About the rainfall trend maps

Analyses available

Australian and regional rainfall trend maps are available for annual and seasonal rainfall, covering periods starting from the beginning of each decade from 1900 to 1980. These different starting times demonstrate the importance of the analysis period on rainfall trend values. For example, the post-1900 maps look quite different to the post-1980 maps. Prior to 1900 the rainfall network is generally too sparse for reliable analysis, and analysis periods starting after 1980 are considered too short to calculate meaningful trend values.

Interpreting the analyses

The trend maps are a useful way to compare how the rainfall has changed in different regions of Australia over time. However, they need to be interpreted with caution. Trend values have been determined from a linear (straight line) fit to the data, but the change may not have been gradual. For example, a calculated trend could be due to a relatively rapid "step" change, with the remainder of the series being fairly flat (see some of the timeseries graphs). Also, because rainfall is such a variable element, trend values are highly dependent on the start and end dates of the analysis. Consequently trend maps starting in different decades can look remarkably different. Users are advised to keep in mind the period over which trend values have been calculated and interpret them alongside the timeseries of spatially averaged values.

In addition, the trend values calculated here using past observations should not be used to imply future rates of change. Due to the complex interactions between the natural and human drivers of climate change and variability, the climate of any location is always changing. Future rates of change will depend on how these drivers interact in future, which will not necessarily be the same as in the past.

Data used

The Bureau of Meteorology has upgraded its monthly gridded rainfall analysis. The enhanced analysis known as Australian Gridded Climate Data (AGCD) employs state-of-the-art statistical modelling and improvements in scientific techniques, to provide the Australian community with a more accurate representation of monthly, seasonal and annual rainfall in their region.

The Bureau has a vast network of manually-read and automated rain gauges across Australia but it's not possible to place this type of infrastructure every few kilometres. While these rain gauges provide rainfall data at point locations, gridded analysis allows the Bureau to provide an accurate estimate of rainfall in much wider areas.

The enhanced analysis has been applied to the Bureau's historical rainfall data which is crucial to ensuring consistency between past and future monthly gridded rainfall data.

Further details about the gridded rainfall analysis