1301.0 - Year Book Australia, 1988
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DROUGHT IN AUSTRALIA
This article has been contributed by the National Climate Centre, Bureau of Meteorology.
INTRODUCTION
The incidence of drought in Australia to 1968 was
surveyed in the 1968 Year Book No. 54. The purpose of this article is to
bring that survey up to date with information to 1986 inclusive. While
broadly summarising the material from the earlier article, the most
recent widespread and severe drought in Australia, the drought of
1982-83, is given a special mention. Developments in the Australian
Drought Watch Service, operated by the Bureau of Meteorology, and in the
monitoring of variations in the climate that can lead to drought are
also briefly described.
DEFINITION OF DROUGHT
Drought in general refers to an acute water shortage.
However the term is relative because water availability, which depends
on supply and demand, is affected by regional differences in both the
climate and the activities of the water user. To a large extent, users
adapt to a perception of what is the normal supply for an area but there
are other differences. A farmer, for example, is concerned with
insufficient water during a season for crops, pastures and stock. A
civil engineer in the same area may be more concerned with longer term
aspects associated with the storage and managing of water in a
reservoir.
On the supply side of the drought equation the main
determinants are meteorological and hydrological. It is the former that
is given emphasis in this article. A comprehensive coverage of
Australia's water resources, including the impact of drought, is given
in the series of publications, Water 2000 prepared for the Australian
Water Resources Council in 1983. The broader subject of drought in
Australia and the mitigation of its adverse effects has been the topic
of many papers and symposia, for example, see the report and
recommendations of a drought workshop held in Melbourne by the Royal
Meteorological Society in 1986.
The amount of water available for the great majority
of users depends on the storage, whether it be in the soil, farm dams,
artesian basins, reservoirs and so on. In addition. water availability
is affected by losses due to run-off, evaporation and wasteful usage.
However the primary indicator of water availability in Australia is
rainfall and, given its extensive measurement across the country,
rainfall is the most suitable starting point to assess the incidence of
drought.
One important aspect of rainfall or more specifically
the lack of it, is the difference between aridity and drought,
distinguished by Coughlan and Lee (1978) thus: Aridity
implies a high probability of rainfall for a given period below a low
threshold. Drought implies a low probability of rainfall for a given
period below a relatively low threshold.
Thus establishing drought criteria is less meaningful
for arid zones since the prospects of receiving useful rainfall are
significantly lower there than in more abundant rainfall zones. During
the dry seasons of the seasonal rainfall zones, e.g. northern Australia,
the expectation of useful rainfall can also be quite low and one may
think in terms of seasonal aridity. Defining drought criteria for areas
with highly seasonal rainfall requires separate consideration and the
problem of delineating the onset and retreat of drought in such areas
can be quite complex.
PASTORAL DROUGHT AND CLIMATIC ZONES
From a practical viewpoint then, drought is
intrinsically related to climatic zones and to the resistance of plants
to water shortages. Generally, natural pastures and herbage have evolved
to become highly resistant to extended periods of low rainfall
particularly in the arid zone. On the other hand, cereal crops such as
wheat, being more sensitive to water limitations, require specific
treatment in the establishment of criteria for drought.
There are many ways of delineating climatic zones.
Those shown for Australia in the diagram on page 211 are based
principally on seasonal rainfall characteristics, although evaporation
has been taken into account to some extent in the derivation of the
zonal boundaries (Bureau of Meteorology, 1975).
In the winter and uniform zones of Australia where
agricultural and pastoral activities are more intensive, three
consecutive months, although an arbitrary figure, has been found to be
an appropriate minimum period for a significant deficiency in the
rainfall to develop. Rainfall in the summer rainfall zone is generally
more variable when compared with the winter and uniform zones. Coughlan
and Lee (1978) have used the summer rainfall zone in northern Australia
to illustrate how probabilities of water stress in sown crops may be
affected by the expected variability within any one season. Native
pastures, in contrast, have evidently evolved to respond more
effectively to seasonal rainfall as a whole and are less likely to be
affected by the distribution of variable quantities throughout the
season. Soil type and the degree to which it has been worked are also
significant factors in this regard.
