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The runoff calculation formula estimates the amount of surface runoff in centimeters from a 24-hour rainfall event based on soil type coefficient and rainfall intensity. This is particularly useful for determining the Phi Index in practical hydrological applications.
The calculator uses the formula:
Where:
Explanation: The formula calculates runoff based on the non-linear relationship between rainfall intensity and surface runoff, with the soil coefficient accounting for different soil permeability characteristics.
Details: Accurate runoff estimation is crucial for hydrological modeling, flood prediction, water resource management, and designing drainage systems. The Phi Index derived from this calculation helps in determining infiltration rates and designing effective water management systems.
Tips: Enter the soil coefficient (α) and rainfall intensity in cm/hr. Both values must be positive numbers. The soil coefficient typically ranges from 0.1 to 0.9 depending on soil type and conditions.
Q1: What is the Phi Index?
A: The Phi Index represents the constant infiltration rate that would yield the measured runoff volume for a given rainfall event. It's a practical parameter used in hydrology to simplify complex infiltration processes.
Q2: How is the soil coefficient determined?
A: The soil coefficient (α) depends on soil type, land use, and antecedent moisture conditions. It's typically derived from empirical studies and varies from sandy soils (lower values) to clay soils (higher values).
Q3: What are typical values for rainfall intensity?
A: Rainfall intensity can vary widely from light drizzle (0.1-0.5 cm/hr) to heavy storms (2.5+ cm/hr). Extreme events can reach intensities of 10+ cm/hr in tropical regions.
Q4: Are there limitations to this formula?
A: This formula provides an empirical estimation and may not account for all factors affecting runoff, such as slope, vegetation cover, or initial abstraction. It works best for preliminary calculations and should be validated with field measurements.
Q5: How accurate is this calculation for practical use?
A: The formula provides a reasonable estimate for many practical applications, but for critical engineering designs, more comprehensive hydrological models incorporating additional factors should be used.