Rainfall and snow

Rainfall in Australia varies a great deal from year to year, and from decade to decade. There are also marked regional differences, both in long-term rainfall trends and in local responses to broader influences on the climate (such as the El Niño–Southern Oscillation).

In general, rainfall is declining in the south of Australia and increasing in the north, but there has been considerable variability over time and substantial seasonal variations in rainfall trends in some regions.

Regional differences also mean that nationally averaged rainfall, which shows a slight upward trend from 1900 to 2020, is of limited value as an indicator of the state and trend of the climate. Averaged over the country, 2019 was Australia’s driest year on record, but the mean for 2011−20 was slightly above the average of the 1961−90 reference period. High levels of decadal variability, particularly in the drier parts of Australia, also mean that observed trends in rainfall can be sensitive to the start and end dates of the period over which the trend is measured.

The most clear-cut regional trend in rainfall is over the south-west of Western Australia. This region has been drying steadily since the 1970s, with area-averaged rainfall decreasing by 15–20% between 1970 and 2020 (Figure 8). Although some individual years have been extremely dry, more noteworthy has been the almost total absence of very wet years; the 25 wettest years on record for the region over 1900−2020 all occurred earlier than 1975. Tree-ring reconstructions indicate that the recent dry period is not unprecedented in a multicentury context, and the post-1900 rainfall decline may at least partially reflect unusually wet conditions in the 1900–70 period (O’Donnell et al. 2021).

The south-east of Australia also shows declining rainfall, although it was not until the 1990s that a sustained downward trend began. The decrease is most pronounced in southern Victoria, where post-1997 rainfall is widely 10–20% below pre-1997 levels. Decreases have also occurred elsewhere in Victoria, Tasmania, and some parts of eastern South Australia and southern New South Wales. As in the south-west of Western Australia, rainfall decreases on the mainland have been concentrated in the cool season (April–October), with little change in the summer; conversely, Tasmanian rainfall changes have generally been largest in the summer.

In contrast, there have been large increases in rainfall in many parts of north-western Australia. These are largest in the Kimberley region of Western Australia, where rainfall in some places has risen by 40% or more since the 1960s, but also extend to the western half of the Northern Territory. Rainfall has also increased since the 1960s in interior regions of Western Australia, extending south as far as the Nullarbor, but limited data from this region make it difficult to establish changes over the full 20th century with confidence. The increasing rainfall trend in north-western Australia largely reflects extremely wet multiyear periods in the 1970s, around 2000 and, to a slightly lesser extent, around the 2010−11 La Niña, which have no equivalents in the data before 1970 (Figure 9).

In most of New South Wales, Queensland and northern South Australia, there has been little change in rainfall over the full 1900–2020 period. However, there has been considerable variability over time. The first half of the 20th century was relatively dry over most of the region, followed by a wet period from 1950 to the early 1990s (with the 1950s and 1970s being especially wet). Since the 1990s, rainfall has returned to levels more typical of the pre-1950 period, although in some regions the 2017−19 drought was more severe than any other drought during the 20th century (see case study: The 2017−19 Australian drought). Drier parts of the region are historically prone to prolonged dry periods – for example, there were 17 consecutive years with rainfall below the 1961−90 average in South Australia in 1922–38.

Figure 8 Australian rainfall trends: (a) 1900–2020; (b) 1960–2020
Figure 9 Timeseries of annual rainfall (anomalies from 1961–1990 mean) with 11-year running mean for (a) south-eastern Australia; (b) northern Australia; (c) south-western Australia; (d) the Murray–Darling Basin

Snow

The higher parts of the mountains in New South Wales, Victoria and Tasmania have regular winter snow cover. In New South Wales, which has the best long-term records of alpine snow depth, the average annual maximum snow depth at Spencers Creek had decreased by 10% in 2000−13 relative to 1954−99 (Pepler et al. 2015). Data availability for Victoria is more limited, beginning only in the 1980s, and very limited data are available for Tasmania. There are also limited data for snowfall outside the alpine regions, although a study (Hague & Trewin 2014) of records at a site at 790 m elevation near Bombala found a significant decrease in various snow season indicators between 1954 and 2001; a long-term decline in snow events in South Australia from 1838 to 2019 has also been found (Gergis et al. 2020).

Assessment Rainfall changes
2021
2021 Assessment graphic showing the environment is in poor condition, resulting in diminished environmental values, and the situation is deteriorating.
Somewhat adequate confidence

Rainfall has decreased since 1970 in south-western and south-eastern Australia. The strongest decreases have been in the cool season, placing substantial stresses on water availability in those regions. Over the same period, rainfall has increased in north-western Australia. In other parts of Australia, a clear trend has not emerged outside natural variability. However, the 2017–19 drought was of unprecedented intensity in some regions, particularly in the northern Murray–Darling Basin.
Related to United Nations Sustainable Development Goal target 13.2

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How was this assessment made

Impacts of rainfall changes

Reduced levels of rainfall and changes in rainfall seasonality in parts of Australia, along with increased frequency of storms and floods (see High-rainfall extremes), will all affect the environment and human communities. Reduced and altered seasonality of rainfall is:

  • decreasing water availability in rivers, associated waterways, wetlands, groundwater and aquifers (see the Inland water chapter)
  • reducing the availability of water for species, habitats and ecosystems, and therefore contributing to ecosystem changes and loss of biodiversity
  • reducing the availability of water for human uses, including consumption and agriculture
  • leading to air quality issues (see the Air Quality chapter)
  • driving spatial shifts in agriculture (see the Biodiversity and Land chapters)
  • affecting traditional plants needed for harvesting and materials (see case study: Yorta Yorta weaving).