4. The process of water entry into the soil through the soil profile
and vertically downward is termed infiltration.
Infiltration rate is the rate at which water is passing through the
soil surface and flowing into the soil profile (mm/hr).
The actual rate at which water is entering the soil at any given
time is termed is infiltration velocity.
The infiltration capacity (infiltrability) is the rate at which soil
profile can absorb water through its surface when it is
maintained in contact with water at atmospheric pressure.
The infiltration rate, which is high in a dry soil during the
initial stages, decreases with time and eventually approaches a
constant rate, which is often termed final infiltration rate or
steady state of infiltration.
5. The path of downward movement
Colman and Bodman (1945)
1.Saturation zone: A zone presumed saturated which reached a
maximum depth of 1.5 cm
2. Transition zone: A region of rapid decrease of water content
extending to a depth of about 5 cm from
the surface.
3. The main transmission zone: A region in which small
changes in water content occurred.
4. Wetting zone : A region of fairly rapid change in water
content.
5. The wetting front : A region of very steep gradient in water
content which represents the visible limit
of water penetration.
6. There is little change in water content from top to
bottom of the transmission zone, through which water
moves by gravity, but a very steep decrease in water
content and in matric potential at the wetting front
(Bodman and Colman, 1944)
Saturation zone 1.5 cm
Transition zone 4.5 cm
Transmition zone
(Water movement
caused by gravity)
Wetting front
Dry soil
7. Table : Typical values of the final (steady state)
infiltration
S.No. Soil type Final infiltration (mm hr-1)
1. Sandy >20
2. Sandy and Silty 10 to 20
3. Loams 5 to 10
4. Clayey soil 1 to 5
5. Sodic clayey < 1
9. Water movement in the soil
S.No. Particular Saturated flow Unsaturated
flow
Vapour
movement
1. Major force Gravitational Metric Vapour
pressure
2. Water form Liquid Liquid Vapour
3. Major direction
of flow
Downward Lateral All directions
4. Pore space All pores filled
with water
Micropores
filled with water
All pores are
empty
5. Rate of flow Fast (1-100
cm/day)
Slow (0.01-
0.00001 cm/day)
-
6. Volume of water
movement
Large quantities
(3,75,000 kg/ha in
15 cm depth)
Small (1,00,000
kg/ha in 15 cm
depth of soil)
Negligible (15
kg/ha in 15 cm
depth of soil)
10. Movement of water within the soil
• Liquid water:
(a) Saturated flow: Water flow through the water filled
pores spaces under the influence of gravity. This
flow is occur at the time of high rainfall or irrigation
water is free from tension.
(b) Unsaturated flow: Water flows through the partial
air filled pore spaces. Water is under tension.
• Saturated conditions : (i) Poiseuille’s law
(ii) Darcy’s law
• Unsaturated conditions
11. • Water Vapour
(a) Diffusion: Water vapour diffuses through the air
filled pore spaces due to differences in
vapour pressure gradient.
(b) Mass flow: Water vapour also flows in mass with
other gases due to differences in total
pressure.
12. Concept of flow
Q œ DK
Q = CDK
Q = Flow velocity C = Proportionality factor
D = Driving force K = Conductivity of the medium
Driving force of water is controlled by
(i) Gravity (ii) Difference in film tension or tension gradient
Only downward movement is affected by gravity while
tension gradient can be act all the directions
13. • The rate of movement is controlled by the size and
continuity of the pores containing the water and by its
viscosity.
• Under a given pressure gradient water moves fastest
through a soil when it is saturated that is when all the
pores are full of water, but as the becomes
unsaturated that is as the proportion of pores
containing air increases, the rate of flow decreases
usually very rapidally. This is because the pores
which get emptied of water first are the widest so as
the soil dries out, water flow takes place in
increasingly thinner films along path becoming
increasingly more tortuous and as the films become
thinner so the effect of viscosity rapidally becomes of
more importance.
