Enter the previous day’s Antecedent Precipitation Index, decay constant, and daily precipitation amount into the calculator to determine the Antecedent Precipitation Index.

## Antecedent Precipitation Index Formula

The following formula is used to calculate the Antecedent Precipitation Index (API).

API = API_prev * k + P

Variables:

• API is the Antecedent Precipitation Index
• API_prev is the previous day’s Antecedent Precipitation Index
• k is the decay constant (0 ≤ k ≤ 1)
• P is the daily precipitation amount (mm)

To calculate the Antecedent Precipitation Index, multiply the previous day’s API by the decay constant, then add the daily precipitation amount. The decay constant is a value between 0 and 1 that represents the rate at which the influence of previous precipitation diminishes over time. The daily precipitation amount is the total amount of rainfall recorded in a day.

## What is an Antecedent Precipitation Index?

The Antecedent Precipitation Index (API) is a hydrological index used to quantify the amount of moisture in the soil. It is calculated based on the amount of rainfall that has occurred over a certain period of time, typically a few days to a few weeks, prior to a specific event. The API is used to predict the likelihood of flooding or landslides, as higher values indicate more saturated soil that is less able to absorb additional rainfall.

## How to Calculate Antecedent Precipitation Index?

The following steps outline how to calculate the Antecedent Precipitation Index (API).

1. First, determine the previous day’s API (API_prev).
2. Next, determine the decay constant (k) within the range of 0 to 1.
3. Next, determine the daily precipitation amount (P) in millimeters (mm).
4. Next, gather the formula from above = API = API_prev * k + P.
5. Finally, calculate the Antecedent Precipitation Index (API).
6. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem :

Use the following variables as an example problem to test your knowledge.

Previous day’s API (API_prev) = 0.5

Decay constant (k) = 0.8

Daily precipitation amount (P) = 10 mm