This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Water Requirements of Crops – Soil-Moisture-Irrigation Relationship”.
1. Which zone is the most important from an irrigation point of view?
a) Soil Zone
b) Intermediate Zone
c) Capillary Zone
d) Saturation Zone
View Answer
Explanation: From this soil zone plants take their water requirements which are needed during their growth. As in this zone roots of the plants are present.
2. What is the name of water which flows under gravity?
a) Hygroscopic Water
b) Gravity Water
c) Capillary Water
d) Residue Water
View Answer
Explanation: When water flows over the land, some water is absorbed in the root zone and remaining water flows underground due to the action of gravity, hence the name gravity water.
3. Field capacity is different from gravity water.
a) True
b) False
View Answer
Explanation: Immediately after application of irrigation water or sudden rain the gravity water drains down to the water table. But certain amount of water gets retained on the surfaces of soil grains and this water cannot be easily drained under action of gravity. This water is called field capacity.
4. How many days are generally taken to obtain field capacity after free gravity drainage?
a) 3 to 7 days
b) 4 to 8 days
c) 5 to 9 days
d) 2 to 5 days
View Answer
Explanation: Generally field capacity is defined as the water content of soil that is left after free gravity drainage for certain period of time. This period of free gravity drainage is generally taken as 2 to 5 days.
5. By what type of forces the soil grains retain water on their surfaces?
a) Capillary Forces
b) Cohesive Forces
c) Molecular Attraction and by Loose Chemical Bonds
d) Compressive Forces
View Answer
Explanation: Due to these forces like molecular attraction between adjacent soil grains and by their loose chemical bonds, i.e adsorption, the soil grains are able to retain water against the action of gravity on their surfaces.
6. Based on surface tension, what is the name of the part of field capacity water?
a) Hygroscopic Water
b) Gravity Water
c) Capillary Water
d) Residue Water
View Answer
Explanation: Capillary water is that part of field capacity water, which is attached to the soil molecules by surface tension against the gravitational forces.
7. Based on loose chemical bonds, what is the name of the part of field capacity water?
a) Hygroscopic Water
b) Gravity Water
c) Capillary Water
d) Residue Water
View Answer
Explanation: The water which is attached to the soil molecules due to the influence of their loose chemical bonds is called hygroscopic water. This is a part of field capacity water.
8. Capillary water is available to plants and hygroscopic water is not.
a) True
b) False
View Answer
Explanation: The capillary water can be extracted by the plants by capillary action or capillarity. But, hygroscopic water cannot be extracted by the capillarity.
9. Determine the field capacity, when the unit weight of water is 9.8 KN/m3, and volume of water stored in unit area of soil is 28 m3, the dry unit weight of soil is 18 KN/m3, and the depth of root zone of the plant is 7 m.
a) 2.178 m2
b) 2.135 m2
c) 2.125 m2
d) 2.25 m2
View Answer
Explanation: We have, field capacity (F) = (weight of water retained in unit area of soil / weight of same volume of dry soil)
Weight of water retained in unit area of soil = γw x volume of water stored in unit area
= 9.8 x 28
= 274.4 KN
Weight of same volume of dry soil = γd x d (where, γw = unit weight of water, γd = dry unit weight of soil)
= 18 x 7
= 126 KN/m2
Now field capacity (F) = (274.4 / 126)
= 2.178 m2.
10. Determine the volume of water stored in unit area if unit weight of water is 9.8 KN/m3, dry unit of soil is 17 KN/m3, field capacity (F) is 2.35 m2, and depth of root zone of the plant is 9 m.
a) 36.69 m3
b) 36 m3
c) 36.12 m3
d) 36.25m3
View Answer
Explanation: We have field capacity (F) = (γw x volume of stored in unit area of soil / γd x d)
Therefore volume of stored water = (γd. d. F / γw)
= (17 x 9 x 2.35 / 9.8) (where, γw = unit weight of water, γd = dry unit weight of soil)
= 359.55 / 9.8
= 36.69 m3.
11. Up to which point plants can extract water?
a) Saturation Point
b) Permanent Wilting Point
c) Hydration Point
d) Dissolution Point
View Answer
Explanation: At permanent wilting point plants can no longer extract sufficient water for its growth and wilts up. Plants are able to extract water from the soil till this point is reached. This is the point at which permanent wilting of plants take place.
12. What is the name of actual available water to the plants?
a) Available Moisture
b) Moisture
c) Capillary Water
d) Hygroscopic Water
View Answer
Explanation: The amount water available to the plants is the difference between field capacity water and permanent wilting point water. This amount of water is known as Available moisture or maximum storage capacity of soil.
13. By what percentage of available moisture is easily extracted by plants?
a) 80 to 90%
b) 75 to 80%
c) 70 to 80%
d) 65 to 70%
View Answer
Explanation: Readily available moisture is that part of available moisture which can be easily extracted by plants and of 75 to 80% of available moisture.
14. Which type of moisture is slightly less or most equal to field capacity?
a) Equivalent Moisture
b) Soil Moisture Deficiency
c) Available Moisture
d) Readily Available Moisture
View Answer
Explanation: Equivalent moisture is similar to field capacity, in such a way that field capacity water is retained by saturated soil after gravity whereas equivalent moisture is retained by saturated soil after applying centrifugal force 1000 times more than the gravity and centrifuged for 30 minutes.
15. Which type of moisture acts as a bridge between soil moisture content and field capacity?
a) Equivalent Moisture
b) Soil Moisture Deficiency
c) Available Moisture
d) Readily Available Moisture
View Answer
Explanation: Soil moisture deficiency is the water that is required to bring the soil moisture content of a given soil to its field capacity. It is also known as field moisture deficiency.
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