Measurement or Computation of Runoff- Runoff Calculation

You know that runoff is the stormwater that flows through the surface of the earth to a reservoir or dam. In this article, I am going to teach you how to do measurement of runoff.

We have learned what are the factors and effects of runoff in an earlier post.If you haven’t read it, please move through that topic also…

There are several formulas for runoff calculation.Before that, you should have to learn other terms which is related to run off.

Infiltration is the movement of water through the pores of soil and the rate of flow is called infiltration rate. During the initial time, the infiltration rate will be equal to the rainfall rate. The infiltration capacity is the maximum rate at which water enters through the soil at a given condition.

The capacity of a soil to absorb water from rainfall falling continuously at an excessive rate goes on decreasing with time. Therefore the infiltration rate will start decreasing and infiltration rate will be equal to the minimum of infiltration capacity or rainfall rate

Computation of Runoff

I going to give you several formulas for the easy calculation of runoff.

1.Runoff Coefficient.

The runoff and rainfall can be linked with runoff coefficient.


R is the runoff in cm

P is the rainfall in cm

k is the runoff coefficient

The value of runoff coefficient will change from different place to place depeding upon the type of ground or type of soil.Below we have mentioned several values for k.

Area  K value
Urban areas 0.4
Commercial,industrial 0.9
Forested areas 0.2
Parks,farm land 0.3
Asphalt or concrete 0.85

2.Inglis formula

Inglis gives the formula for ghat areas and non-ghat areas.

For ghat areas

R=0.85P -12”( When R and P are in inches)

R=0.85pP-30.5 ( When R and P are in cm)

For Non ghat areas

R= ( (P – 7)  100) × P ( When R and P are in inches)

R=  ( (P – 17.8)  254  )  × P ( When R and P in centimetres)



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