Changes to the Intensity-Frequency-Duration (IFD) design rainfalls across Tasmania

The Bureau of Meteorology released new design rainfall estimates (IFDs) for Australia in July 2013 as part of the Australian Rainfall and Runoff revision project. There were changes in method and data availability between the old IFDs and the new IFDs. Previous comparisons showed that these changes have resulted in differences in IFDs across Australia. This paper explores the new IFDs from a Tasmanian perspective, comparing grids of the old and new IFDs for different durations and probabilities, highlighting observable differences and similarities and presenting possible reasons for these features.

For Tasmania, an additional 102 daily Bureau sites, 28 subdaily Bureau sites and 67 subdaily sites operated by other organisations were included in the rainfall database for the new IFDs. Many of these new sites are located in the data sparse region around the west coast and central plateau increasing the accuracy of the IFDs in these areas. The elevation enhancement used in the regionalisation and gridding process was found to have a significant impact in the areas of Tasmania with high topographic variation. There has also been a general decrease in subdaily IFDs at the 1% AEP frequency across much of the state. Spatial patterns across Tasmania correlated with prevailing weather patterns are evident by calculating ratios of standard durations.

The new IFDs are examined using recent rainfall events. The first example is an elevated site with high topographic variation in the northeast of the State. The second is an elevated site in the west of the State. The final example used is a lower site in a relatively flat area of the northern midlands. These examples show the changes in the IFDs at each of these specific locations, as well as demonstrating the ratios for the longer duration events and the shape of the IFD curves.