Step 1: Understanding the Concept:
This question asks for the application of the "Weibull Method" in the context of hydrology and water resources engineering.
Step 2: Detailed Explanation:
1. Frequency Analysis: In hydrology, frequency analysis is a statistical technique used to analyze historical data (like annual maximum flood flows, rainfall depths, etc.) to predict the probability of occurrence of future events.
2. Return Period: The return period (or recurrence interval), T, of an event is the average time interval between occurrences of an event of a certain magnitude or greater. It is the reciprocal of the exceedance probability, P ($T = 1/P$). For example, a "100-year flood" has a 1% chance of being equaled or exceeded in any given year.
3. Plotting Position Formulas: To perform a frequency analysis, the historical data is first ranked. Then, a "plotting position" formula is used to assign a probability of exceedance (or a return period) to each ranked event. This allows the data to be plotted on probability paper to fit a statistical distribution.
4. Weibull Method: The Weibull formula is one of the most common and simple plotting position formulas. It is given by:
\[ P = \frac{m}{n+1} \]
or for return period:
\[ T = \frac{n+1}{m} \]
where 'n' is the total number of years of record, and 'm' is the rank of the event (m=1 for the largest event, m=2 for the second largest, etc.).
5. Therefore, the Weibull method is used specifically for estimating the return period or probability of a flood event (or other extreme hydrological events) as part of a frequency analysis.
The other options are incorrect:
- (B) Infiltration rates are measured using infiltrometers (like double-ring infiltrometer) or estimated using models (like Horton's or Green-Ampt equation).
- (C) Surface runoff is calculated using methods like the Rational Method or the SCS-Curve Number method.
- (D) Groundwater recharge is determined using water balance studies, tracer methods, or groundwater modeling.
Step 3: Final Answer:
The Weibull Method is a plotting position formula used for estimating the return period of a flood event in hydrological frequency analysis. Therefore, option (A) is the correct answer.