to the Hume Reservoir, 1891-1991, in GL per month

The storages and other regulatory structures smooth out many of the smaller flow variations, but they have only limited impact on the major floods (see The Impacts of Water Regulation and Storage on the Basin's Rivers). In times of drought, the storages, provided they contain water, add to river flows, as illustrated by the contributions of the Snowy Mountains reservoirs to flows in the Murray during periods of drought*.

For the Murray and Murrumbidgee, the high and relatively reliable precipitation in their source areas means that stream flow is much more reliable than in other parts of the Basin. But even for some of their tributaries, there are significant exceptions, notably the Broken and Avon Rivers. However, these variations are small in comparison with those of the Darling River and its tributaries (Figure 5). These rivers not only experience massive floods, as indicated above; they can also cease flowing for extended periods. The Darling provides some of the most extreme examples. At Menindee, between 1885 and 1960, it ceased to flow on 48 occasions. The longest no-flow period was 364 days in 1902-03.

Figure 5 Variations in average annual streamflow for selected rivers in the MDB (source: MDBMC 1987, 79)

Major flooding can affect particular catchments or groups of catchments, such as on the King, Ovens and Broken Rivers in early October, 1993, the latter resulting in major flooding at Benalla, and the Namoi in July, 1998. Because much of the Basin is in effect a vast floodplain, flooding can be very extensive. In April 1990, large areas along the Darling and a number of its tributaries, as well as other areas to the north in Queensland, were devastated. The flooding was among the most extensive and dramatic recorded, covering in total (within and beyond the Basin) more than one million km², with a total damages bill well in excess of $250 million. Record floods on the Warrego and Bogan Rivers resulted in the total inundation of Charleville and Nyngan (respectively) (NSWDWR 1990) (Figure 6). Floods, like droughts, are part of the Basin's environment, the extreme manifestations of Australia's climatic variability. In order to better live with them, strategies are being developed to manage floods and limit the damage they inflict, such as the Swan Hill Regional Flood Strategy (DCNR 1994).

Figure 6 The April 1990 flood (source: based on inkjet image provided by Mapping & Monitoring Technology Pty Ltd., Townsville)

This figure is based on satellite data collected during the afternoon of April 24, 1990, by the Northeast Australian Satellite Imagery System in Townsville. It covers an area from Cape York to south of Nyngan, from 10°S to 34°S. It is important to note that the figure is based on work of an experimental nature, and is neither a map or a photograph. However, it provides a graphic indication of the areal extent of the April 1990 floods in south-west Queensland and northern New South Wales.

* : Even with the many storage and regulatory structures, there are particular features of flooding in at least some parts of the Basin. For example, "River Murray flood flows and flow patterns change dramatically along the length of the river. Large volumes which escape from the river are either temporarily ponded in former lakes or swamps or return hundreds of kilometres downstream along flood plains formerly occupied by ancestors of the present river system. The retention, diversion or reversal of flood flow can be attributed to the coincidence between volumes of discharge from the various river systems with the dimensions of the flood plains. These flow patterns can be related to five distinct geomorphic tracts occupied by the modern river from its headwaters to the South Australian border" (Currey & Dole 1978).

AWRC (1987): 1985 Review of Australia's Water Resources and Water Use. Department of Primary Industries and Energy / Australian Government Publishing Service, Canberra.

Currey, D.T. & Dole, D.J. (1978): "River Murray flood flow patterns and geomorphic tracts". Proceedings of the Royal Society of Victoria, 90(1), 67-77.

CNR (1994): Swan Hill Regional Flood Strategy. Department of Conservation and Natural Resources, Melbourne.

Kurian, G.T. (Editor)(1989): Geo-Data: the world geographical encyclopaedia. Gale Research Co., Detroit.

Maheshwari, B.L. et al. (1995): "Effects of regulation on the flow regime of the River Murray, Australia". Regulated Rivers: research and management, 10, 15-38.

McMahon, T.A. et al. (1992): Global Runoff: continental comparisons of annual flows and peak discharges. Catena Verlag, Cremlingen-Destedt.

MDBMC (1987): Murray-Darling Basin Environmental Resources Study. Murray-Darling Basin Ministerial Council, Canberra.

MDBMC (1995): An Audit of Water Use in the Murray-Darling Basin. Murray-Darling Basin Ministerial Council, Canberra.

NSWDWR (1990): Nyngan April 1990 Flood Investigation: summary report. New South Wales Department of Water Resources, Sydney.

NSWWRC (1991): Water Facts. New South Wales Water Resources Council, Sydney.

Table 1 Some of the world's major river systems* (source: Encyclopaedia Britannica, Volume 26, 846)

* These are river systems with similar features to the Murray-Darling in terms of length, catchment area, and discharge.

Table 2 Australia's surface water resources, by Drainage Divisions (source: AWRC 1987, Volume 1, 40)

* rounded totals

Table 3 Illustrations of travel time of flow wave in the Murray and Darling Rivers (source: MDBC)

Table 4 Major rivers of the Murray-Darling Basin (source: MDBC)

 Table 5 Surface water resources data for the Basin's 26 major catchments (source: AWRC 1987, Volume 1, 50-51)

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

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 12,200 GL under natural conditions. However, the mean annual runoff within the Division is 24,300 GL. This indicates that 50 per cent of the water that originates in the Division is lost through natural process before reaching the sea.

Divertible surface water resources

A divertible resource is defined as the volume of water that can be diverted on a sustained basis into conventional water supply systems or to substantial private users, utilising existing storages and potential dam sites identified by investigation or indicated by preliminary reconnaissance. In general, it is any source capable of yielding more than 500 ML per annum.

Developed resource

This is defined as the portion of the divertible resource currently available for use, estimated for storages existing or under construction, and including licensed withdrawals from streams.