Drought in general terms refers to an acute deficiency of water supply to meet a specified demand. The best single measure of water availability is rainfall but this measure should be tempered with the parameters of evaporation and soil moisture which can be dominant in some situations. The declaration of drought conditions generally, depends on the effect that the lack of water will have on the principal local primary industry.
Australia is well known for periods of low rainfall and drought which severely reduces production of crops and animal forage and exposes soils to erosion. For example, between 1965 and 1980 almost all of southern Australia experienced drought conditions that is, received less than 10 percentile long term rainfall for between 30% and 40% of all months. Because the extent of drought was not known during the initial stages of settlement and expansion over the past 154 years, cropping activities were often extended well beyond sensitive climatic limits.
This caused such hardship and environmental damage that government decrees were enacted to limit the extent of cultivation in southern Australia. For example, the Western Division of NSW was designated “non agriculture” in 1901. Even steep sloping lands were often overcleared encouraged by taxation advantages and then used for cultivation resulting in severe erosion. For example, on the southern and northern tablelands of NSW.
In southern pastoral lands were saltbush and bluebush provided forage for sheep and cattle, a lack of understanding of annual rates of shrub growth led to gross overstocking which, together with a huge rabbit population led to large losses of vegetation and consequently very serious erosion. Such lands never fully recovered and even today sheep numbers are still below those at the turn of the century.
Since the 1860s there have been 10 major droughts in Australia. Some of these major droughts could be described as periods consisting of a series of dry spells of varying length, but overlapping in time and space and totalling up to about a decade.
The drought periods of 1895-1903,1958-68 and 1982-3 were the most devastating in terms of their extent and effects on primary production.
The remaining 7 major droughts occurred in 1864-66 (and 1868), 1880-86, 1888, 1911-16, 1918-20, 1939-45 and 1994-95. In this same period 6 droughts of lesser severity caused significant losses over a large area of several states. They occurred in 1922-2 and 1926-9, 1933-38, 1946-49, 1951-52, 1970-73 and 1976.
Southeastern Australia (NSW, southern Queensland, Victoria Tasmania and the settled parts of SA) contains about 75% of the nation's population and droughts affecting this region have a markedly adverse impact on the economy.
There have been 8 severe droughts in southeastern Australia since 1888 and these were encompassed within the major Australian droughts specified above except for the severe drought in 1972-73.
The diagram below shows the severity and extent of the 1982-83 drought in terms of rainfall deficiency:
SEVERITY AND EXTENT OF THE 1982-83 DROUGHT