ACT State of the Environment 2007

Indicator: Weather – Rainfall

See also: | Temperature | Sunshine | Relative humidity | Evaporation | Wind |

Figure 1  shows the trends in actual monthly rainfall during the reporting period in terms of annual rainfall, monthly time series, anomalies from the 1961–90 monthly means and rainfall accumulation against the monthly mean. The deficit between the actual rainfall accumulated over 2003–07 and the mean is around 536 mm (or 107 mm/year).

Figure 1 : Trends in rainfall at Canberra International Airport, 2003–07

Four graphs showing trends in rainfall at Canberra International Airport, 2003 to 2007

Source: Bureau of Meteorology

Long-term trends

The correlation between Queanbeyan and Canberra International Airport is extremely high, to the extent that Queanbeyan can be used as a proxy for Canberra International Airport for long-term analysis. Figure 2 indicates the correlation between these sites while Figure 3 shows the long-term time series of rainfall at Queanbeyan 1871–2007 with a five-year moving average.

Figure 2 : Comparison of annual rainfall at Canberra International Airport and Queanbeyan, 1940–2007

Graph comparing annual rainfall at Canberra International Airport and Queanbeyan, 1940 to 2007

Source: Bureau of Meteorology

Figure 3 : Time series of annual rainfall at Queanbeyan with a five-year running mean, 1987–2006

Graph showing the time series of annual rainfall at Queanbeyan with a five-year running mean, 1987 to 2006

Source: Bureau of Meteorology

The ACT has now experienced six out of eight years of below average rainfall since 2000 and there have been no considerably wet years since 1998–99 to replace the loss in ground water. While the five-year moving average suggests there has been a drying tendency over the last 10–15 years, it is not necessarily out of kilter with long-term variability, such as the late 1970s and early 1980s and the long dry period of 1895–1915. However, if climate change is occurring, any future trend may not follow past trends and the concern is whether the present drying trend will persist.

While there are no apparent annual trends, there are interesting seasonal trends. Figure 4 shows the seasonal time series for Queanbeyan and indicates that while there has been a drying trend in autumn over the last decade, this has been ameliorated by slight increases in the other seasons over this period. (skip graph)

Figure 4 : Time series of seasonal rainfall at Queanbeyan with a five-year running mean, 1870–71 to 2007

Four graphs showing a time series of seasonal rainfall at Queanbeyan with a five-year running mean, 1870/71 to 2007

Source: Bureau of Meteorology

Rainfall in the ACT and region, particularly during winter and spring, tends to be affected by the El Niño Southern Oscillation (ENSO) phenomenon. The main measure the Bureau of Meteorology uses to determine an ENSO event is the Southern Oscillation Index (SOI) which is an index based on the pressure differences between Darwin and Tahiti. Table 1 lists the average winter–spring SOI values and Figures 5 and 6 indicate the correlations between the winter and spring average SOI and rainfall.

Under the criterion that an El Niño event is defined when the June–November average SOI is within the 20th percentile (-8.3), 2006 is identified as an El Niño event while 2004 was just below this level.

Table 1: Average Southern Oscillation Index, June/November, 2003–07
Year 2003 2004 2005 2006 2007
Average June/November SOI -3.1 -7.5 1.5 -8.7 3.4

Source: Bureau of Meteorology

Figure 5: Comparisons of winter and spring rainfall at Queanbeyan with June–November Southern Oscillation Index, 1876–2007

Graphs showing comparisons of winter and spring rainfall at Queanbeyan with June to November Southern Oscillation Index, 1876 to 2007

Source: Bureau of Meteorology

Figure 6: Scatterplot comparing winter and spring rainfall at Queanbeyan with June–November Southern Oscillation Index, 1876–2007

Scatterplot comparing winter and spring rainfall at Queanbeyan with June to November Southern Oscillation Index, 1876 to 2007

Source: Bureau of Meteorology

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