The relationship between ARI and AEP
Let's clarify the relationship between Average Recurrence Interval (ARI) and Annual Exceedance Probability (AEP).
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Average Recurrence Interval (ARI): This is the average time, in years, between events of a specific magnitude. An ARI of 5000 years means that on average, an event of that size (or larger) occurs once every 5000 years.
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Annual Exceedance Probability (AEP): This is the probability that an event of a specific magnitude will be equaled or exceeded in any given year. It's usually expressed as a percentage.
The relationship between these two concepts is an inverse one, and for rare events (those with a long ARI), the formula is straightforward:
DEPENDENCY OF AEP
Estimation of the Annual Exceedance Probability (AEP) is highly dependent on changes in a basin's or catchment's characteristics. AEP represents the probability that a certain hydrologic event (like a flood) will be equaled or exceeded in any given year. This probability is calculated based on historical data and the physical properties of the watershed.
Here's why changes in the basin affect AEP:
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Urbanization: When natural land is converted to urban areas, more impervious surfaces (like roads and roofs) are created. This reduces the ground's ability to absorb rainfall, leading to faster and larger runoff, which increases the likelihood of flooding and alters the AEP.
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Deforestation: Removing trees reduces the amount of rainfall intercepted and the soil's capacity to infiltrate water. The result is an increase in surface runoff and a higher risk of floods, which directly changes the AEP.
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Climate Change: Changes in rainfall patterns, intensity, and frequency due to climate change can significantly alter the hydrologic regime of a basin. Since AEP is based on the statistical analysis of these rainfall events, any shift in the climate will change the calculated AEP for a given event.
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Structural Changes: The construction of dams, levees, or other flood control structures can alter a basin's flow regime, decreasing the likelihood of floods and thus changing the AEP.
Essentially, any human or natural change that alters how a watershed collects, stores, and releases water will affect its hydrologic response and, consequently, the AEP.
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