Movement of Water in Soil: 3 Types

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

Type # 1. Saturated Flow:

Water moves because of water potential gradients in the soil caused mostly by gravity, salt content and water usage and the direction of flow is from a zone of higher to zone of lower moisture potential. When soil water moves mainly due to gravity, which is at moisture potentials greater than -1/3 bar, the movement is called “Saturated flow”.

Saturated flow starts with water infiltration, which is the movement of water into the soil when precipitation of irrigation water is on the soil surface. When the soil profile is completely saturated with water, the movement of more water flowing through the saturated soil is termed percolation.

The flow of water under saturated conditions is determined by two major factors—the hydraulic force driving the water through the soil and the ease with which the soil pores permit water movement.

This can be expressed as follows:

V = kf

where, V = is the total volume of water moved per unit time,

f = is the water moving force and

k = is the hydraulic conductivity of the soil or permeability of the soil.

The hydraulic conductivity or permeability of the saturated soil is essentially constant being dependent on the size and configuration of the soil pores. The driving force, hydraulic gradient, is the difference in height of water above and below the soil column (hydraulic head difference).

The volume of water moving down the soil profile will depend upon the forces due to driving force and hydraulic conductivity of soils. In this flow of water, more vertical flow occurs than that of horizontal flow because the force of gravity does not help the later flow.

An equation of this same type was used by Darcy and is presented below:

Qw = (∆dw)At/(∆ds)

Qw = Quantity of water in c.c.

or

k = Qw(∆ds)/At(∆ds)

where k = Rate constant or hydraulic conductivity in cm sec^-1

∆dw = Water head in cm

A = Soil area in sq cm

t – Time in seconds

∆ds = Soil depth used in cm.

*[Since flow is downward and water thus loses potential (work must be done to put the water back where it started), the downward values are negative]

Darcy stated that the rate of flow was increased with an increased depth of water above the bottom of the soil through which it flowed. The flow decreased with an increased depth of soil through which water flowed. The hydraulic conductivity (k) will be different for the different types of soils.

Factors Affecting Saturated Flow of Water:

(a) Texture:

The flow of water is proportional to the size of the soil particles. The larger the size of the particles, the more rapid is the rate of movement of water. Sandy soils generally have higher saturated conductivities than fine textured clay soils.

(b) Structure:

Soils with stable granular structure conduct water much more rapidly than that of unstable structural units. In platy soil structure saturated flow of water is poor in comparison to spheroidal soil structure.

(c) Organic Matter:

Organic matter helps to maintain a high proportion of macro-pores in soils which increases the saturated flow of water.

(d) Inorganic Colloids:

Some types of clay are especially conducive to fine pores. Soils high in montmorillonite (2:1) generally have low conductivities compared to soils with 1: 1 type of clays.

(e) Pressure:

The movement of gravitational water is also influenced by the resistance offered by the entrapped air in the soil. As a result of pockets of air, the soil-air pressure is increased and percolation decreased and this usually found in the sub-soil horizons.

Type # 2. Unsaturated Flow:

It is the flow of water held with water potentials lower than -1/3 bar. Water will move toward the region of lower potential (towards the greater “pulling” force). In a uniform soil this means that water moves from wetter to drier areas. The water movement may be in any direction (Fig. 7.13).

The rate of flow is greater as the water potential gradient (the difference in potential between wet and dry) increases and as the size of water filled pores also increases.

The two forces responsible for this movement are the attraction of soil solids for water (adhesion) and capillarity. Under field conditions this movement occurs when the soil macro-pores (capillary) pores filled with air and the micro-pores (capillary) pores with water and partly with air.

Due to presence of air phase in the soil system of unsaturated conditions the equation for the movement of water through the saturated soil can be modified as follows by introducing a dimensionless factor λ with k. The factor λ varies from 0 to 1 and disappears when the soil is saturated. In this flow gravity, matric and pressure potentials are involved.

v = -k λ grad H

The negative sign appears in order to make the flux positive, since flow is from regions of higher potential to region of lower potential which give rise to gradients with negative signs.

H = H_M + H_P + z = φ + z, where, if z = 0

at the water table, H is positive below and negative above the water table. It is composed of two parts: (a) the pressure potential φ, which above the water table is the matric potential (H_M), below the water table is a potential due to positive water pressure head (H_P), and (b) the gravity potential (H_Z) or merely z in head units, which accounts for potential changes associated with change in elevation.

The symbol ‘grad’ denotes the gradient of H in all directions so that it can be replaced with φ + z as follows:

where the value of kλ depends only upon the nature and water content of soil pores at the point v and is independent of direction of flow, v is positive when flow is in the positive x or positive z directions and also z is taken as positive in the upward direction. Again negative v denotes downward flow of water.

Factors Affecting the Unsaturated Flow:

Unsaturated flow is also affected in a similar way to that of saturated flow. Amount of moisture in the soil affects the unsaturated flow. The higher the percentage of water in the moist soil, the greater is the suction gradient and the more rapid is the delivery.

Type # 3. Water Vapour Movement:

The movement of water vapour from soils takes place in two ways:

(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.

The movement of water vapour through the diffusion mechanism takes place from one area to other soil area depending on the vapour pressure gradient (moving force). This gradient is simply the difference in vapour pressure of two points a unit distance apart. The greater this difference, the more rapid is the diffusion and the greater is the transfer of water vapour during a unit period.

Soil conditions affecting water vapour movement. There are mainly two soil conditions that affect the water vapour movement namely moisture regimes and thermal regimes. In addition to these, the various other factors which influence the moisture and thermal regimes of the soil like organic matter, vegetative cover, soil colour etc. also affect the movement of water vapour.

The movement takes place from moist soil having high vapour pressure to a dry soil (low vapour pressure). Similarly the movement takes place from warmer soil regions to cooler soil region (Fig. 7.14).

In dry soils some water movement takes place in the vapour form and such vapour movement has some practical implications in supplying water to drought resistant plants.