Rainfall in arid zones, as well as being low, is
usually highly variable in space and time, and natural pastures and
herbage are strongly resistant to such stresses. Drought in an arid zone
is generally more appropriate to longer periods, e.g. a year or more,
rather than to periods as short as three consecutive months.
RAINFALL DEFICIENCY AND THE AUSTRALIAN DROUGHT WATCH SERVICE
There have been many attempts to arrive at a
satisfactory method of objectively defining drought, establishing
criteria for its onset, monitoring its course and declaring a drought
ended. Perhaps the most successful approach, and one of the simplest in
concept, uses the first decile of accumulated rainfall for a given
period as an indicator of drought (Gibbs and Maher, 1967; Lee and
Gaffney, 1986). The first decile is simply that amount of rainfall which
is exceeded on ninety per cent of occasions for the period of the year
specified, e.g. winter, spring or indeed any period of consecutive
months. The concept of rainfall deficiency employed by the Bureau of
Meteorology is based on a comparison of the rainfall total for at least
three months in a specific area with the historical long period record
for those three or more months. Thus an area is categorised as having a
rainfall deficiency when the rainfall for a period of at least three
months falls within the lowest ten per cent (below the first decile) of
the historically recorded rainfalls for the same period of the year.
The terms serious and severe rainfall deficiency are defined as follows:
- a serious rainfall deficiency
exists for a specific period of three (or more) months when the
rainfall is above the lowest five per cent of recorded rainfalls, but is
less than the ten per cent value;
- a severe rainfall deficiency exists for a
specific period of three (or more) months when the rainfall is among the
lowest five per cent of recorded rainfalls.
When serious or severe deficiencies exist in an area they continue as such until:
(a) rainfall for the past month is already
sufficient to rank in the 30th percentile or greater of the recorded
rainfalls for the three month period starting with that month (a break
due to relatively heavy rainfall), or
(b) rainfall for the past three months ranks in the
70th percentile or greater of the recorded rainfalls for the
corresponding three month period (a break due to a series of lesser but
overall significant falls).
Rainfall deficiency criteria based on decile values
provide the basis for alerting to incipient drought and monitoring the
course of extant drought. The procedures, which have been in use in
Australia since 1965, have also been adopted by the World Meteorological
Organization to monitor drought on a worldwide scale (World
Meteorological Organization, 1985). The Drought Watch Service, operated
by the National Climate Centre in the Bureau of Meteorology, uses
rainfall data from around 800 individual stations throughout the country
to provide a monthly statement supported by maps and figures on the
distribution of existing rainfall deficiencies (Coughlan, 1986).
MAJOR DROUGHTS IN AUSTRALIA
Foley (1957), on the basis of rainfall analyses,
classified major droughts in Australia from the early period of European
settlement to 1955. He referred to these droughts, summarised in Table
1, as major, severe and widespread and his broad descriptive material
indicates that each affected several States covering about one quarter
of Australia or more, for varying periods of one or more years. Some of
these droughts could be described as drought periods consisting of a
series of dry spells of various lengths, overlapping in time and space,
and totalling up to about a decade, as in the case of the 1895-1903
drought.
Subsequent to Foley's work, major droughts in
Australia have been assessed from time to time using rainfall decile
analyses. Typically they have been described as areas of at least
serious rainfall deficiency (below the first decile), collectively
encompassing at least one quarter of Australia for periods in excess of
10 months. The drought period of 1958-68 and the drought of 1982-83 met
these criteria.
MAJOR DROUGHTS IN AUSTRALIA |
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| Description |
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The little data available indicate that this drought period was rather
severe in Victoria, South Australia, New South Wales, Queensland and
Western Australia.
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Victoria (northern areas and Gippsland);
New South Wales (mainly northern wheat belt, northern tablelands and
south coast); Queensland (1881-86, in south-east with breaks - otherwise
mainly in coastal areas, the central highlands and central interior in
1883-86); and South Australia (1884-86, mainly in agricultural areas).
