Question

Difference between phi index and w index

Answer 1

The "W index is actually the average rate of infiltration  and it is when the intensity of the rainfall increase and go beyond the rate of infiltration .You can calculate by either simple mathematical calculation  or can also calculate by using the data of  runoff as well as the rainfall that is observed. 

While the "Phi index'" is also the rate of infiltration but it is when the infiltrate rate exceeds the  value at which  volume  of runoff become equals to the volume of rainfall.

Answer 2

In Hydrology, the average value of infiltration (the process of water on the ground entering the soil) is known as “Infiltration Index”. 
There are 2 types of infiltration indices – Phi index and W index.

Phi-index – It is the rate of infiltration exceeding which the volume of rainfall becomes equal to the runoff volume.

W-index – It is the average infiltration rate that is calculated during the time when the intensity of rainfall is exceeding the infiltration rate.

The W-index is much more accurate than the phi-index because it subtracts the initial losses also.