What should you know?
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Unique properties of liquid water
How soil water content is measured
The forces that drive water movement
Types of pores that transport water
Soil water classification
Influence of texture and other properties on
available water and water holding capacity
• An appreciation for the status of soil in the global
water cycle
• Factors that affect soil water status and
movement
Soil Water
ESS 210
Chapter 5
p. 176-218
(and Chapter 6, p. 219-271)
1
The Water Molecule
δ+ ≈ +0.42
Important Properties
• Attraction and interaction with ions,
polar compounds, and mineral surfaces:
ADHESION
• Ions, especially cations, are hydrated
• Results in the dissolution of soil
minerals; ions have stronger affinity for
water than for each other
Al(OH)3 + 3H3O+ → Al(H2O)63+
Covalent O–H
H-bond O–H
470 kJ mol–1
23.3 kJ mol–1
H
δ+
O
97 pm
H
H
2δ– ≈ –0.84
O
109.5°
186 pm
Water is a non-linear molecule and oxygen
is highly electronegative relative to the
proton → an unequal distribution of charge
→ polarity
2
H
3
Hydrated Cation
R [Mg2+] = 86 pm
R [Mg(H2O)62+] = 452 pm
Mg2+
4
Important Properties
• Strong attractive forces
between ions and water
molecules
• Water molecules form a shell
around the ion called the
primary hydration sphere
• Water molecules in the primary
hydration sphere form bonds
with other water molecules to
form a secondary hydration
sphere
• Strength of the bonds
decreases with increasing
distance from the ion until it is
equivalent to the bonding
forces in the bulk water
• Strength of hydration of an ion
is dependent on the size and
the valence of the ion
5
• Strong attraction of water for water:
COHESION
• H-bonding
– A low energy bond between the
electronegative oxygen atom of one water
molecule and the electropositive hydrogen
atom of another
– H-bonding also occurs with other
electronegative atoms, e.g., N
• H-bonding lends liquid water internal
structure
6
1
H-Bonding Lends Water:
•
•
•
•
Cohesion and Adhesion
High boiling point (100 ºC)
High freezing point (0 ºC)
High specific heat (4.1813 J g–1 K–1)
Expansion upon freezing
• Cohesion: attraction of water for other
water molecules
• Adhesion: attraction of water for other
substances and surfaces
• Results in the ability of soil to hold water
• Results in water movement in soil
– Water structure contracts until 4 ºC
– Expands from 4 ºC to 0 ºC
• These properties result in thermal
buffering, ice floating
7
8
Capillary Fundamentals
Clay mineral
• Two forces cause capillarity
Adhesion
– Attraction of water for the solid (adhesion)
– The surface tension of water (cohesion)
• Capillary rise: water moving against gravity
• Height of rise is inversely proportional to capillary
diameter, surface tension, degree of attraction to
the soil surface
O
OM
C – O–
– Hydrophobic: water-hating
– Hydrophilic: water-loving
Cohesion
9
10
Capillary Rise
h=
•
•
•
•
Capillary Rise Example
2t
ρ w gr
Calculate the capillary rise of water in a soil pore
with a radius of 0.1 mm (a macropore!)
h = height of rise in meters
t = surface tension of the soil solution (0.073 N m–1)
ρw = density of water
g = acceleration due to gravity (9.8 N kg–1)
11
h=
2(0.073 N m −1 )
= 0.15 m (15 cm)
(1,000 kg m − 3 )(9.8 N kg −1 )(0.0001 m)
12
2
Capillary Rise Questions
Soil Water
• When a small amount of water is placed
on soil, will the water enter into the
macropores or into the micropores?
• When water flowing in a micropore comes
in contact with a macropore, will it flow
out of the micropore and into the
macropore?