15. Poiseuille’s Law
The law of Poiseuille’s express the flow of water in a narrow tube
q = P π r4
8 l µ
q = Volume of flow per unit time (cm3/sec.)
P = Pressure difference between two ends of the tube of length l
(dynes/cm2)
r = Radius of tube (cm)
l = Length of tube (cm)
µ = Viscosity of liquid (Poises)
P1 P2
l
16. q œ r4
Equation shows that the pore size is of outstanding
significance, as its fourth power is proportional to the
rate of saturated flow. This indicates that as the size of
pore space decreases the rate of flow in saturated
conditions decreases.
Saturated flow œ Pore size
Generally the rate of flow in soils of various
texture is in the following sequence
Sand > loam > Clay
Poiseuille equation express the effect of viscosity and
temperature in rate of flow. The viscosity of water
decreases 10% of every 1oC increasing in temperature.
17. Darcy’s Law
q = K i a
q = Volume of flow per unit time (cm3/sec.)
K = Hydraulic conductivity (cm/sec.)
i = Hydraulic gradient (dimensionless)
a = Cross section of flow area (cm2)
i = h1 – h 2/l
h1 = hydraulic head at point of measurement 1
h2 = hydraulic head at point of measurement 2
h1 – h2 = Difference in hydraulic head
h1
h2
l
18. q = K i a
If a=1, i = Gradient of hydraulic head
q œ i
Darcy’s law states that the quantity of water passing a unit
cross section of soil is proportional to the gradient of hydraulic head.
q = K i a
q/a = Ki = v
v = Velocity of flow (cm/sec.)
q/a = also called velocity flux (v)
v = Ki If i = 1
v = K
If hydraulic gradient is 1 then hydraulic conductivity will be equal to
velocity flux
19. • If the hydraulic head is replaced by difference in total
potential (ᴪ)
v = K . i
v = K . h1-h2
l
v = K . ᴪ 1- ᴪ 2
l
v =- K ᴪ
v œ - K ᴪ
l
Rate of movement œ Potential gradient
Negative represents that movement is in the direction of
decreasing potential
20. Limitation of Darcy’s law :
The flow through the porous medium must be laminar
Soil - Laminar flow
Pipe & Tube – Turbulent flow
The usual index used to determine the tendency of flow to
be laminar is the Reynold’s number (Rn)
Rn = evd/µ or pvd/µ
e = Density of fluid v = Velocity of flow
d = Mean diameter of the soil particles
µ = Viscosity of the fluid
Rn < 1 = Laminar flow = Apply Darcy’s law
22. In unsaturated condition
• Soil moisture movement also called capillary movement
• K is often termed as capillary conductivity
• Unsaturated conductivity = f (Soil moisture content,
no., size & continuity of
soil pores)
• If moisture content is below field capacity, the capillary
conductivity is so low that capillary movement is of
little or no significance in relation to plant growth.
• Wetter the soil the greater is conductivity
Sand < loam < Clay
23. Principle of capillary:
h = Height of water in tube
T= Surface tension (dyne/cm) r = Radius of tube (cm)
d = Density of liquid g = Gravity (dyne/cm)
When a dry, clean capillary tube kept in water
then water is move to upward direction and this process
is know as capillary it happen with cohesion and
adhesion force.
h = 2T
rdg
24. Movement of water under unsaturated condition
ᴪ = ᴪm + ᴪg
Horizontal movement – Only ᴪm applies
Downward movement - ᴪm + ᴪg
Upward movement - ᴪm & ᴪg oppose one another
25. • As drainage proceeds in a soil and the larger pores are
emptied of water, the contribution of the hydraulic
head or the gravitational component to total potential
becomes progressively less important & the
contribution of the matric potential (ᴪm) becomes
more important.
• The effect of pressure is generally negligible because
of the continuous nature of the air space.
