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Duration of Rainfall Excess is the total time during which the rainfall intensity exceeds the average rate of infiltration (measured in hours). It represents the period when surface runoff occurs due to rainfall exceeding the soil's infiltration capacity.
The calculator uses the formula:
Where:
Explanation: The formula calculates the time duration during which rainfall intensity exceeds the infiltration capacity, resulting in surface runoff.
Details: Calculating rainfall excess duration is crucial for hydrologic analysis, flood forecasting, and designing drainage systems. It helps determine the period when surface runoff occurs, which is essential for water resource management and flood control planning.
Tips: Enter total storm precipitation and total direct runoff in centimeters, and Φ-Index in cm/h. All values must be valid (P ≥ Rd, φ > 0).
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. It represents the average infiltration rate during a storm event.
Q2: How is direct runoff different from total runoff?
A: Direct runoff is the portion of runoff that enters stream channels promptly after rainfall or snowmelt, while total runoff includes both direct runoff and baseflow.
Q3: What factors affect the duration of rainfall excess?
A: Soil type, land use, antecedent moisture conditions, rainfall intensity, and duration all affect the duration of rainfall excess.
Q4: Can this calculation be used for all types of storms?
A: This calculation works best for single storm events with relatively uniform rainfall intensity. For complex storm patterns, more sophisticated hydrologic models may be needed.
Q5: What are typical values for Φ-Index?
A: Φ-Index values vary widely depending on soil type and conditions, typically ranging from 0.25 cm/h for clay soils to 2.5 cm/h or more for sandy soils.