• Water in soil is held by surface attractive
forces – adhesion
• As the soil surface area increases, the
amount of water a soil can hold increases
• The strength with which water is held can
be measured in energy terms
• Related to free energy of water – the
ability to do work
13
Measuring Soil Water Content
Measuring Soil Water Content
• Gravimetric water content (θm)
– Mass of moist sample
– Dry for 24 hr at 105 ºC
– Mass of oven-dry soil
• Volumetric water content (θv)
– Plants use water from a volume of soil
– θv is more utilitarian then θm
moist soil (g) − oven dry soil (g)
× 100
oven dry soi l( g )
%θ m =
%θ m =
14
mass of water (g)
× 100
oven dry soi l( g )
%θ v =
volume of water (cm −3 )
× 100
volume of soil (cm − 3 )
%θ v =
mass of water (g) ρb (g cm − 3 )
×
× 100
mass of soil (g) ρ w (g cm − 3 )
%θ v = %θ m × ρb × 100
15
Soil Water Depth
16
Example Computation
• θv used in irrigation management to determine
– How much water to add
– How much has evaporated
• If θv is known, the depth of water in a given depth
of soil can be computed
depth of water (cm)
depth of soil (cm)
∴ depth of water (cm) = θ v × depth of soil (cm)
θv =
17
Question: How deep will a 0.5 inch rainfall
wet a uniform soil that contains 28% water
(wt.%)? Assume that the water will enter
the soil and wet each layer from its initial
water content of 28% up to field capacity
(42%), and that the excess water
(gravitational water) will leach to lower soil
layers. Also, assume ρb = 1.30 g cm–1 and
no runoff.
18
3
Example Computation
Soil Water Potential
Solution:
a) Storage capacity (wt.%) = field capacity –
initial water content = 42 % – 28 % = 14 %
b) θv = 0.14 × 1.30 = 0.18 (1 cm3 soil will
store 0.18 cm3 of water)
c) Depth of wetting: 0.5 inches of rainfall
divided by 0.18 inches of water stored per
inch of soil = 2.78 inches of soil depth
• Soil water potential is the work water can
do as it moves from its present state to a
free pool of standing water (reference
state)
• Adsorbed water in soil is less free to move
relative to the water in the pool
– Soil water has a LOWER Free Energy than
water in the pool
– Energy is required to move the water from the
soil to the pool
19
20
Soil Water Potential Measurement
Soil Water Potential
The Pressure Plate
• Energy term, units are in kPa or MPa
– Free pool = 0 kPa
– Adsorbed water = negative kPa values
– Range from 0 to < –100,000 kPa
– More negative = more tightly held
• Soil water potential is an ENERGY TERM,
NOT A VOLUME
21
22
Soil Moisture Curves
23
Water potential
Work to access
water
> 0 kPa
None
Ponded
0
None
Saturated
–33
Very low
Field capacity
–1,500
High
Permanent
wilting point
–100,000
Very high
Air dry
< –800,000
Extremely high
Oven dry
Water content
24
4
Components of Soil Water
Potential
Matric Potential – Ψm
• Attraction of water for soil surfaces and for
itself: adhesive and cohesive forces
• Always negative
• As soil dries, energy of the least tightly
held water decreases (Ψm decreases)
• Causes movement of water from moist to
dry soil
• Three different potentials commonly
contribute to the total soil water potential
(Ψt)
• Ψt ≡ Ψ m + Ψo + Ψg + …
• Ψm ≡ matric potential
• Ψo ≡ osmotic potential
• Ψg ≡ gravitational potential
25
Matric Potential
Osmotic (or Solute) Potential – Ψo
Water moves from moist to dry;
dry soil has higher attraction for
water than wet soil (water may
move in any direction)
Moist soil = high Ψm (-1 kPa)
(less negative, high free
energy)
26
Dry soil = low Ψm (-100 kPa)
(more negative, low free
energy)
• Attraction of water for dissolved solutes
• As salt concentration increases, free energy of
water decreases
• Always negative
• Water moves from areas of low salt (high free
energy) to high salt (low free energy)
• Important in arid, semi-arid regions, and saltaffected soils
• Semipermeable membrane is required for water
movement; otherwise, solutes move
27
28
Unsaturated Water Movement
Gravitational Potential – Ψg
• Results in the downward movement of
water in response to gravity
• Always positive
• Water in macro-pores responds to this
• Water in micropores is influenced, but
matric is more important
• Most important in saturated flow conditions
Describe how the
water (drip-applied)
will move into the soil
shown in the diagram.
