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.

Month	Jan	Feb	Mar	Apr	May	June	July	Aug	Sep	Oct	Nov	Dec
Temp (°C)	12	16	21	27	31	34	31	29	28	29	19	14
Rainfall	4	4	2	0	2	12	32	29	16	2	1	2

Solution:

In Khosla’s formula,

R_m = P_m – L_m

Where,

• R_m = Monthly runoff
• P_m = Monthly rainfall in cm
• L_m = (0.48 × T) Or P_m whichever is less.
• T = Monthly temperature in degrees.

1. For January month

L_m = (0.48 × 12) = 5.76

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

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

Now,

R_m = (P_m – L_m) = (4 – 4) = 0

2. For July month

L_m = (0.48 × 31) = 14.88

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

So, we will take the value of L_m = 14.9

Now,

R_m = (P_m – L_m) = (32 – 14.88) = 17.1

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

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Rainfall (Pm)	4	4	2	0	2	12	32	29	16	2	1	2
Temp (°C)	12	16	21	27	31	34	31	29	28	29	19	14
Lm	4	4	2	0	2	12	14.9	13.9	13.4	2	1	2
Rm	0	0	0	0	0	0	17.1	15.1	2.6	0	0	0

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

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