One of the methods to compute runoff is  "infiltration method" explained below.
Infiltration Capacity

Maximum rate at which water enters the soil in a given condition 

Infiltration Rate 

Rate at which water actually enters the soil during a storm and is equal to the infiltration capacity or the rainfall rate, whichever is less 

Infiltration Index  

Average rate of loss such that volume of rainfall in excess of that rate will be equal to direct runoff 

Infiltration Indices

—W-Index or Average Infiltration Rate

W = (P-R)/tr cm/hr and k = (i-W)/i

W = Index cm/hr,

P = Precipitation, cm

R = Runoff, cm

tr = duration of infiltration, hr

k = Runoff Coefficient

i = Rainfall intensity, cm/hr

Phi Index

Rate of rainfall above which rainfall volume equals the runoff volume


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