Lag Time Of 1-h Unit Hydrograph Given Duration Of Rainfall Excess Calculator

Formula Used:

\[ \text{Lag Time of a 1-h Unit Hydrograph} = \frac{\text{Duration of Excess Rainfall in Hours}}{1.1} \] \[ T_{p1} = \frac{D}{1.1} \]

Lag Time of a 1-h Unit Hydrograph:

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is the Lag Time of a 1-h Unit Hydrograph?

The Lag Time of a 1-h Unit Hydrograph represents the time interval from the midpoint of the rainfall excess to the peak discharge in the hydrograph. It is a crucial parameter in hydrological modeling for predicting watershed response to rainfall events.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ T_{p1} = \frac{D}{1.1} \]

Where:

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

Explanation: This formula provides a simplified relationship between rainfall duration and the resulting lag time in the unit hydrograph response.

3. Importance of Lag Time Calculation

Details: Accurate lag time estimation is essential for flood forecasting, watershed management, and designing hydraulic structures. It helps predict when peak flows will occur following rainfall events.

4. Using the Calculator

Tips: Enter the duration of excess rainfall in hours. The value must be a positive number representing the time period of continuous rainfall excess.

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 at a constant rate for a specified duration.

Q2: Why is the divisor 1.1 used in the formula?
A: The factor 1.1 is an empirical coefficient that accounts for the relationship between rainfall duration and the resulting lag time in the hydrograph response.

Q3: What are typical lag time values?
A: Lag time values vary significantly depending on watershed characteristics, but typically range from a few minutes for small urban watersheds to several hours for large rural watersheds.

Q4: How does watershed size affect lag time?
A: Generally, larger watersheds have longer lag times due to the greater distance water must travel to reach the outlet, while smaller watersheds typically have shorter lag times.

Q5: Are there limitations to this simplified formula?
A: Yes, this formula provides a simplified estimation and may not account for all watershed characteristics such as slope, land use, soil type, and channel morphology that can affect lag time.