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Rainfall, or the lack of it, is the most important single factor determining land use and rural production in Australia. The scarcity of both surface water and groundwater resources, together with the low rates of precipitation which restrict agriculture (quite apart from economic factors), has led to extensive programs to regulate supplies by construction of dams, reservoirs, large tanks and other storages.
The major topographical feature affecting the rainfall and drainage patterns in Australia is the absence of high mountain barriers. Australia's topographical features encompass sloping tablelands and uplands along the east coast Main Divide, the low plain and marked depression in the interior, and the Great Western Plateau.
Only one-third of the Australian land area drains directly to the ocean, mainly on the coastal side of the Main Divide and inland with the Murray-Darling system. With the exception of the latter, most rivers draining to the ocean are comparatively short, but account for the majority of the country's average annual discharge. Surface drainage is totally absent from some arid areas of low relief.
Australia's large area (just under 7.7 million square kilometres) and latitudinal range (3,680 km) have resulted in climatic conditions ranging from alpine to tropical. Two-thirds of the continent are arid or semi-arid, although good rainfalls (over 800 mm annually) occur in the northern monsoonal belt under the influence of the Australian-Asian monsoon, and along the eastern and southern highland regions under the influence of the great atmospheric depressions of the Southern Ocean. The effectiveness of the rainfall is greatly reduced by marked alternation of wet and dry seasons, unreliability from year to year, high temperatures and high potential evaporation.
The availability of water resources controls, to a large degree, the possibility and density of settlement; this in turn influences the quality of the water through production and disposal of waste. Most early settlements were established on the basis of reliable surface water supplies and, as a result, Australia's population is concentrated along the coast, mainly in the comparatively fertile, well-watered east, south-east and far south-west.
As settlement spread into the dry inland grazing country, the value of reliable supplies of underground water was realised. Observations of the disappearance of large quantities of the rainfall precipitated on the coastal ranges of eastern Australia eventually led to the discovery of the Great Artesian Basin which has become a major asset to the pastoral industry. Development, however, has not been without costs. Significant environmental degradation and deterioration in water quality are becoming evident. Table 1.12 summarises Australia's major groundwater resources.
1.12 AUSTRALIA'S MAJOR GROUNDWATER RESOURCES
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 | | Major divertible resource | |
| |
| |
| Area of aquifers | Fresh | Marginal | Brackish | Saline | Total | Abstraction
during 1983-84 |
| State/territory | km2 | GL | GL | GL | GL | GL | GL |
|
| New South Wales | 595,900 | 881 | 564 | 431 | 304 | 2,180 | 242 |
| Victoria | 103,700 | 469 | 294 | 69 | 30 | 862 | 146 |
| Queensland | 1,174,800 | 1,760 | 683 | 255 | 144 | 2,840 | 962 |
| South Australia | 486,100 | 102 | 647 | 375 | 86 | 1,210 | 504 |
| Western Australia | 2,622,000 | 578 | 1,240 | 652 | 261 | 2,740 | 355 |
| Tasmania | 7,240 | 47 | 69 | 8 | – | 124 | 5 |
| Northern Territory | 236,700 | 994 | 3,380 | 43 | 10 | 4,420 | 24 |
| Australia | 5,226,440 | 4,831 | 6,877 | 1,833 | 835 | 14,376 | 2,238 |
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| Source: Australian Water Resources Council, 1987. |
Permanent rivers and streams flow in only a small part of the continent. The average annual discharge of Australian rivers has been recently assessed at 387 thousand gigalitres, of which 100 thousand gigalitres are now estimated to be exploitable on a sustained yield basis. This is small in comparison with river flows on other continents.
In addition, there is a pronounced concentration of run-off in the summer months in northern Australia, while the southern part of the continent has a distinct, if somewhat less marked, winter maximum.
Even in areas of high rainfall, large variability in flow means that, for local regional development, most streams must be regulated by surface storage. However, in many areas evaporation is so great that storage costs are high in terms of yield. Extreme floods also add greatly to the cost of water storage, because of the need for adequate spillway capacity.
Table 1.13 provides a broad comparison of rainfall and run-off by continent. Map 1.14 shows the location of Australia's Drainage Divisions, and table 1.15 summarises Australia's surface water resources by Drainage Division. The Drainage Division with the highest intensity of run-off is Tasmania with 11.8% of the total from only 0.9% of the area. Conversely, the vast area of the Western Plateau (2,450,000 square kilometres, approximately 32% of Australia) has no significant run-off at all.
