1. What is Total Direct Runoff Depth?

Total Direct Runoff Depth represents the depth of water that flows over the land surface and enters stream channels promptly after rainfall or snowmelt, excluding infiltration and other losses.

2. How Does the Calculator Work?

The calculator uses the Total Direct Runoff formula:

Where:

• \( R_d \) — Total Direct Runoff Depth (cm)
• \( P \) — Total Storm Precipitation (cm)
• \( \phi \) — Φ-Index (cm/h)
• \( t_e \) — Duration of Rainfall Excess (hours)

Explanation: The formula calculates runoff by subtracting the infiltrated water (Φ-Index multiplied by duration) from the total precipitation.

3. Importance of Runoff Calculation

Details: Accurate runoff estimation is crucial for flood prediction, water resource management, drainage system design, and environmental impact assessment.

4. Using the Calculator

Tips: Enter total storm precipitation in cm, Φ-Index in cm/h, and duration of rainfall excess in hours. All values must be non-negative.

5. Frequently Asked Questions (FAQ)

Q1: What is Φ-Index?
A: Φ-Index is the constant infiltration capacity that would yield the actual total runoff for a given rainfall amount, representing the average infiltration rate during a storm.

Q2: What is Duration of Rainfall Excess?
A: It's the total time during which rainfall intensity exceeds the average infiltration rate, resulting in surface runoff generation.

Q3: What are typical values for Φ-Index?
A: Φ-Index values vary by soil type, land use, and antecedent moisture conditions, typically ranging from 0.25 to 2.5 cm/h.

Q4: When is this calculation most accurate?
A: This method provides reasonable estimates for moderate rainfall events on relatively uniform catchments with consistent soil conditions.

Q5: What are the limitations of this approach?
A: The method assumes constant infiltration rate and doesn't account for spatial variability, initial abstraction, or changing soil moisture conditions during the storm.