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Climate Variability and Change (continued)

Average conditions and trends

Mean precipitation

In Australia, most precipitation occurs as rainfall, with only small amounts of hail and snow. During the winter, snow generally covers much of the Australian Alps above 1500 m and the Tasmanian mountains. Snow below 500 m is occasionally experienced in southern Australia. Snow may persist through summer in sheltered locations around Mount Kosciuszko.

Australia's annual mean rainfall is calculated from an area-weighted average of the total annual precipitation at 379 high-quality rainfall stations across the country (Lavery et al. 1997) (Figure 14). The records of rainfall from these stations provide a basis for monitoring long-term trends in Australian rainfall. Australia is a dry country with high interannual variability of rainfall. Figure 13 shows the spatial pattern of average annual rainfall based on long-term records. About 80% of the country has an average annual rainfall of less than 600 mm; about half has an average of less than 300 mm; and over one-third has an annual average of less than 200 mm. The driest part of the continent is around the Lake Eyre Basin, with an annual average of less than 125 mm.

The highest annual rainfall occurs on the east-facing slopes of north-east Queensland in the area surrounding Cairns and Tully, with Tully having an annual median rainfall of 4048 mm (Hobbs 1998). However, there are some stations that receive more than 10 000 mm of annual total rainfall. For example, the highest annual total rainfall is recorded at Bellenden Ker in Queensland, which received 12 461 mm of rain in 2000 and 11 852 mm in 1999. Annual falls in this region can exceed 5000 mm, while on west-facing slopes in Tasmania and above 2000 m in the Australian Alps, annual totals reach 4000 mm. Mountain areas of north-east Victoria and some parts of the eastern coastal slopes have median annual rainfall of over 2500 mm.

Rainfall trends [A Indicator 1.4]

Australia's rainfall shows strong variations from season to season and from year to year. Strong year-to-year variations in rainfall have been occurring for thousands of years. As a result, many native animals and plants have evolved to cope with extended periods of drought and floods. Australian annual mean rainfall since 1900 is shown in Figure 15. Australian annual rainfall shows a weak upwards trend that is largely as a result of several wet years during the 1970s. However, Australia's high year-to-year rainfall variability dominates any background trends.

Much of this variability can be accounted for by the influence of ENSO. Rainfall variability is expressed by the ratio of the 90th to the 10th percentile range to the 50th percentile (median value). High to very high annual rainfall variability lies mostly in the summer rainfall arid zones of central and western Australia (e.g. near Alice Springs and the Simpson Desert). Variability is low over the south-west of Western Australia and extreme south-east Australia (Figure 16).

Regional rainfall trends from 1910 to 1999 show both increases and decreases (Figure 17). Large increases (greater than 20 mm per 10 years) have been observed over northern and eastern New South Wales. A decrease has occurred in south-west Western Australia. During the past 50 years, the largest increases (more than 30 mm per 10 years) have occurred in the north-west of Australia, while largest decreases (more than 30 mm per 10 years) occurred in the east of the country (Figure 18).

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