1.13 RAINFALL AND RUN-OFF OF THE CONTINENTS
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| | Run-off |
| Average yearly rainfall |
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| Continent | mm | mm | % | km3 |
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| Africa | 690 | 260 | 38 | 7,900 |
| Asia | 600 | 290 | 48 | 13,000 |
| Australia | 465 | 57 | 12 | 440 |
| Europe | 640 | 250 | 39 | 2,500 |
| North America | 660 | 340 | 52 | 6,900 |
| South America | 1,630 | 930 | 57 | 16,700 |
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| Source: O'Brien WT, McGregor A & Crawshaw B 1983, 'In-stream and Environment Issues', "Water 2000: Consultant's Report No. 9", AGPS, Canberra. |
1.14 DRAINAGE DIVISIONS AND RUN-OFF
Source: National Land and Water Resources Audit, 2000.
1.15 SURFACE WATER RESOURCES
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| Area | Mean annual run-off | Mean annual run-off | Mean annual outflow | Volume diverted |
| Drainage division | km2 | GL | % | GL | GL |
|
| North-East Coast | 451,000 | 73,411 | 19.0 | n.a. | 3,182 |
| South-East Coast(a) | 274,000 | 42,390 | 10.9 | 40,591 | 1,825 |
| Tasmania(b) | 68,200 | 45,582 | 11.8 | 45,336 | 451 |
| Murray-Darling(a) | 1,060,000 | 23,850 | 6.2 | 5,750 | 12,051 |
| South Australian Gulf(c) | 82,300 | 952 | 0.2 | 797 | 144 |
| South-West Coast | 315,000 | 6,785 | 1.8 | 5,925 | 373 |
| Indian Ocean | 519,000 | 4,609 | 1.2 | 3,481 | 12 |
| Timor Sea | 547,000 | 83,320 | 21.5 | 81,461 | 48 |
| Gulf of Carpentaria | 641,000 | 95,615 | 24.7 | 24,748 | 52 |
| Lake Eyre | 1,170,000 | 8,638 | 2.2 | n.a. | 7 |
| Bulloo-Bancannia | 101,000 | 546 | 0.1 | - | <1 |
| Western Plateau | 2,450,000 | 1,486 | 0.4 | n.a. | 1 |
| Total | (d)7,680,000 | 387,184 | 100.0 | . . | 18,147 |
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(a) South-East Coast and Murray-Darling Division. The volume diverted represents the sum of available data (NSW has not reported water use for unregulated surface water management areas).
(b) Tasmanian Division. Volume diverted does not include the Hydro scheme diversions.
(c) South Australian Gulf Division. Mean annual outflow includes the flow from surface water management areas Willochra Creek and Lake Torrens, which do not flow to the sea, but flow into the terminal lake, Lake Torrens.
(d) Total area differs slightly from that in table 1.1, due to improvements in mapping reflected in that table, but not in this table. |
Source: National Land and Water Resources Audit, 2000. |
To summarise, the mean annual run-off across Australia is 387 thousand gigalitres. The portion of run-off able to be diverted for use is very low compared to that in other continents, and results from the high variability of stream flow, high rates of evaporation and the lack of storage sites on many catchments. On an Australia-wide basis, only about a quarter of the divertible resource has currently been developed for use; much of the remaining resource is available in remote regions where development is impractical and uneconomic. In areas such as the Murray-Darling Division, where water is scarce, there are few resources not yet developed, and management is focusing on greater efficiency in water use.
Water resources are assessed within a framework comprising four levels:
- The total water resource is the volume of water present in the environment, measured as mean annual run-off for surface water, and mean annual recharge for groundwater.
- The divertible resource is the portion of run-off and recharge which can be developed for use.
- The developed resource is the portion of the divertible resource which has been developed for use.
- Resource utilisation is a measure of the portion of the developed resource which is actually used.
Emphasis is given to the second level of assessment, the divertible resource, as the prime measure of the resource. The divertible resource is defined as the average annual volume of water which, using current technology, could be removed from developed or potential surface water or groundwater sources on a sustained basis, without causing adverse effects or long-term depletion of storages.
Australia's water resources are managed by a large number of resource management agencies, irrigation authorities, metropolitan water boards, local government councils and private individuals. State authorities dominate the assessment and control of water resources as, under the Commonwealth Constitution, primary responsibility for management of water rests with the individual state governments. The Commonwealth Government is responsible for matters relating to the territories, and participates indirectly through financial assistance or directly in the coordination or operation of interstate projects through bodies such as the Murray-Darling Basin Commission.
A description of the management, main storage and use of water resources across the states and territories is contained in the chapter Water resources in the 1994 and earlier editions of Year Book Australia.
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