SOIL WATER- SATURATED AND UNSATURATED FLOW

1. SOIL WATER- SATURATED AND
UNSATURATED FLOW
NAMITHA M R
2015664502
M. TECH (LWME)
TNAU
SAC 604. SOIL PHYSICS (2+1)

2. SOIL WATER
• Soils contain considerable amounts of water.
• Soil can take in water, and will keep doing so until the rate at
which they can transmit water into and through the pores is
exceeded.
• Much of the water will be retained, away from the influence
of gravity, for use of plants and other organisms to contribute
to land productivity and soil health

3. Soil water retention
• Pores provide for the passage and/or retention
of gases and moisture within the soil profile.
• Soil water retention depends on:
– Particle size
– Clay type
– Organic content
– Soil structure

4. Types of soil water
• 3 basic types:
– Gravitational water --- -1/3 bar
– Capillary water --- -1/3 to -31 bars
– Hygroscopic water --- -10,000 bars

5. Contd…
– Gravitational water:
– Moves through the soil due to the force of gravity.
– Found in the macropores.
– Not considered to be available to plants.
– Drains out of the soil in 2-3 days

6. Contd…
– Capillary water:
– Found in the micropores.
– Most, but not all, of this water is available for plant growth
– Held in the soil against the pull of gravity
– Capillary water is held by cohesion and adhesion
– Amount of water held is a function of the pore size and
pore space.
– Tension (in bars) increases as the soil dries out.

7. Contd…
– Hygroscopic water:
– Forms very thin films around soil particles.
– Not available to the plant.
– Not held in the pores, but on the particle surface.
– Clay will contain much more of this type of water than sands
because of surface area differences.
– Held very tightly, by forces of adhesion this water is not
available to the plant.

8. Soil moisture constants
• Soil moisture is always being subjected to pressure
gradients and vapour pressure differences that cause it
to move.
• Certain moisture contents are of particular significance
in agriculture --- Soil moisture ‘constants’.

9. Contd…
• Saturation capacity:
• When all the pores of the soil are filled with water, the soil
is said to be under saturation capacity or maximum water
holding capacity.
• Tension of water at saturation capacity is almost zero and
it is equal to free water surface.

10. Contd…
• Field capacity:
• The field capacity is the upper limit of available moisture
range in soil moisture and plant relations.
• Large soil pores are filled with air, the micro pores are filled
with water.
• Soil moisture tension at field capacity varies from soil to soil,
but it generally ranges from 1/10 to 1/3 atmospheres.

11. Contd…
• Moisture equivalent:
• Moisture equivalent is defined as the amount of water
retained by a sample of initially saturated soil material
after being subjected to a centrifugal force of 1000
times that of gravity for a definite period of time,
usually half an hour.

12. Contd…
• Permanent wilting percentage:
• Also known as permanent wilting point or wilting co-efficient
• Soil moisture content at which plants can no longer obtain
enough moisture to meet transpiration requirements; and
remain wilted unless water is added to the soil.
• Moisture tension of a soil at the permanent wilting point
ranges from 7 to 32 atmospheres.

13. Contd…
• Wilting range:
• Range in soil-moisture content through which plants undergo
progressive degrees of permanent or irreversible wilting, from
wilting of the oldest leaves to complete wilting of all leaves.
• Moisture content at which the wilting is complete and the
plants die is called the ultimate wilting.
• At the ultimate wilting point soil-moisture tension may be as
high as 60 atmospheres.

14. Contd…
• Available water:
• Soil moisture between field capacity and permanent wilting
point
• It is the moisture available for plant use.
• Fine-textured soils have a wide range of water between field
capacity and permanent wilting point than coarse textured
soils.

15. Fig: Soil moisture constants

16. Table: Range of available water holding capacity of soils

17. Fig: Response of water to different soils

18. Movement of water within soils
• Moves along gradients of decreasing water potential.
• Water in the liquid phase:
– flows through the water filled pore space under the influence
of gravity.
– moves under the influence of surface tension forces (under
unsaturated conditions).
• Water in vapour phase:
– moves through the air-filled pore spaces along gradients of
decreasing vapour pressure.