Water
A horizon - air dry soil
29
5
Saturated Water Movement
Unsaturated Water Movement
Water
Describe how the
water (ponded) will
move into the soil
shown in the diagram.
The water moves sideways
and downward at the same
rate. This is because of
adhesion and cohesion.
Water
A horizon - air dry soil
Saturated Water Movement
The movement would
mainly be downward
due to gravity.
Soil Water Classification
• Saturation = 0 kPa
Water
– All pores are filled
• Field Capacity = –10 to –33 kPa
– Greatest amount held against gravity
– All micropores filled
– Macropores are drained
34
Soil Water Classification
Soil Water Classification
• Permanent Wilting Point = –1,500 kPa
• Plant-available water
– Water held so tightly that plants cannot extract
– Plant transpiration can’t exert this much
energy
– Plants wilt irreversibly
• Gravitational water
– Held between –10 to –33 and –1,500 kPa
– Total plant available = FC – PWP
– Water stored in larger micropores
• Unavailable water
– Water held at SWP of < –1,500 kPa
– Held in smaller micropores
– Water between 0 and –10 to –33 kPa
– Drains freely
– Plant unavailable - moves too fast
35
36
6
Unavailable
Water
Air
Soil as a Reservoir
–33 kPa
Soil
Particle
–1,500 kPa
–800,000 kPa
Available
Water
Air
• Total water at any soil water potential
depends on:
• Texture
– Clay increases, surface area increases, total
water holding capacity (WHC) increases
– Medium textured soils have highest
AVAILABLE WATER – WHY?
37
38
Texture and Water Holding
Capacity
Available Water
30
Unavailable
Vol.%
25
20
10
12
15
10
Available
16
8
10
5
4
0
Clay
Loam
Sandy loam
39
Available Water
40
Soil as a Water Reservoir
• Clay Mineralogy
– 2:1 expansive > 2:1 non-expansive > 1:1 >
sesquioxides
• Organic matter
– ↑ OM increases, ↑ WHC
• Structure
– Strong structure decreases WHC and
increases macroporocity
41
42
7
Water Flow in Soil
Measuring Water in the Field
• Rapid movement due to gravity potential
A tensiometer is a sealed
tube filled with water. The
end has a porous ceramic
cup. The soil “sucks” the
water out of the tube. The
gauge then measures the
suction (read in kPa, etc.)
which is related to the
volumetric water content.
– This is SATURATED FLOW
– Preferential flow → flow through macropores
– Ψt > –33 kPa
• Slow movement due to matric and osmotic
potentials
– This is UNSATURATED FLOW
– Independent of gravity
– Movement through micropores
• Vapor movement
43
44
What Controls Infiltration?
