# How to Calculate Annual Runoff? Problem & Solution

## How to Calculate Annual Runoff?

Mathematical Problem:

For a catchment in M.P., India, the mean monthly rainfall and temperatures are given. Calculate the annual runoff by Khosla’s formula.

Solution:

In Khosla’s formula,

Rm = Pm – Lm

Where,

• Rm = Monthly runoff
• Pm = Monthly rainfall in cm
• Lm = (0.48 × T) Or Pm whichever is less.
• T = Monthly temperature in degrees.

1. For January month

Lm = (0.48 × 12) = 5.76

So, 5.76 is greater than the corresponding Pm = 4.

So, the value of Lm will be taken as 4.

Now,

Rm = (Pm – Lm) = (4 – 4) = 0

2. For July month

Lm = (0.48 × 31) = 14.88

So, 14.88 is less than the corresponding value of Pm = 32

So, we will take the value of Lm = 14.9

Now,

Rm = (Pm – Lm) = (32 – 14.88) = 17.1

I think you can able to solve the rest of the months by using the above method.

So, the total annual runoff = ( 17.1 + 15.1 + 2.6) = 34.8 in cm.

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