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Victoria (northern areas and Gippsland);
Tasmania (1887-89 in the south); New South Wales; Queensland (1888-89);
South Australia and Western Australia (central agricultural areas).
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Practically the whole of Australia was
affected but most persistently the coast of Queensland, inland areas of
New South Wales, South Australia, and central Australia. This was
probably Australia's worst drought to date in terms of severity and
area. Sheep numbers, which had reached more than 100 million, were
reduced by approximately half and cattle numbers by more than 40 per
cent. Average wheat yields exceeded 8 bushels per acre in only one year
of the nine, and dropped to 2.4 bushels per acre in 1902.
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Victoria (1913-15 in north and west);
Tasmania (1913-15); New South Wales, particularly inland areas;
Queensland; Northern Territory (mainly in the Tennant Creek-Alexandria
Downs area); South Australia (some breaks in agricultural areas); and
Western Australia (1910-14).
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Queensland, New South Wales, South
Australia, Northern Territory (Darwin-Daly Waters area and central
Australia), Western Australia (Fortescue area), Victoria, and Tasmania.
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New South Wales (severe on the coast),
South Australia (persistent in pastoral areas), Queensland and Tasmania;
also (more particularly in 1940 and 1944-45) in Western Australia,
Victoria, and central Australia; Tennant Creek-Alexandria Downs area in
1943-45.
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This drought was most widespread and
probably second to the 1895-1903 drought in severity. For more than a
decade from 1957, drought was consistently prominent and frequently made
news head-lines from 1964 onwards. This was treated as one major
drought period, but could be subdivided into two which overlapped, both
in time and space. Central Australia and vast areas of adjacent
Queensland, South Australia, Western Australia, New South Wales, and
northern Australia were affected, with varying intensity, 1957-66; and
south eastern Australia experienced a severe drought, 1964 68.
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This extensive drought affected nearly
all of eastern Australia, and was particularly severe in south eastern
Australia. Lowest ever 11 month rainfall occurred over most of Victoria
and much of inland New South Wales and central and southern Queensland;
and lowest ever 10 month rainfall occurred in much of South Australia
and northern Queensland. Total losses were estimated in excess of S3,000
million.
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| (a)
Major droughts to 1939-45 were classified by Foley (1957). Subsequent
droughts were classified by the Drought Watch Criteria (1986). |
Australia's most severe drought periods since the
beginning of European settlement appear to have been those of 1895-1903
and 1958-68. The 1982-83 drought was possibly the most intense with
respect to the area affected by severe rainfall deficiencies. These
periods were comparable in their overall impact, but differed
appreciably in character.
The 1895-1903 drought period was probably Australia's
worst to date, in terms of both its severity and area - affecting
practically the whole of Australia at various times but more
persistently in parts of eastern and central Australia. Stock and crop
losses were apparently the highest in Australian history.
The 1958-68 drought period is described in the
article contained in the 1968 Year Book No. 54. That drought period was
widespread and probably second only to the 1895-1903 drought period in
severity. The areas affected and their duration's of drought were
variable and overlapping.
The 1982-83 drought was notably severe also,
especially in south-eastern Australia. This drought was monitored
closely and is discussed more fully below.
Droughts of a lesser degree of severity categorised
by Foley (1957) are given in Table 2. The droughts of 1970-73 and 1976
were analysed by rainfall deficiency methods based on decile analysis
and are appropriate for inclusion in this category.
DROUGHTS IN AUSTRALIA OF LESSER SEVERITY |
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| Description |
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Queensland (severe); New South Wales
(intermittent); Western Australia (more particularly Fortescue:
1922-29); South Australia (mainly pastoral areas); central Australia
(1924-29); Northern Territory (1926-29); Victoria (1925-27; severe in
the north 1925-29) and Tasmania (1925-27, not continuous).
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Western Australia (severe in pastoral
and northern agricultural areas); Queensland (breaks on the coast);
Victoria (north and Gippsland); New South Wales (not continuous except
on the northern tablelands); Northern Territory; South Australia
(1935-36 in pastoral areas and 1938 in agricultural areas) and northern
Tasmania (1935-37, not continuous).
