Because the magnitude of the φ index is bounded between the maximum and minimum average infiltration capacities of the catchment during a rainfall event.

Nilesh Patidar Thank you for replying. I still don't understand how does the duration of rainfall excess comes into the picture? Phi-index should still be equivalent to depth of infiltration/time of infiltration.

Harsh Upadhyay lets go to the basics..

what is phi index ?.. The φ index of a catchment is defined as the constant infiltration capacity that would yield the actual total runoff for a given rainfall amount.

Now what is Rainfall Excess ..?

Rainfall that is neither retained on the land surface nor infiltrated into the soil.

NOW

phi-index = (P - R)/te

P = depth of total precipitation, R = depth of rainfall excess and te = duration of rainfall excess.

Now what is (P-R)..??.. Rainfall which infiltrate into the ground in Te time which is duration of rainfall excess..?

This is how duration of rainfall excess comes into the picture.

Nilesh Patidar So can I say that phi-index is that infiltration rate above which rainfall excess is produced unconditionally. Like every phi-index is an infiltration rate but not every infiltration rate can be a phi-index. For e.g., the infiltration rates which are higher than a given rainfall intensity thus, not producing rainfall excess, can't be considered as phi-index?

You can consult this ------

"Computing Excess Rainfall using Phi Index - SERC - Carleton

https://serc.carleton.edu › steps › computing_exces

26-Feb-2020"