Phi Index Given Total Runoff Depth Calculator

Φ-Index Formula:

\[ \Phi\text{-Index} = \frac{\text{Total Storm Precipitation} - \text{Total Direct Runoff}}{\text{Duration of Rainfall Excess}} \]

Φ-Index:

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1. What is the Φ-Index?

The Φ-Index of a catchment is the constant infiltration capacity that would yield the actual total runoff for a given rainfall amount. It represents the average rate of infiltration during the period of rainfall excess.

2. How Does the Calculator Work?

The calculator uses the Φ-Index formula:

\[ \Phi\text{-Index} = \frac{\text{Total Storm Precipitation} - \text{Total Direct Runoff}}{\text{Duration of Rainfall Excess}} \]

Where:

Total Storm Precipitation — Annual precipitation of the current year (cm)
Total Direct Runoff — Runoff entering stream channels promptly after rainfall or snowmelt (cm)
Duration of Rainfall Excess — Total time when rainfall intensity is greater than the average infiltration rate (hours)

Explanation: The Φ-Index represents the constant infiltration rate that would produce the observed runoff volume from the given rainfall event.

3. Importance of Φ-Index Calculation

Details: Accurate Φ-Index calculation is crucial for hydrologic modeling, flood prediction, watershed management, and designing drainage systems. It helps in understanding the infiltration characteristics of a catchment.

4. Using the Calculator

Tips: Enter total storm precipitation and total direct runoff in centimeters, and duration of rainfall excess in hours. All values must be positive numbers with precipitation greater than runoff.

5. Frequently Asked Questions (FAQ)

Q1: What is the typical range of Φ-Index values?
A: Φ-Index values typically range from 0.1 to 2.5 cm/hour, depending on soil type, land use, and antecedent moisture conditions.

Q2: How does Φ-Index differ from infiltration capacity?
A: Φ-Index represents the average infiltration rate during the rainfall excess period, while infiltration capacity is the maximum rate at which soil can absorb water.

Q3: When should Φ-Index be used?
A: Φ-Index is particularly useful for flood forecasting, urban drainage design, and watershed management studies where simplified infiltration modeling is appropriate.

Q4: What factors affect Φ-Index values?
A: Soil type, vegetation cover, land use, antecedent soil moisture, and rainfall intensity all influence Φ-Index values.

Q5: Are there limitations to the Φ-Index method?
A: The method assumes constant infiltration capacity throughout the storm, which may not accurately represent real-world conditions with varying infiltration rates.