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Queensland (central coast and highlands
and central interior, elsewhere mainly in 1946); Northern Territory and
New South Wales (mainly in 1946-47); Western Australia (more
particularly in central agricultural areas, 1947-50), and northern
Tasmania (1948-49).
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Queensland and Northern Territory; and Western Australia, especially pastoral areas (1951-54).
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Prolonged drought over the north-eastern
goldfields of Western Australia and adjacent areas, caused by
successive below average rainfall years.
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Western New South Wales, most of Victoria and South Australia due to failure of autumn-winter rains; break in September 1976.
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| (a) The drought to 1951-52 inclusive, were classified by Foley (1957). The subsequent droughts, 1970-73 and 1976, were classified by the Drought Watch Criteria (1986) |
Severe droughts in south-eastern Australia
South-eastern Australia is taken to include New South
Wales, southern Queensland, Victoria, Tasmania and the settled parts of
South Australia; it contains about 75 per cent of the nation's
population, and major droughts affecting the region have a markedly
adverse impact on the economy. Severe droughts in south-eastern
Australia are usually caused by a failure of the winter-spring rains and
may extend through summer to the following autumn.
A severe drought is defined here in general terms as a
drought in which ten or more rainfall districts are substantially
affected by rainfall deficiencies for eight or more months. The onset of
drought is taken as the month in which rainfall drops below average,
and which marks the start of a period with serious rainfall deficiencies
(below the first decile) lasting three months or more. A drought is
considered broken when rainfall meets the criteria defined previously.
SEVERE DROUGHTS IN SOUTH-EASTERN AUSTRALIA |
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Average duration and month of break
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Southern Queensland, most of New South Wales, Victoria, South Australia and parts of Tasmania
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9-10 months to January 1889
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In parts of northern New South Wales, not broken until autumn 1889
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New South Wales, Victoria, parts of southern Queensland, South Australia and Tasmania
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Victoria, South Australia and Tasmania: 9 months to December 1902
New South Wales and southern Queensland 12 months to 1902
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Considerable overlapping of affected areas
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Victoria, New South Wales west of the tablelands, settled areas of South Australia and most of Tasmania
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South Australia 11-12 months to June 1915
Northern Victoria and New South Wales 10-12 months to June/July 1915
Southern Victoria 16 months to May/June 1915
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Rainfall during 1913 also below average
in parts of south-eastern Australia; and much of Victoria and western
New South Wales had some relief in the summer of 1914-15
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Most of New South Wales, Victoria, South Australia and eastern Tasmania
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South Australia 6 months to January 1941
Tasmania 8-9 months to January 1941
Victoria 11 months to January
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Variable durations in New South Wales
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Most of New South Wales, Victoria and South Australia
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South Australia and south-western Victoria 4-6 months to summer 1944-45
Southern Victoria 12 months to August 1945
Northern Victoria and southern New South Wales 15-19 months to August 1945
Northern New South Wales 15-17 months to June 1945
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Well below average rainfall in parts of
South Australia in April-June 1945; and 1943 was also a dry year in
parts of south-eastern Australia
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Victoria, southern New South Wales, South Australia and Tasmania
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South Australia 12- 13 months to March 1968
Tasmania 15- 16 months to May 1968
Victoria and New South Wales I4 - 15 months to May 1968
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Other extensive parts of Australia affected during 1958-67
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Most of Victoria, western and central New South Wales, South Australia and north eastern Tasmania
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9-10 months ending February 1973
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Drought broke in February 1973; except in north-eastern Tasmania, where it broke in autumn 1973
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Victoria, most of New South Wales, South Australia, southern Queensland and Tasmania
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Generally 11 months ending February 1983
Tasmania: 9 months ending February 1983
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Drought broke in autumn 1983
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| (a)
The drought periods prior to 1965 inclusive, occurring prior to the
operation of the Drought Watch Criteria, have been re-assessed applying
those criteria. The specified severe droughts in south eastern Australia
are actually encompassed within the major droughts in Australia
contained in Table I (except 1972-73). |
These past, severe droughts were investigated (Bureau of Meteorology,
1983) using seasonal rainfalls over south-eastern Australia based on a
limited network of rainfall stations and previously published material.
