The phi-index (phi) method for estimating direct runoff from a rainfall event is defined as:
Choose the correct answer
The average rainfall intensity over the event
The constant infiltration rate (uniform loss rate) above which the total rainfall excess equals the observed direct runoff depth
The initial loss due to interception
The baseflow abstraction rate
Correct Answer
B. The constant infiltration rate (uniform loss rate) above which the total rainfall excess equals the observed direct runoff depth
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Phi-index (phi): assume a constant loss rate phi (mm/h). Rainfall exceeding phi at each time step contributes to runoff. Find phi by trial so that: sum of (I - phi) x dt (for I > phi) = observed storm runoff. Simple but assumes uniform soil throughout the catchment. W-index includes initial infiltration capacity.
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