Only one-third of Australia, or 1,000,000 square miles of area, receives more than 20 inches of rainfall annually. Another third receives between 10 and 20 inches each year, and the final third receives less than 10 inches. The average annual rainfall for the continent is estimated to be only about 17 inches.
Severe droughts were experienced in Australia in 1866, 1876, 1888, 1902, 1914, 1929, and 1944, or at 10 to 15-year intervals. Some of these droughts lasted for two or three successive years. The 19441945 drought was perhaps the most disastrous because of the unprecedented expansion of agriculture into the drier districts, many of which are characterized by light soils.
Although rainfall is fairly well distributed throughout the year in the eastern portion of the continent, precipitation in a wide belt along the north coast is limited to the summer season, and that along the south coast comes in winter. To make matters worse, there is a wide variation in the amount of rainfall from year to year. In years of severe drought two-thirds of the continent receives less than 10 inches of rainfall. In years of moderate drought one-half of the continent receives less than 10 inches. In many years of drought even the better-watered portions of the continent receive only half or twothirds their normal rainfall, and hence these regions also suffer important losses. As a result of all these conditions water supply is relatively scarce in most parts of the continent, and it is the major limiting factor in the general development of most regions.
The scarcity of water is reflected in the scant number and ephemeral nature of streams and other surface-water bodies. Perennial streams are found only along the north, east, southeast, and southwest coasts of Australia and in Tasmania. Many of them are subject to large flows during the summer rainy season and to severe diminution during the long season of drought. No rivers occur in a vast area in the interior or in the Nullarbor Plain. Rivers along most of the west coast and those in the Lake Eyre and Darling River basin flow only occasionally. Even the Murray has on rare occasions practically ceased to flow. On these bases the continent may be divided into large regions with (1) no runoff, (2) internal drainage, and (3) external drainage. Numerous significant facts about water supply, climatic conditions, and soil and vegetational responses are intimately related.
Numerous lakes appear on the physical maps of Australia although they scarcely exist in reality. The large lakes appearing in South Australia, such as Eyre, Torrens, and Gardiner, are really only lake beds ("playas," "salt pans," or "clay pans") covered by a shallow layer of water after occasional wet periods and by glistening salt deposits during most of the year. The salt-lake region of Western Australia and its hundreds of "lakes" present the same picture. In all these instances not only is surface water ephemeral but also its saline character makes it of little use for plant, man, and beast. Small accumulations of water are available on the Western Plateau in rock basins for brief periods after rains.