Duty of canal water is calculated: 

Concept:

Duty (D): 

• It is defined as the area irrigated per cumec (m3/s) of discharge running for the base period.

\(D={Area(A)\over Discharge(Q)}\)

Where area (A) is in hectares, discharge is in m3/s, and duty (D) is in hac/cume.

• Canal water duty can be calculated at all important control points of the canal system;

1. At the headworks, the canal emerges from the source.
2. At the junction of a canal and a branch canal.
3. At an outlet through which water is ultimately supplied to the farmers.

Hence option (4) is the correct answer.