Trends in rainfall indices for six Australian regions: 1910-2005

Abstract

Trends in rainfall indices from January 1910 to August 2005 were examined using an updated high-quality rainfall data-set from the Australian Bureau of Meteorology. Nine indices, reflecting changes to mean annual/seasonal rainfall and extreme daily rainfall (defined by the 95th and 99th percentiles), were evaluated for six regions in the east and southwest of Australia. Seasonal and annual trends were calculated over two periods: long term (1910-2005) and medium term (1950-2005). In the Central region, since 1910 there have been significant increases in spring and annual rain days and extreme rainfall intensity (95th percentile), but significant decreases in spring and annual rain per rain day and the proportion of rainfall from extreme events. During spring, the Western Tablelands showed a decrease in rain per rain day over the period 1910-1930 and an increase over the period 1970-2005, most likely due to heavy-rain events. No significant changes were detected in the West region. In the Southwest, annual total rainfall has significantly decreased by 21 into per decade since 1910 and by 24 nun per decade since 1950. These declines were accompanied by decreases in rain days and extreme rainfall indices. In the eastern Coastal region, since 1950, there has been a significant decrease of almost 55 mm per decade in annual total rainfall, along with decreases in rain days and extreme rain, particularly in summer and winter. In the Southeast, a significant decrease in annual total rainfall of 20 mm per decade since 1950 stems mainly from decreases during autumn. Generally, the direction of changes in extreme rainfall is consistent with changes in the mean.