From 1970 to 2010, total annual rainfall declined over much of eastern Australia and south-west Western Australia (Figure 3.4). This decline in rainfall affected all capital cities except Darwin. In contrast, rainfall increased over north-west and central Western Australia.
The 13-year period from April 1997 to March 2010 (156 months) shows rainfall deficiencies for much of south-western and south-eastern Australia and south-eastern Queensland (Figure 3.5). Most notable are the large areas of lowest rainfall on record for this period: large parts of Western Australia’s south-western coast, western Tasmania and large areas in Victoria received the lowest rainfall on record for the 13-year period.
For the more recent eight-year period from April 2002 to March 2010 (96 months), much of south-eastern Australia still experienced severe, long-term rainfall deficiencies. Approximately 95% of Victoria received rainfall in the lowest 10% of historical totals when considered over such a period. The south-eastern corner of Queensland also had serious to severe rainfall deficiencies over this period. Serious to severe deficiencies also remained in central to eastern coastal districts of South Australia, large areas of Tasmania (especially in the north), and a large area covering the south-west coast and adjacent inland regions of Western Australia. Rainfall deficiencies across the south-western and south-eastern corners of the continent have been most severe in autumn and winter.
For the 12 months from April 2009 to March 2010 (Figure 3.6), serious to severe rainfall deficiencies remained evident over much of the central Western Australian coast, reaching inland to cover much of the Pilbara and Gascoyne districts, where they intensified to some extent. Serious to severe rainfall deficiencies also remain over the south-east coastal and Great Southern districts of Western Australia, with a small area near Esperance reporting the lowest rainfall on record for the period.
However, from March 2010, large parts of the continent experienced above-average rainfall, associated with an extremely strong La Niña event (Figure 3.7). (A La Niña event refers to a periodic cooling of ocean surface waters off the western coast of South America. This leads to low rainfall in countries along that coast and in the south-west of the United States, and above-average rainfall in countries of the western Pacific, including the Philippines and northern and eastern Australia—outcomes that are the opposite of those associated with an El Niño event.) Most notably, eastern Australia received widespread record-breaking rains, with associated loss of life and massive damage to agriculture, homes and infrastructure. For the Murray–Darling Basin averaged as a whole, 2010 was the seventh wettest start to the year since records began in 1900. This rainfall has effectively ended a prolonged (decade or longer) sequence of very low rainfall years across parts of eastern Australia, most notably in the central and lower Murray–Darling Basin and south-east Queensland. Although Victoria experienced its wettest summer since 1974 in 2010–11, long-term rainfall deficiencies remained during autumn and winter.
South-west Western Australia missed out on La Niña–driven rainfall for most of 2010, experiencing its lowest winter rainfall on record.20-21 However, this situation changed markedly over the summer of 2010–11. Averaged across the state, summer rainfall was the second highest on record. In the north, this mainly reflected the influence of the monsoon that was active throughout the summer. Across the state more generally, it reflected the impact of the particularly strong La Niña. In much of the lower south-west, the Bureau of Meteorology characterised summer rainfall as ‘very much above average’, due mainly to rainfall in January.22 Despite this, during the 12 months from March 2010, rainfall in the lower south-west ranged from below average to the lowest on record.