Question

The probability of the occurrence of a rainfall event is calculated by

• A. Arithmetic mean
• B. Isohyetal method
• C. Thiessen method
• D. Weibull method

Hint:

When you see "probability" or "return period" of rainfall/floods, think of statistical frequency analysis methods like Weibull, Gumbel, or Log-Pearson. When you see "average rainfall over an area," think of spatial averaging methods like Arithmetic Mean, Thiessen Polygon, or Isohyetal.

Answer 1

The Correct Option is D

Step 1: Analyze the purpose of each method listed.
- Arithmetic mean, Isohyetal method, Thiessen method: These are all methods used to calculate the average rainfall depth over an area from point measurements at different raingauge stations. They deal with the spatial distribution of a rainfall event that has already occurred.
- Weibull method (and similar plotting position formulas like Gringorten, Cunnane): This is a statistical method used in frequency analysis. It is used to estimate the probability of occurrence (or exceedance probability) and the return period of a hydrological event, such as a rainfall of a certain magnitude. It calculates the likelihood of future events based on historical data.
Step 2: Match the method to the task.
The question asks for a method to calculate the probability of occurrence, which is the definition of frequency analysis. The Weibull method is a standard technique for this purpose.