Soil Water Movement and Retention

1
Soil Water Movement and Retention
2
Functions of Soil
Medium for plant growth Regulator of water
supplies Recycler of raw materials Habitat for
soil organisms Engineering medium
3
Functions of Soil
Medium for plant growth
Physical Support Gas exchange Water Temperature Nu
trient source
4
Functions of Soil
Regulator of water supplies
Infiltration Run-off Storage/Movement Distribution
Purification
Integral to hydrologic cycle
5
Water Movement
6
(No Transcript)
7
Two Forces Responsible for Water Movement in
Soils
Gravity Capillarity
8
Gravity
9
Capillarity
Spontaneous movement of water into and through
pore spaces in soil without the aid of
gravity.
10
Adhesion and Cohesion
11
Cohesion
12
Adhesion and Cohesion
adhesion
S U R F A C E
H
oxygen
H
13
Adhesion and Cohesion
droplet
adhesion
Cohesion (H-bonding)
Surface
14
Adhesion and Cohesion
Weak adhesion
Strong adhesion
15
Weak Adhesion
16
Adhesion to Soil Particles
Strong Adhesive Forces
17
Soil Pores
capillarity
Adhesion and Cohesion
Adhesion to the tube or pore wall Cohesion
between water molecules
18
Capillarity
h 0.15 r
Tube/Pore wall
adhesion

cohesion
Force down
19
Capillarity
h 0.15 r
Small pores
h
20
(No Transcript)
21
Capillarity
22
Soil Pores and Pore Size Distribution
Texture Density Structure
23
Texture
Particle Size Large/coarse Medium
Fine/Small
Sand Loamy Sand Sandy Loam Silt
Sandy clay Loam Silty clay Loam Silt Loam Loam
Clay Loam Sandy Clay Silty Clay Clay
Pore Size Large/Macro
Meso/Medium Micro/Small
Capillarity Weak Moderate Strong
24
Soil Pores
Silty
Clayey
Sandy
25
Gravity Dominated
Capillarity Dominated
26
Density
Depth in Profile Arrangement of Particles
Compaction
27
Structure
Macropores
Micropores
28
Examples
29
Sand
Water
Clay
30
Initial Saturation
Sand
Clay
31
Initial Saturation
Sandy Loam
Uncompacted
Compacted
32
Aggregates
33
Same Texture and Density
Wet
Moist
34
transpiration
Relevance
35
Quantification Soil Water Energy
36
Potential Energy
Energy waiting to be used or exploited
37
Gravitational Potential Energy
Water moves in response to differences in
potential energy, from high potential energy to
low potential energy.
High potential Energy
The greater the difference in height The greater
the difference in Gravitational potential energy.
Low potential Energy
38
Gravitational Potential
?g
The potential energy of a unit quantity of water.
Unit quantities volume mass weight
mg
?g mgh
The greater the height, the greater the potential
energy.
39
Gravitational Potential
Independent of soil properties
Height (cm)
100
a
?ga 100 cm ?gb 40 cm
50
40
b
soil
?g 0
Reference level
Difference in energy determines movement
40
Gravitational Potential
?ga 60 cm ?gb 0 cm
Height (cm)
100
a
Reference level
(?g 0)
40
b
?ga ?gb 60 - 0 60 cm
0
41
Gravitational Potential
1. Gravitational potential energy is due only
to the height of an object (water) above some
reference point. 2. Gravitational potential
energy is independent of soil properties.
42
Capillary Potential Energy
(Matric Potential Energy)
43
Matric Potential
suction potential - capillarity
Narrow capillary tube high capillary rise
h 0.15 - strong force r
- compared to free water
Small particles, small pores
Applies to unsaturated soils
44
Primary Factors in Matric Potential
Texture, Density, Aggregation
Moisture Content
Which Pores are Filled
Pore Size Distribution
45
Capillarity and Soil Texture
Small pores Strong suction Strong capillarity
Large pores Weak suction Weak capillarity
46
Capillary Potential Energy
water
Dry soil
Suction potential energy
Matric potential energy
47
Capillary Potential
Suction (capillarity)
Dry soil
48
Soil Texture
Sandy Soil
suction
1000 cm
Dry soil
49
Soil Texture
Fine-textured soil
suction
10,000 cm
Dry soil
50
Soil Texture
suction
suction
Clay
Sand
Unsaturated soils have negative matric potential
energy
51
Submergence Potential
52
Submergence Potential (?s)
Equal to the distance below a free water surface
Water Table
10 cm
53
Units of Potential
Centimeters of water Bars Pascals
1 bar 1020 cm water (4oC) 1 KPa 10 cm water 1
bar 100 kPa
54
Total Potential Energy is the sum of the
gravitational, submergence, and matric potential
energies.
?g ?m ?s ?T
55
Gravitational Potential Matric Potential
Total Potential
Height (cm)
50
?g 50 cm
a
?T -15 cm
20
10
?g 0
Reference level
56
Gravitational Potential Matric Potential
Total Potential
Height (cm)
50
?g 50 cm
a
?T -15 cm
20
?g 10 cm
10
b
?T 5 cm
?g 0
Reference level
57
Energy Differences
Height (cm)
50
a
?Ta -15 cm
20
10
?Tb 5 cm
b
?g 0
Reference level
?Ta ?Tb (-15cm) - 5cm -20 cm
58
Which way will water move?
Height (cm)
50
a
?Ta -15 cm
20
10
?Tb 5 cm
b
?g 0
Reference level
?Ta ?Tb (-15cm) - 5cm -20 cm
59
Determining the Direction of Water Flow

1. Sum the individual potentials at each point
2. Determine if there is a difference in potential

3. Water will move from the higher to the lower
energy
4. Point A Point B 5. Water moves from high
to low energy
Positive
Point A to Point B
Negative
Point B to Point A
60
Next Characterizing Water Status