Question

A reservoir with a live storage of 300 million cubic metre irrigate 40000 hectares (1 hecatre = 10⁴ m²) of a crop with two fillings of the reservoir. If the base period of the crop is 120 days, the duty for this crop (in hectares per cumec, round off to integer) will then be _______

Answer 1

Concept:

Duty: The duty of water is the relationship between the volume of water and the area of the crop it matures. It is defined as the area irrigated per cumec of discharge running for base period B. The duty is generally represented by D.

 \(D = \frac{{8.64B}}{\Delta }\)

Where,

Δ = Total depth of water required (in meter)

D = Duty in ha/cumec

B = Base period (in days)

Calculation:

Given:

B = 120 days

Live storage = 300 Mm³

Crop Area = 40000 hectares

Since the tank gets filled twice a year.

∴ Total Volume of water required for the crop area = 2 × 300 = 600 Mm³

⇒ Total depth of water required (Δ), \(\Delta = \frac{{600 \times {{10}^6}}}{{400 \times {{10}^4}}} = 1.5\ m\)

⇒ \(\rm D = \frac{{8.64 \times 120}}{{1.5}} = 691.2\ ha/cumec\)