Question

A crop requires a total water depth of 90 cm for a base period of 120 days. The discharge required to irrigate the crop in 2880 hectares is
1. 0.25 m³ / s
2. 1.0 m³ / s
3. 2.5 m³ / s
4. 10.0 m³ / s

Answer 1

Correct Answer - Option 3 : 2.5 m³ / s

Concept:

Duty: The area of land expressed in hectares that can be irrigated with unit discharge, that is, 1 cumec flowing throughout the base period, expressed in days.

Base period: The time between the first watering of a crop at the time of its sowing to its last watering before harvesting is called the base period or the base of the crop.

Crop period: The time period that elapses from the instant of its sowing to the instant of its harvesting is called the crop period.

Note:

The crop period is slightly more than the base period, but for all practical purposes, they are taken as one and the same thing, and generally expressed in days.

Delta: The total quantity of water required by the crop for its full growth may be expressed in hectare metre (ha.m) or simply as depth to which water would stand on the irrigated area if the total quantity supplied were to stand above the surface without percolation or evaporation. This total depth of water (in cm) required by a crop to come to maturity is called its delta (Δ).

If the water supplied is just enough to raise the crop within D hectares of the field, then a relationship may be found out amongst all the variables as:

Volume of water supplied = B × 60 × 60 × 24 m³, Here B = Base period of the crop

Area of crop irrigated = D × 10⁴ m²

Volume of water supplied per unit area = \(\frac{{86400 \times B}}{{1000 \times D}} = \frac{{8.64 \times B}}{D}\)

Calculation:

Given,

Total water depth = 90 cm

Base period = 120 days.

Area = 2880 hectares = 2880 x 10⁴ m²

Discharge = \(\frac{{Volume}}{{Time}} = \frac{{2880\times10000\times0.9}}{{120\times86400}} = 2.5\) m³/s