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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
15m
21.0 mm
12:30pm 23/07/2026
12:45pm 23/07/2026
1 in 15.3 years
6.52 %
30m
33.6 mm
12:15pm 23/07/2026
12:45pm 23/07/2026
1 in 39.8 years
2.51 %
45m
44.0 mm
12:00pm 23/07/2026
12:45pm 23/07/2026
1 in 85.6 years
1.17 %
60m
44.0 mm
11:45am 23/07/2026
12:45pm 23/07/2026
1 in 47.6 years
2.1 %
90m
44.0 mm
11:15am 23/07/2026
12:45pm 23/07/2026
1 in 24.2 years
4.14 %
120m
0.0 mm
4:15am 23/07/2026
6:15am 23/07/2026
1 in 1 years
100.0 %
3h
0.6 mm
12:15am 23/07/2026
3:15am 23/07/2026
1 in 1 years
100.0 %
4h
23.0 mm
8:00am 23/07/2026
12:30pm 23/07/2026
1 in 1.2 years
84.38 %
6h
44.2 mm
6:45am 23/07/2026
12:45pm 23/07/2026
1 in 4.3 years
23.52 %
9h
44.2 mm
3:45am 23/07/2026
12:45pm 23/07/2026
1 in 2.6 years
38.35 %
12h
44.2 mm
12:45am 23/07/2026
12:45pm 23/07/2026
1 in 2.1 years
48.21 %
24h
44.2 mm
3:15am 23/07/2026
3:15am 24/07/2026
1 in 1.4 years
73.61 %
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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