Water Requirement of Crops MCQ Quiz - Objective Question with Answer for Water Requirement of Crops - Download Free PDF

Concept:

The relation between duty, base period and delta is given by:

\(\Delta=\frac{8.64 \times B}{D}\)

Calculation:

B = 120 days

D = 1428 hectares/cumec

\(\Delta=\frac{8.64 \times 120}{1428}=0.726m\)

Explanation:

Gross Command Area (GCA):

• It is the total area that can be economically irrigated from an irrigation system, irrespective of water availability.

• It includes all land types within the command — both cultivable and uncultivable (roads, settlements, barren lands, etc.).

• It forms the basis for further classifications like Culturable Command Area and Net Irrigated Area.

 Additional Information

Culturable Command Area (CCA):

• Part of the Gross Command Area that is suitable for cultivation.

• Excludes uncultivable land such as forests, rocky terrain, and infrastructure.

• Water availability is also not a limiting factor in its definition.

Gross Cultivable Area:

• Refers to the total cultivable land in a region, which may not be linked to a specific irrigation system.

• It includes both rain-fed and irrigated areas.

Mean Sea Level (MSL):

• A standard reference for measuring elevations and altitudes.

• It has no relevance in defining irrigable command areas.

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
\(γ_d = 14 kN/m³\)
\(γ_w = 10 kN/m³\)

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

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

Concept: 

Storage Efficiency of Irrigation

It is a measure of how effectively water is stored in the root zone during irrigation compared to the amount of water that was required in the root zone before irrigation. It is calculated using the ratio of water store to the water required, expressed as a percentage.

Calculations:

Given: Water stored in the root zone = 60 cum

Water required in the root zone prior to irrigation = 80 cum

Storage efficiency = (60/80) x 100 % = 75 %

Concept:

Time factor: 

It is the ratio of no. of days canal has actually run during a watering period to that of no. of days of watering period.

Calculation:

Given:

Time factor = 0.75

Watering period = 120 days

The number of days for which a distributary will receive full is  0.75 × 120 = 90 days.

Explanation

We know,

a) Consumptive Irrigation Requirements (CIR) = Consumptive Use (Cu) – Effective Rainfall (Re)

b) Net Irrigation Requirement (NIR) = CIR + other requirements such as leaching

c) Field Irrigation Requirements (FIR) = NIR + Surface runoff losses + deep percolation losses

Or

\({\rm{FIR}} = \frac{{{\rm{NIR}}}}{{{\rm{Water\;application\;efficiency}}}}\)

d) Gross Irrigation Requirements (GIR) = FIR + Conveyance Losses (Seepage and Evaporation)

Or 

\({\rm{GIR}} = \frac{{{\rm{FIR}}}}{{{\rm{Water\;Conveyance\;efficiency}}}}\)

If you look through the above equations we can conclude that

 GIR > FIR, FIR > NIR, NIR > CIR 

∴ GIR > FIR > NIR > CIR

Concept:

Water distribution efficiency \(= {{\rm{\eta }}_{\rm{d}}} = \left[ {1 - \frac{{\rm{y}}}{{{{\rm{d}}_{\rm{w}}}}}} \right] \times 100{\rm{\;}}\left( {{\rm{in\;\% }}} \right)\)

\({{\rm{d}}_{\rm{w}}} = \frac{{{{\rm{d}}_1} + {{\rm{d}}_2} + \ldots + {{\rm{d}}_{\rm{n}}}}}{{\rm{n}}},\) d_w = average depth of water stored

\({\rm{y}} = \frac{{y_1 \ + \ y_2\ + \ ......\ +\ y_n}}{n}\), y = Average of absolute value of deviations

y₁, y₂,......., y_n = absolute deviations from average depth of water stored

Calculation:

\({\rm{d_w}} = \frac{{1.5 + 1.8 + 2.1}}{3} = 1.8\)

_y1 = 0.3 m, y₂ = 1.8 – 1.8 = 0, y₃ = 2.1 – 1.8 = 0.3 m

\({\rm{y}} = \frac{{0.3 + 0.3 + 0}}{3} = 0.2\)

Duty: It is the number of hectares of land irrigated for full growth of a given crop by a supply of 1 cumec of water continuously during the entire base period of that crop.

Duty of water changes from place to place, it will be maximum at the field and minimum at the head of the main canal.

