1. What is Duration of Excess Rainfall?

Duration of Excess Rainfall in Hours is the time during which continuous rain is observed at a given point or within a given area that contributes to direct runoff.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( D \) — Duration of Excess Rainfall in Hours
• \( T_{p1} \) — Lag Time of a 1-h Unit Hydrograph

Explanation: The formula establishes a relationship between the duration of excess rainfall and the lag time of a 1-hour unit hydrograph, with a multiplication factor of 1.1.

3. Importance of Calculation

Details: Accurate calculation of duration of excess rainfall is crucial for hydrological modeling, flood forecasting, and water resource management. It helps in determining the time period during which rainfall contributes to direct runoff.

4. Using the Calculator

Tips: Enter the lag time of a 1-h unit hydrograph in hours. The value must be valid (greater than 0).

5. Frequently Asked Questions (FAQ)

Q1: What is a unit hydrograph?
A: A unit hydrograph represents the direct runoff response of a watershed to a unit depth of excess rainfall occurring uniformly over the watershed for a specified duration.

Q2: Why is the factor 1.1 used in the formula?
A: The factor 1.1 is an empirical coefficient that relates the lag time of a 1-hour unit hydrograph to the duration of excess rainfall based on hydrological studies and observations.

Q3: What are typical values for lag time?
A: Lag time values vary depending on watershed characteristics such as size, shape, slope, and land use. They typically range from a few minutes to several hours.

Q4: Can this formula be used for all watersheds?
A: While the formula provides a general relationship, it may need calibration for specific watershed conditions and characteristics.

Q5: How is lag time determined?
A: Lag time can be determined from historical streamflow data, through hydrological modeling, or using empirical formulas based on watershed characteristics.