Phi Index for Practical Use Calculator

Φ-Index Formula:

\[ \Phi\text{-Index} = \frac{(I - R_{24h})}{24} \]

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 infiltration rate during a rainfall event.

2. How Does the Calculator Work?

The calculator uses the Φ-Index formula:

\[ \Phi\text{-Index} = \frac{(I - R_{24h})}{24} \]

Where:

\( I \) — Intensity of Rainfall (cm/hr)
\( R_{24h} \) — Runoff in Cm from 24h Rainfall (cm)

Explanation: The formula calculates the constant infiltration capacity by subtracting the total runoff from the total rainfall intensity and dividing by the duration (24 hours).

3. Importance of Φ-Index Calculation

Details: Accurate Φ-Index estimation is crucial for hydrological modeling, flood prediction, water resource management, and understanding catchment response to rainfall events.

4. Using the Calculator

Tips: Enter rainfall intensity in cm/hr and runoff in cm from 24-hour rainfall. Both values must be valid (non-negative, and intensity should be greater than or equal to runoff).

5. Frequently Asked Questions (FAQ)

Q1: What does the Φ-Index represent?
A: The Φ-Index represents the constant infiltration capacity that would produce the observed runoff volume for a given rainfall event.

Q2: How is Φ-Index different from infiltration rate?
A: While both measure water infiltration, Φ-Index is a calculated average value that produces the observed runoff, whereas infiltration rate is the actual rate at which water enters the soil.

Q3: What are typical Φ-Index values?
A: Φ-Index values vary widely depending on soil type, land use, and antecedent moisture conditions, typically ranging from 0.1 to 2.5 cm/hr.

Q4: When should this calculation be used?
A: This calculation is particularly useful for hydrological studies, flood forecasting, and designing drainage systems where understanding catchment response is important.

Q5: Are there limitations to this equation?
A: The main limitation is assuming constant infiltration capacity throughout the rainfall event, which may not reflect actual variable infiltration rates in natural conditions.