Rainfall probability terminology

BoM updates Storm Intensity Frequency Duration Terminology

The probability terminology used in InSite Water for the 2016 design rainfalls (IFD rainfall or Intensity Frequency Duration) is consistent with the probability terminology for the new edition of Australian Rainfall and Runoff (ARR2019). Further details on the new probability terminology can be found in Book 1; Chapter 2; Section 2.2 Terminology of ARR2019 http://arr.ga.gov.au/arr-guideline.

The main terms used to describe design rainfalls are:

  • Exceedances per year (EY): the number of times an event is likely to occur or be exceeded within any given year.
  • Annual exceedance probability (AEP): the probability or likelihood of an event occurring or being exceeded within any given year, usually expressed as a percentage.

The table below lists the probability terminology used for the 2016 design rainfalls and shows in bold the standard EY and AEP values for which design rainfalls are available. Generally, EY terminology is used for Very Frequent design rainfalls used in Water Sensitive Urban Design sizing, AEP (%) terminology is used for Frequent and Infrequent design rainfalls, and AEP (1 in x) terminology is used for Rare design rainfalls (major floods).

IFD Equivalencies for Exceedances per Year (EY), Annual exceedance probability (AEP) and Average Return Interval (ARI) source: http://www.bom.gov.au/water/designRainfalls/ifd/ifd-faq.shtml

Note:

  • The 50% AEP IFD does not corresponds to the 2 year Average Recurrence Interval (ARI) IFD. Rather it corresponds to the 1.44 ARI.
  • The 20% AEP IFD does not corresponds to the 5 year Average Recurrence Interval (ARI) IFD. Rather it corresponds to the 4.48 ARI.
  • The 10% AEP IFD does not corresponds to the 10 year Average Recurrence Interval (ARI) IFD. Rather it corresponds to the 9.49 ARI.

For more information and to view the IFD data visit the Bureau of Meteorology (BoM) design rainfalls page.