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