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: Determine the function of each listed method.
- Arithmetic mean, Isohyetal method, Thiessen method: These are techniques for determining the average rainfall depth across a region, utilizing data from multiple raingauge stations. They focus on the spatial distribution of past rainfall events.
- Weibull method (and analogous plotting position formulas such as Gringorten, Cunnane): This statistical approach is employed in frequency analysis. Its purpose is to estimate the likelihood of a hydrological event (e.g., a rainfall of a specific intensity) occurring or exceeding a certain threshold, and to determine its return period. It projects the probability of future events based on historical records.
Step 2: Align the method with the objective.
The query seeks a method for calculating the probability of occurrence, which is the core concept of frequency analysis. The Weibull method is a recognized approach for this task.