Test: Soil Moisture Relationship - Civil Engineering (CE) MCQ

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.

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.

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.

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.

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.

Capillary water is that part of field capacity water, which is attached to the soil molecules by surface tension against the gravitational forces.

We have field capacity (F) = (γ_w x volume of stored in unit area of soil / γ_d x d)
Therefore volume of stored water = (γ_d. d. F / γ_w)
= (17 x 9 x 2.35 / 9.8) (where, γ_w = unit weight of water, γ_d = dry unit weight of soil)
= 359.55 / 9.8
= 36.69 m³.

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 = γ_w x volume of water stored in unit area
= 9.8 x 28
= 274.4 KN
Weight of same volume of dry soil = γ_d x d (where, γ_w = unit weight of water, γ_d = dry unit weight of soil)
= 18 x 7
= 126 KN/m²
Now field capacity (F) = (274.4 / 126)
= 2.178 m².

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.

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.