Water Flow
• Infiltration: movement of water INTO the
soil surface (after rain, irrigation)
• Percolation: downward movement of water
through the soil profile
• Evapotranspiration (ET): upward
movement of water into the atmosphere by
evaporation from the soil surface and plant
transpiration
• Texture: coarse > fine
• Structure: strong granular structure is good
• Organic matter:
– Humus acts as a glue to provide structure
– Surface residues protect structure
• Amount of water already in soil
– Saturated soil can’t take more
• Compaction: destroy macroporocity
• Crusting: destroys microporosity
45
Percolation
Infiltration rate (cm/hr
Infiltration and Texture
Sandy Soil
14
12
Clay loam, good structure
10
Clay loam, poor structure
8
46
6
4
• Excess water moves down through profile
• Percolating water carries dissolved
substances (salts), e.g., Ca2+, Mg2+, K+,
Na+, NH4+, Cl–, SO42–, NO3–, H4SiO40
• Loss of soluble salts → leaching
• As air replaces water, flow rates slow
– Water closer to soil at lower SWP
2
0
0
30
60
120
180
240
300
Time (minutes)
47
• Rates highest in larger pores
– Rate increases by 4× for 2× increase in pore
diameter
48
8
Saturated Water Flow
• The rate of saturated water flow is described by
Darcy’s Law → describes the vertical movement
of water due to gravity
Q = K sat tA
ΔΨ
L
Q ≡ water flux (cm3)
A ≡ area of soil surface (Q/A = flux in cm)
ΔΨ ≡ water potential (pressure head, cm)
L ≡ depth of water column (cm)
ΔΨ/L ≡ hydraulic gradient
Ksat ≡ saturated hydraulic conductivity; permeability (cm s–1)
t ≡ time (s)
Representative Ksat
values (cm h–1)
– Gravel: 54 to 7.2
– Sand: 43 to 0.72
– Loam:
6.1×10–2 to 6.1×10–5
– Clay:
9×10–7 to 3.6×10–7
Ψ1
A
ΔΨ = Ψ1 – Ψ2
Saturated
Soil profile
L
Ψ2
Q /t
49
50
Unsaturated Flow
Unsaturated flow
Bulk soil
• Movement from high Ψt to low Ψt
• Moist to dry
• Accounts for downward flow, lateral flow,
and upward flow of water in unsaturated
soil
Rhizosphere soil
Root
– From moist soil to roots
– From saturated zones to dry zones
Relatively moist
soil: less
negative kPa
Relatively dry
soil: more
negative kPa
51
52
Unsaturated Flow
Unsaturated Flow
Dry soil
Capillary fringe:
matric potential causes
“wicking” from saturated
zone below
H2O
uptake
Vadose
zone
Richardson Equation: a
simple expression that
describes onedimensional vertical water
flow
∂θ v ∂ ⎡
⎛ ∂h
⎞⎤
= ⎢ K (hm )⎜ m + 1⎟⎥
∂z ⎣
∂t
⎝ ∂z
⎠⎦
•
•
•
•
θv = volumetric water content
t = time
z = vertical dimension
K(hm) = unsaturated hydraulic conductivity
(K is a function of hm)
• hm = soil water matric potential
Saturated zone:
shallow groundwater
or perched water
53
54
9
Flow Constraints
Clay loam horizon over sandy horizon: water
does not enter sand until clay loam is nearly
saturated
• Unsaturated flow slower in fine textures
• Slower in compacted soils
• Water will not move from fine texture to
coarse texture readily
Water added
– Macropores must be nearly saturated first
– Results in ponded water in soils with horizons
of differing textures
Clay loam
Sand
Discontinuity
Time
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56
Transpiration:
evaporative
loss from
plant leaves
Plant Uptake of Water
• Passive absorption (>90%):
– Due to transpiration
– Evaporative loss of water at leaves results in
water being pulled up the xylem tubes (soil →
roots → leaves is a continuous water column)
• Active absorption
Precipitation
Infiltration
– Plant takes up more ions (takes energy)
– Osmotic potential results in water uptake
Passive & active
absorption
Saturated flow
(gravity - percolation)
57
Unsaturated flow
(matric potential)
58
Soil in the Global Water Cycle
Oceans – 97.25%
Ice – 2 %
Groundwater – 0.7%
The rest – < 0.05 %
The rest
Lakes (60%)
Soil (33%)
Rivers (1%)
Atmosphere (6%)
59
60
10
SPAC
Ψair = –20,000 kPa
Condensation
Transpiration
Transport
Ψleaf = –500 kPa
Ψxylem = –85 kPa
Precipitation
Evaporation
Runoff
Ψxylem = –75 kPa
Infiltration
Percolation
Drainage
Ψsoil = ~0 kPa
Capillarity
Ψroot = –70 kPa
Ψsoil = –50 kPa
61
Things to Think About
Chapter 6
• What is the fate of precipitation?
• What soil and landscape properties affect
infiltration?
• What soil and landscape properties affect
evapotranspiration?
• What is the fate of soil water?
• What can humans do to manage the fate
of soil water?
62
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