Duty is the area that can be irrigated by the discharge of 1 cumec of water.

At the head of the canal, there are numerous losses to occur later which requires more amount of water to irrigate a particular field. However, if considered on the field, all losses have already occurred and a lesser amount of water is required to irrigate the same considered area.

Concept:

For an irrigation land:

SC = Saturation capacity, FC = Field capacity, OMC = Optimum moisture content, PWP = Permanent welting point and UWP = Ultimate welting point

1. Equivalent depth of water held at field capacity (x) = S × d × Fc

2. Equivalent depth of water held at PWP (x’) = S × d × (PWP)

3. Available moisture/storage capacity of soil (y) = S × d × (Fc - PWP)

4. Readily available moisture content (dw) = S × d × (Fc - OMC)

Given:

Field capacity (F. C) = 25 % = 0.25

Permanent willing point (PWP) = 15 % = 0.15

Depth of the root zone (d) = 80 cm

Dry Unit weight of soil (γ_d) = 1.5 g/cc

Calculations:

The storage capacity \(= \frac{{{\gamma _d} \times d \times \left( {F.C - PWP} \right)}}{{{\gamma _w}}}\)

The storage capacity \(= \frac{{1.5 \times 80 \times \left( {0.25 - 0.15} \right)}}{1} = 12\;cm\)

Hence, the storage capacity of the soil is 12 cm.

Explanation:

Alkali or alkaline soil:

• Alkaline soils are clay soils with high pH (> 8.5), a poor soil structure, and a low infiltration capacity. 
• it usually contains a great deal of sodium, calcium, and magnesium ions.
• Alkaline soil is less soluble than acidic or neutral soil, availability of nutrients is often limited.
• In arid or desert areas where rainfall is slim and places where there are dense forests, soil tends to be more alkaline.

Classification of different types of soil:

Explanation: 

Frequency of irrigation: It is defined as the ratio of available soil moisture depletion to rate of consumptive use. 

Wilting coefficient: The moisture content at which plant can no longer extract the water from the soil for their growth and finally wilts up, It is known as the wilting coefficient. 

Explanation:

Annual irrigation intensity:

By adding intensities of irrigation for all the crop seasons we obtain the annual intensity of irrigation.

Annual intensity of irrigation can be more than 100%.

The annual irrigation intensity is usually found to be in the range of 40 to 60%. But needs to be raised in the range of 100 to 180% by cultivating larger parts of CCA with more than one crop in a year.

Important Points

Intensity of irrigation is the percentage of Culturable Commanded c (CCA) to be irrigated annually for a particular crop season.

CCA is portion of gross commanded area that is cultivable.

CCA = Gross Command Area – Uncultivable area (pasture lands, ponds, townships, waste land)

Concept:

Duty:

It is the area of land that can be irrigated with a unit volume of water supplied across the base period. It is generally expressed in hectare/cumec.

Delta:

It is the depth of water required to a crop over a unit area across the base period for full growth of the crop. It is generally expressed in cm or m.

Calculation:

Given:

Dicharge (Q) = 3 cumec

Area of land (A) = 5100 hectare

Than, duty = 5100/3 = 1700 hectare/cumec

For delta:

Water depth = 10 cm

Number of watering = 12

Then, delta = 12 x 10 = 120 cm

Concept:

Gross Irrigation Requirement, GIR = \(\frac{{Net\;Irrigation\;Requirement\left( {NIR} \right)}}{{{\eta _a} \times {\eta _c}\;}}\)

Where η_a and η_c are the application and conveyance efficiency respectively.

Calculation:

Given,

ηa = 80 %

ηc = 60 %

NIR = 15 cm

\({\rm{GIR}} = \frac{{15\;cm}}{{0.8 \times 0.6}} = 31.25\;cm\)

Concept:

SC = Saturation capacity, FC = Field capacity, OMC = Optimum moisture content, PWP = Permanent welting point and UWP = Ultimate welting point

• Field capacity is the water that cannot be easily drained under the action of gravity and having its presence in the root zone.
• The permanent Wilting point is the water content in the root zone below which the roots can no longer extract water for their growth and dries up.

• Readily available moisture is that portion of available moisture that can be most readily extracted by plants.

• In general, readily available moisture is approx 75 % of the available moisture.

• The difference in water content of soil between field capacity and the permanent wilting point is called available moisture.