19. Contd…
• Flow rate of water depends on:
– Gradients in soil water potential (Ψm) caused by
differences in height, pressure, dissolved solutes and soil
wetness.
– Hindrances: friction between water and particle surfaces
as well as pore constrictions and other interruptions in
flow path

20. Contd…
• It is generally recognized that three types of water
movement occur in soil:
– Saturated
– Unsaturated
– Vapour

21. SATURATED FLOW OF SOIL WATER
• Saturated flow occurs when soil pores are completely filled
with water (i.e.Ψm > 33 J/Kg).
• Occur in aquifers (water - bearing sediments and rock layers),
in flooded soil and in lower horizons of soil with limited
drainage
• Pore size is of outstanding significance, as its fourth power is
proportional to the rate of saturated flow

22. Contd…
• Saturated flow decreases as the pore size decreases.
• Generally the rate of flow in soils of various textures is
in the following sequence.
Sand > loam > clay
• Poiseuille’s law forms the basis.

23. UNSATURATED FLOW OF SOIL WATER
• Water flow when larger pores in soil are filled with air is said
to be unsaturated.
• Smaller pores hold and transmit water.
• Contribution of hydraulic head or the gravitational
component to total potential becomes progressively less
• Negligible effect of solute potential is due to the fact that
both solutes and water are moving.

24. Contd…
• Driving force for water flow in these conditions is through
the matric potential gradient (i.e.Ψm)
• Movement will therefore be in the direction of moist soil
to dry soil and from thick moisture- to thin moisture-films
• In horizontal movement, only ψm applies.
• In downward movement, capillary and gravitational
potentials act together.

25. Contd…
• In upward capillary movement ψm and ψg oppose one
another.
• For unsaturated flow may be rewritten as:
V=
− k Δ (ψm + ψg)
ΔI
• Darcy’s law is applicable if k is regarded as a function of water
content.

26. Contd…
• As the soil moisture content and soil moisture potential
decreases, the k decreases rapidly, so that ψsoil is – 15 bars,
k is only 10-3 of the value at saturation.
• Rapid decrease in conductivity occurs because the larger
pores are emptied first, which greatly decreases the cross-
section available for liquid flow.

27. Contd…
• Unsaturated conductivity is a function of soil moisture
content , number, size and continuity of soil pores
• When the continuity of the films is broken, liquid flow no
longer occurs.
• Movement of unsaturated flow ceases in sand at a lower
tension than in fine textured soils, as the water films lose
continuity sooner between the larger particles.
• Wetter the soil, the greater is the conductivity for water

28. Contd…
• In the ‘moist range’, the range of unsaturated flow in
soils of various textures is in the following order:
Sand < loam < clay
• In the ‘wet range’ the unsaturated conductivity occurs
in the same or similar order as saturated conductivity

29. WATER VAPOUR MOVEMENT
• Vapour transfer increases as void space increases.
• At a soil moisture potential of about-15 bars, the continuity of
the liquid films is broken and water moves only in the form of
vapour.
• Vapour pressure gradient acts as the driving force.
• Vapour pressure of soil moisture increases with the increase in
soil moisture content and temperature, it decreases with the
increase in soluble salt content.

30. Contd…
• Water vapour movement is significant only in the ‘moist
range’.
• Rate of diffusion of water vapour through the soil is
proportional to the square of the effective porosity,
regardless of pore sizes.
• Finer the soil pores, the higher is the moisture tension
under which maximum water vapour movement occurs

31. Contd…
• In a coarse textured soil pores become free of
liquid water at relatively low tensions and when the
soil dries out there is little moisture left for vapour
transfer
• Fine textured soil retains substantial amounts of
moisture even at high tensions, thus permitting
vapour transfer

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