INFILTRATION INDEX

In hydrological calculation, the average infiltration rate is called infiltration index.

Ø Index     

• The value of ø index can be derived from the rainfall hyetograph and the resulting surface runoff volume by trial and error.

• The unshaded area below the horizontal line is assumed that all losses are due to infiltration only.
• For determination of ø index, a horizontal line is drawn on the hyetograph such that the shaded area above that line is equal to the volume of surface runoff.
•  If the shaded area is not equal to the volume of measured surface runoff, the horizontal line is shifted upwards or downwards till this condition is satisfied.

Procedure for Determination of the ø Index

•  Draw the hyetograph of the storm rainfall and compute the total volume of rainfall.
•  Compute the excess runoff volume from the storm hydrograph by separating the base flow from it.
• Subtract (2) from (!) to determine the total intake of rainwater into the soil.
• Divide the value at (3) with the effective time period to get the ø index (cm/h).

FORMULA

Example Calculation

Example 1

A storm with 20cm of precipitation produced a surface runoff of 11.6cm; estimate the ø index of the storm if the data recorded is as follows.

Storm time (hour)	1	2	3	4	5	6	7	8
Incremental rainfall per hour (cm)	0.8	1.8	3.0	4.6	3.6	3.2	2.0	1.0

Solution:

P = 20cm

R = 11.6cm

t = 8 hour

However, the value of ø index is more than the rainfall recorded during the first and the eighth hour respectively, which is not possible. Therefore, by assuming again that the effective time period is 6 hours, we eliminate the chance for occurrence of any excess rainfall during the first and the eighth hour respectively. Then, for the 6 hour effective period, the infiltration loss is

= 20cm – 0.8cm – 1.0cm – 11.6cm

= 6.6cm

Therefore, 

This value of ø is not more than any of the values of precipitation from the second to the seventh hour of the storm, and is, therefore ACCEPTABLE.