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You can find out more about Design Rainfall and IFD here.
Observations for this analysis :
Maximum observed rainfall by duration, estimated return period and AEP
Duration
Rainfall
Start
End
Years
AEP
60m
11.8 mm
4:00pm 17/07/2026
5:00pm 17/07/2026
1 in 1.2 years
81.89 %
120m
18.8 mm
3:00pm 17/07/2026
5:00pm 17/07/2026
1 in 1.7 years
59.79 %
3h
23.2 mm
3:00pm 17/07/2026
6:00pm 17/07/2026
1 in 1.9 years
51.3 %
6h
38.4 mm
3:00pm 17/07/2026
9:00pm 17/07/2026
1 in 4.2 years
23.81 %
9h
41.8 mm
2:00pm 17/07/2026
11:00pm 17/07/2026
1 in 3.6 years
27.74 %
12h
48.8 mm
9:00am 17/07/2026
9:00pm 17/07/2026
1 in 4.4 years
22.86 %
18h
65.8 mm
3:00am 17/07/2026
9:00pm 17/07/2026
1 in 8.1 years
12.31 %
24h
72.2 mm
1:00am 17/07/2026
1:00am 18/07/2026
1 in 8.5 years
11.72 %
30h
76.6 mm
7:00pm 16/07/2026
1:00am 18/07/2026
1 in 8.7 years
11.54 %
36h
79.2 mm
4:00pm 16/07/2026
4:00am 18/07/2026
1 in 8.4 years
11.85 %
Note: A '1 in 1.0 years' estimate means that it is almost certain to occur at least once in a year, and could occur much more than once per year on average. For smaller values like this the Annual Recurrance Interval (ARI) may be of more relevance, but it is complex to calculate for common events.
IFD describes how rainfall intensity relates to how long it lasts and how often it happens. It is the basis for design rainfall used in engineering and flood studies.
- Duration: the length of the rainfall burst, from a few minutes to several days.
- AEP (Annual Exceedance Probability): the chance, in any one year, of a rainfall of that depth or greater. A 1% AEP event has a 1 in 100 chance each year - what is often loosely called a '1 in 100 year' event.
- The curves show design rainfall depths for a range of AEPs. The observed maximums for this location and period are plotted against them so you can see how rare the event was.
Design rainfall data is sourced from the Bureau of Meteorology.
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