Droughts after 1914 were identified using also the district rainfall
data. Two earlier droughts affected south-eastern Australia, in 1864-66
and 1880-86, but rainfall data for these are incomplete. The 1918-20
period was also significantly drought affected without quite meeting the
criteria.
Of these eight severe droughts in south-eastern
Australia, four ended in summer (1888, 1902, 1940-41, 1972-73). Two
droughts (1967-68 and 1982-83) broke in autumn. The remaining two
(1914-15 and 1944-45) generally persisted until the following winters,
although there were useful summer rains over a significant portion of
the drought affected areas.
The 1982-83 major drought
The following figure indicates the severity and extent of the 1982-83
major drought in terms of rainfall deficiency over the extensive areas
where rainfall for the duration of the drought, approximately ten to
eleven months, was the lowest on record. This was due to a widespread
failure of the winter and spring rains of 1982. By the end of February
1983, in this vast area of eastern Australia, only small parts of
south-east Queensland, adjacent north- east New South Wales and parts of
south-west and north-east Tasmania were free from drought.
In the far south-eastern part of the continent the
drought was markedly severe. Virtually all of Victoria and southern New
South Wales had registered record low rainfall for the eleven months,
April 1982-February 1983 inclusive. Much of the settled areas of South
Australia had recorded their lowest ever rainfall for the ten months,
May 1982-February 1983 inclusive.
It is generally agreed that the widespread bushfires
which culminated in the enormous conflagrations of Ash Wednesday, 16
February 1983, were a direct consequence of the preceding drought
conditions. Total losses caused by the drought were estimated by the
Australian Government to exceed $3,000 million; and estimates of losses
in south-eastern Australia exceeded $1,200 million.
Widespread heavy rains in March 1983 significantly
reduced the extent of the drought over eastern Australia. Heavy April
rains further decreased the area of the drought, and record May rains
left only small scattered remnants at the end of autumn 1983.
PHYSICAL CAUSES OF DROUGHT
The physical causes of drought, as distinct from the socio-economic
factors that may induce stress in association with below average
rainfall (e.g. see Coughlan, 1985), have their origins in the
fluctuations of the global climate system. There are many possible
reasons why the weather during a particular month or season will differ
from one year to the next. The climate system as a whole is an extremely
complex mix of different sub-systems all interacting with each other on
a wide range of time and space scales, e.g. the atmosphere, oceans, ice
masses and the biosphere. The potential for variability from year to
year and decade to decade therefore is very high. Given this high level
of 'internal' variability, the significance or even the reality of
possible external influences from sunspots, phases of the moon and so
on, remains highly questionable on time scales shorter than millennia.
The fact that variability in time and space is an
inherent character of the climate means that droughts of varying extent
and severity must also be an inherent part of this variability. With an
increase in understanding of how the various parts of the climate system
fit together and interact with each other, is coming a greater
understanding at least of what causes the larger scale droughts. Perhaps
the most widely known climatic anomaly that has developed every few
years is the so-called EI-Nino phenomenon. EI-Nino, a name given to an
anomalous warm ocean current off the equatorial Pacific coast of South
America is part of a much wider system affecting the whole of the
Pacific Basin and probably the whole globe. The appearance of an EI-Nino
is very often associated with below average rainfall over much of
eastern Australia. El-Nino is linked to a swing in the mean atmospheric
pressure difference across the Pacific Ocean called the Southern
Oscillation. Many of the widespread and severe droughts affecting
eastern Australia identified above were a direct consequence of a marked
swing in the Southern Oscillation.
With a growing international awareness of the social and economic
impacts of climate variability, including drought, the World
Meteorological Organization (WMO) in the late 1970s instituted a World
Climate Programme (WCP) to complement its long established World Weather
Watch Programme. The WCP is the formal framework for international
co-operation in climate data exchange, climate monitoring, applications
of climate data, climate research and the impacts of climate variability
on man and the environment. As a national focus, some countries (e.g.
