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The average daily rainfall for each year and season, together with the percentage of the area with annual rainfall below decile one or above decile nine. Water is a limiting factor in agricultural and other biological systems. Australia's highly variable rainfall thus has substantial effects on natural ecosystems and human activities. An understanding of variations in rainfall is an important adjunct for interpreting several inland waters and land indicators.
Average daily rainfall is monitored by the Bureau of Meteorology (BoM). However, not all stations have the same quality of records, and there are some areas in Tasmania with a less than ideal coverage of sites. Hence some aspects of this indicator can not be measured as specified. The following data relates to seasonal and annual rainfall at several sites around Tasmania. It also only uses BoM recording sites with datasets that have had rigorous quality control techniques applied. Statewide trends A consolidated technical report (Barnes-Keoghan 2002) is available for downloading with annual and seasonal rainfall and temperature information including data and graphs for 13 stations around Tasmania. Details of all sites together with general climatic information are also available from the Climate section of the BoM Web site. The annual mean rainfall in Tasmania over the past century is displayed below. Extreme conditions for El NiÑo and La NiÑa events are also outlined below. When the annual data for the 13 detailed sites are examined, the most consistent features are the relatively dry period in the early to mid 1960s, followed by a relatively wet period in the early 1970s and a subsequent decline to the mid 1980s. After this, most stations show relatively low rainfall.
The seasonal graphs for the 13 sites hint at the complex changes that have occurred in the rainfall distribution. For example, the sites such as Sorell, Swan Island and Ormley have seen relatively high summer rainfall and relatively low winter rainfall over the last 10 to 20 years, but at Queenstown the reverse is true, at least for some of the period. The suggestion is that a change has occurred in the frequency or intensity of the various synoptic conditions that produce rainfall in different parts of the State at different times of the year. The variability warns of the dangers of simple linear trend statistics for a site, let alone for the whole State.
Trends in the drier parts of the State Water is a limiting factor in agricultural and other biological systems. Australia's highly variable rainfall thus has substantial effects on natural ecosystems and human activities. An understanding of variations in rainfall is an important adjunct for interpreting several inland waters and land indicators.
El NiÑo and La NiÑa events El NiÑo and La NiÑa events also affect rainfall patterns across Australia. Prolonged El NiÑo conditions (a low SOI index) are typically associated with drier than average conditions over much of Australia, including north-east Tasmania. The reverse tends to occur during La NiÑa events, with above average rainfall more likely for Tasmania. The strongest El NiÑo and La NiÑa years and their average SOI values are listed in the following table, while the mean rainfall for June to November for the same years is shown in the map below. Average (June to November) SOI values for the twelve strongest El NiÑo and La NiÑa years Caption: Strongly negative SOI values sustained across many months form one of the indicators of an El NiÑo event. Strongly positive SOI values sustained across many months form one of the indicators of a La NiÑa event. El Niño La Niña Year Average SOI Year Average SOI 1905 -15.1 1910 16.3 1914 -14.2 1916 11.9 1940 -16.3 1917 25.0 1941 -15.3 1938 11.9 1965 -15.0 1950 16.1 1972 -11.5 1955 15.8 1977 -13.6 1956 9.4 1982 -22.6 1971 10.0 1987 -11.8 1973 14.3 1991 -8.6 1974 6.8 1994 -14.0 1975 18.6 1997 -16.9 1988 13.0 Source: Bureau of Meteorology 2001
National core indicator A2 (Manton and Jasper 1998) |
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