3 Main Types of Movement of Water within Soil

This article throws light upon the three main types of movement of water within the soil. The types are: 1. Saturated Flow 2. Unsaturated Flow 3. Water Vapor Movement.

Type # 1. Saturated Flow:

Condition of soil, when all large and small pores, are filled with water is called saturated. Saturated flow takes place when the soil is saturated. The direction of flow is from a zone of higher moisture potential to a lower moisture potential.

Generally, water percolates down (vertically) into the lower layers. But horizontal flow also occurs with very less rapidity in comparison to vertical flow. Horizontal movement is much more evident in the clay soil, whereas vertical movement is much more evident in sandy loam.

The movement of percolation water (saturated flow) depends on five factors:

(i) Texture,

(ii) Structure,

(iii) Temperature,

(iv) Organic matter, and

(v) Pressure.

(i) Texture:

The flow of water is proportional to the size of particles. The bigger the particle, the more rapid is the flow or movement. Therefore, water percolates more easily and rapidly in sandy soils than in clay soils.

(ii) Structure:

In clay soils having single-grain structure (structure-less), the gravitational water percolates more slowly than in those having an aggregate structure (granular or crumby structure). In platy structure, saturated flow is poor in comparison to granular structure.

(iii) Organic matter:

Organic matter helps to maintain a high proportion of macro pores. Larger the pore space, greater the flow.

(iv) Pressure:

The movement of gravitational water is also influenced by the resistance offered by the entrapped soil air. As a result of pockets of air, the soil-air pressure is increased and percolation decreased. This is more especially the case in lower layers. From a practical point of view saturated flow is very important.

Type # 2. Unsaturated Flow:

Soil pores contain some air as well as water is called unsaturated soil. Under field conditions most soil-water movement occurs where the soil pores are not completely saturated with water. The soil macro pores are mostly filled with air, and the micro pores with water and some air.

Water movement under these conditions is very slow compared to that occurring when the soil is saturated. Movement will be from a zone of low suction (thick moisture film) to one of high suction (thin moisture film).

The two forces responsible for this suction are the:

(i) Attraction of soil solids for water (adhesion) and the

(ii) Capillarity.

Let us consider the situation when the water supply is stopped. It is, however, observed that the rate of flow decreases with time and at a particular time, becomes very slow, almost negligible. The lowermost portion being more moist than the uppermost one. The force due to gravity is very present.

Therefore, water at any point of the soil column is held by soil particles with a force which is equal to magnitude, by opposite in direction to that of gravity. This force is called Soil suction or soil moisture tension. Water arises from inter-particle pores (Fig. 4.8). Movement is from a zone of low suction to one of high suction or from a zone of thick moisture films to one where the films are thin.

Another explanation of capillary movement is based on free energy concept. When such movement occurs, it does so from an area where the free energy of the soil water is high to one where it is lower. Thus, water movement will occur most easily from soil areas of high moisture level where low attractive forces of the soil, material results in high free energy levels of soil water.

The movement of moisture takes place because of the difference in water potential. The portion of soil that is moist is at a low water potential, while that which is dry is at a high water potential. Moisture moves from high potential to low potential thus water rises upward in the capillary tube.

Type # 3. Water Vapour Movement:

There are two types of water vapour movement:

(a) Internal movement, the change from the liquid to the vapour state takes place within the soil, that is, in the soil pores, and

(b) External movement, the phenomenon occurs at the land surface and the resulting vapour is lost to the atmosphere by diffusion and convection (surface evaporation). The diffusion of water vapour from one area to another in soils does occur.

Water vapour moves for high vapour pressure (generally in moist soil) to low pressure (generally in dry soil). Some vapour transfer does occur within soils. The extent of the movement by this means, however, even from one continuous macro pore to another.