What does Annual Exceedance Probability (AEP) mean?
The Average Recurrence Interval (ARI) and the Annual Exceedance Probability(AEP) are both a measure of the rarity of a rainfall event. In an attempt to explain probability and averages, imagine a stadium of people throwing a 20c coin. On average 50% or ½ the people will throw heads therefore heads has a 1 in 2 ARI. But what happens when two people in isolation throw coins and they both show heads? With 2 coins, there are 4 possible outcomes. Therefore the probability of throwing two heads is 1/4 or 25%. In an attempt to overcome some of the misconceptions of using averages, flood events are also represented by Annual Exceedance Probability (AEP). This is the probability that the average ARI event will be exceeded in a year. Calculated from: The probability of experiencing a flood exceeding 1 in 2 year ARI event is 39.3% based on the above formula.