U.S.A and Canada) have established National Climate Programs.
Australia's Bureau of Meteorology plays a key role in
international data exchange and analysis by operating in Melbourne one
of the three World Meteorological Centres (WMC), the other two centres
being in Washington and Moscow. The Melbourne WMC and a Regional
Meteorological Centre in Darwin, also operated by the Bureau, collect
and process weather and climate data for the southern hemisphere. These
Centres issue daily weather analyses and forecasts for the southern
hemisphere, eastern Asia and the western equatorial Pacific.
The National Climate Centre (NCC), in addition to its
monitoring of fluctuations in Australia's climate, carries out analyses
of monthly and seasonal variations of atmospheric pressure, temperature
and wind over the southern hemisphere as a whole. The analyses are
contained in the NCC's monthly Climate Monitoring Bulletin accompanied
by seasonal indications, outlooks and inferences when feasible.
Information is regularly exchanged between similar climate centres
operating in other countries.
The Bureau of Meteorology Research Centre has also
instituted a program of research into the problems of forecasting
climate fluctuations on monthly and seasonal time scales. Any
improvements in this regard would have far reaching implications for our
ability to cope with drought. Already there are signs of some skill in
using the new found knowledge of the Southern Oscillation/EI-Nino
phenomenon to assess the likelihood of major anomalies in winter/spring
rainfall over eastern Australia several months in advance.
CONCLUSION
Since the 1860s there have been nine major Australian droughts. The
major drought periods of 1895-1903 and 1958-68 and the major drought of
1982-83 were the most severe in terms of rainfall deficiency and their
effects on primary production. In south-eastern Australia the droughts
of 1967-68 and 1982-83 were notably extreme. There have been six other
droughts of a lesser degree of intensity, but nevertheless causing
appreciable losses in large areas of several States. In south-eastern
Australia there have been eight severe droughts, mostly encompassed
within the major Australian droughts.
Droughts will continue to be a prominent feature of
the Australian scene. Improved meteorological drought watch services and
hopefully an improved ability to forecast droughts through local
research and participation in the WCP will help to mitigate their
adverse impacts. The nature of drought, however, and the way in which
the community should deal with it are complex issues incorporating
significant variables in fields such as hydrology, agriculture,
economics and sociology, as well as in the political realities of the
day.
BIBLIOGRAPHY
BUREAU OF METEOROLOGY. Climatic Atlas of Australia. Map Set 5 'Rainfall', Melbourne, 1975. - Drought Review Australia. February 1983, Accompanying Notes, Melbourne, 1983.
COUGHLAN, M. J. and LEE, D. M. 'The Assessment of Drought Risk in Northern Australia ', Natural Hazards Management in North Australia. Printed by A.N.U, Canberra, 1978.
COUGHLAN, M. J. 'Drought in Australia', Natural Disasters in Australia. Printed by Australian Academy of Technical Sciences, Parkville, Victoria, 1985.
- Monitoring Drought in Australia. International Symposium and Workshop on Drought, Lincoln, Nebraska, 1986.
FOLEY, J.C. Drought in Australia. Bulletin No. 43, Bureau of Meteorology, Melbourne, 1957.
GIBBS, W.J. and MAHER, J.V. Rainfall Deciles as Drought Indicators. Bulletin No. 48, Bureau of Meteorology, Melbourne, 1967.
LEE, D.M. and GAFFNEY, D.O. District Rainfall Deciles - Australia. Meteorological Summary, Bureau of Meteorology, Melbourne, 1986.
RESOURCES AND ENERGY, DEPARTMENT OF. 'Water Resources Aspects of Drought in Australia', Water 2000: Consultant's Report No. 13. AGPS,
Canberra, 1983.
ROYAL METEOROLOGICAL SOCIETY, AUSTRALIAN BRANCH. Report and Recommendations of the Drought Workshop, May 1986. Melbourne, 1986.
WORLD METEOROLOGICAL ORGANIZATION, WORLD CLIMATE DATA PROGRAMME. Climate System Monitoring. Monthly Bulletin, Issue No. 15, Geneva, 1985.
