Wheat is to be grown in a field having field capacity equal to 30% and the permanent wilting point equal to 10%. Irrigation water is to be supplied when the moisture content of the soil falls to 20%. The root zone depth is 1 m , dry weight of soil is 14 kN/m³and the specific weight of water is 10 kN/m³. The Net Irrigation Requirement (NIR) for the above cultivation is:

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Wheat is to be grown in a field having field capacity equal to 30% and the permanent wilting point equal to 10%. Irrigation water is to be supplied when the moisture content of the soil falls to 20%. The root zone depth is 1 m , dry weight of soil is 14 kN/m³and the specific weight of water is 10 kN/m³. The Net Irrigation Requirement (NIR) for the above cultivation is:

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28 cm of water
19.6 cm of water
21 cm of water
14 cm of water

Answer 1

Concept:

Net Irrigation Requirement (NIR) is the depth of water required to restore the soil moisture in the root zone to field capacity after depletion.
It is calculated as the difference between moisture at field capacity (FC) and moisture at the time of irrigation, multiplied by root zone depth and adjusted for the weight of soil and water.

\(NIR=(FC-MC)\times RootZoneDepth\times\frac{\gamma_d}{\gamma_w}\)

where:
FC = Field Capacity (%),
MC = Moisture content at time of irrigation (%),
\(\gamma_d\) = Dry unit weight of soil (kN/m³),
\(\gamma_w\) = Unit weight of water (kN/m³).

Calculation:

Given:
Field Capacity (FC) = 30%
Permanent Wilting Point (PWP) = 10%
Moisture at time of irrigation (MC) = 20%
Root zone depth = 1 m = 1000 mm
\(γ_d = 14 kN/m³\)
\(γ_w = 10 kN/m³\)

\(NIR=(30-20)\times1\times\frac{14}{10}\)

\(NIR=10\times1\times1.4=14cm\)

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