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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.

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 387,000 gigalitres, of which 100,000 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 1.11 provides a broad comparison of rainfall and run-off by continent. Map 1.13 shows the location of Australia's Drainage Divisions, and table 1.14 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.

1.11 RAINFALL AND RUN-OFF OF THE CONTINENTS
			Run-off
	Average yearly rainfall
Continent	mm		mm	%	cubic km
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
Source: O'Brien WT, McGregor A & Crawshaw B 1983, 'In-stream and Environment Issues', "Water 2000: Consultant's Report No. 9", AGPS, Canberra.

1.12 MAJOR GROUNDWATER RESOURCES
			Major divertible resource
	Area of aquifers		Fresh	Marginal	Brackish	Saline	Total	Abstraction during 1983-84
State/territory	sq km		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
Source: Australian Water Resources Council, 1987.

1.13 DRAINAGE DIVISIONS AND RUN-OFF

Source: National Land and Water Resources Audit, 2000.

1.14 SURFACE WATER RESOURCES
	Area	Mean annual run-off	Mean annual run-off	Mean annual outflow	Volume diverted
Drainage division	sq km	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	-	(d)
Western Plateau	2,450,000	1,486	0.4	n.a.	1
Total	(e)7,680,000	387,184	100.0	. .	18,147
(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) Less than one gigalitre. (e) 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.

• 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.