V = - k (ᴪm + ᴪg)
l
l = Path of greatest change in (ᴪm + ᴪg)
26. Darcy’s law in unsaturated condition
It is still applied but with some modifications
K is regarded as a function of water content
K = K (θ)
Where, θ = Soil moisture content
As the soil moisture content & soil moisture potential
decreases, the hydraulic conductivity decreases very
rapidly, so that ᴪsoil is -15 bars, K is only 10-3 of the value
at saturation.
According to Philip (1957), the rapid decrease in
conductivity occurs because the large pores are emptied
first, which greatly decreases
28. Factors affecting movement of water
1. Degree of swelling & shrinking of soil colloids
Swelling α Movement of water
Internal surface area >External surface area
2. Soil salinity
Richards & Fireman (1943) found that the permeability of soil
saturated with sodium was much less than when it was saturated with
calcium. Sodium causes dispersion of the clay aggregates into their
constituent micelles reducing the number of large pores through which
water can move rapidly.
Soil with Na < Soil with Ca
Soil type Swelling (cc/g)
Montmorillonite High (1-2)
Illite Medium (0.5-1)
Kaolinite Low (0.1-0.5)
29. Factors affecting movement of water
3. Soil organic matter content
• It promote soil aggregation
• Well aggregated soil transmit water faster than poorly aggregated soils.
• Improve soil permeability and increase infiltration (Pillsbury
and Richards, 1954)
4. Soil conductivity
As per darcy’s law
q α K
q - Volume of flow per unit of time (cm3/sec.)
K – Hydraulic conductivity (cm/sec.)
5. Initial soil moisture content : Infiltration reduces as initial soil moisture
content is more because wetting front reduced and swelling of clay
particles reduces the cross sectional area available for the entrance of
water (Tisdall, 1951).
30. 6. Soil permeability
Soil permeability α Hydraulic conductivity α Rate of flow
According to Smith & Browning (1946)
Permeability class Hydraulic conductivity (cm/hrs)
Extremely slow < 0.0025
Very slow 0.0025 - 0.025
Slow 0.025 - 0.25
Moderate 0.25 – 2.5
Rapid 2.5 – 25
Very rapid > 25
7.Condition of the soil surface (Surface permeability): Infiltration
is greatly decreased by zones of low soil permeability such
surface crust, claypan and hardpan zones
31. 8. Soil water potential
Soil water potential α movement of water
9. Surface tension
Surface tension is the force pulling inward acting on a
surface liquid to make the surface area as small as possible.
Surface tension α Adhesion α 1/transmibility
10. Adhesion force between soil and water
Adhesion α 1/transmibility
11. Capillary conductivity
Large pores transmit water faster than a large number of
small pores.
12. Soil plasticity
Soil plasticity α 1/macro pores α 1/transmibility
13. Vegetative cover
More infiltration rate opportunity
14. Duration of irrigation/precipitation
More duration of irrigation reduces wetting front
32. 15. Specific heat
The calorie required to increase tem. 4oC to 5oC of 1
g water is called specific heat.
Specific heat α 1/viscosity α Permeability
16. CO2 content of soil
CO2 dissolve in water Carbonic acid
Carbonic acid looses the soil and increases the permeability
17. Soil aggregate stability
Aggregate stability is a measure of resistance of
aggregates to breakdown when subjected to potentially
disruptive processes.
Aggregate stability α water transmission
33. Physical properties of soil
1. Soil texture :
Conductivity : Coarse texture > light texture
Conductivity α permeability
• Soil texture
Sand>Silt>Clay
2. Soil structure
Well aggregated > Poorly aggregated
3. Soil density
Soil density α 1/pore space α compaction
4. Soil Air
Soil air α Pore space α Permeability
5. Soil temperature
Soil temperature α Permeability α 1/ Viscosity of water
6. Pore space :
High total pore space and low bulk density
Water transmibility α Large pores α 1/ small pores
