Movement of Water and Vapour in the Soil

After reading this article you will learn about the movement of water and vapour in the soil.

Movement of Liquid Water:

Liquid water moves in the soil under saturated and unsaturated conditions.

Saturated flow of water:

Soil is saturated with water which means that even the macrospores are full of water, and water at the surface of the soil possesses a high positive potential with respect to the water located at the lower level of the soil profile. As water moves from the state of higher positive potential to the state of lower the adjacent drier soil. But water can only flow through the pore spaces which are very irregular or even discontinuous in shape.

Saturated flow of water depends on the difference of potential which is the force causing the flow of water between different points the soil and the case with which the soil allows the ready movement of water. This latter factor is called the Hydraulic Conductivity of the soil which depends on the macrospores and the viscosity of water.

Unsaturated Flow of Water:

The soil is unsaturated with water which means that the macrospores are filled up with air and the micropores are filled up with water. Water flows through micropores and the surface of the soil particles from a zone of lower negative potential to another zone of a higher negative potential.

Clay and humus attract water molecules. Water moves from moist soil where it is under a lower negative potential to dry soil. The water absorbed by dry clay possesses a very high negative potential.

Movement of Water Vapour:

The movement of water vapour is of two kinds:

(i) Mass flow:

Water vapour flow in a mass along with the movement of other soil gases. When the atmospheric pressure decreases soil gases including water vapour expand and move out the soil. When the atmospheric pressure is increased, soil gases and water vapour contracts and therefore gases and water vapour enter the soil.

(ii) Diffusion:

Diffusion is the property by which two gases readily mix with each other when they are brought in contact with each other. Vapour pressure of water is a measure of its tendency to evaporate. Water evaporates from the moist soil and the vapour diffuses into the adjacent dry soil.

Water also evaporates from warm soil and the water vapour diffuses into the adjacent cool soil. Diffusion of water vapour due to the difference in relative humidity may balance the diffusion of water vapour due to the difference in temperature so that the net movement of water vapour may be zero as shown in Fig. 5.4A.

Water vapour moves from one zone to another zone only when both of them work in one direction only as shown in the Fig. 5.4B.