LAST LECTURE
Solid characteristics (2):
Factors affecting soil porosity & aggregation
Soil consistency
Soil classification (1)
Diagnostics
THIS LECTURE
Soil Water
EFFECTS OF WATER ON SOILS
THE WATER MOLECULE
•Shrinkage and swelling
•Particles adhere to one another
•Encourages aggregate formation
•Chemical reactions releasing or tying up nutrients
•Chemical reactions causing acidity
•Chemical reactions that break down minerals
•Affects rate of change of temperature
•Leads to freeze-thaw activity
•Affects metabolism of soil organisms
•Basic requirement for all plants
One of only three inorganic liquids normally found on Earth
POLARITY OF H2O
HYDROGEN BONDING
• Exhibits polarity - side with hydrogen atoms
electropositive, oxygen side electronegative
• H atom of one molecule attracted to O molecule of another
• High boiling point for its molecular weight molecules cluster together due to uneven polarity
• Causes high boiling point, specific heat and viscosity
• Polarity explains electrostatic attraction to
charged ions and colloidal surfaces
• Polarity encourages dissolution of salts
(attracted more to H2O than to each other)
• Heat of solution - molecules more tightly packed
when attracted to electrostatically-charged ions or
clay (energy status lower than in pure water)
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SURFACE TENSION
COHESION versus ADHESION
Attraction of water molecules to one another = cohesion
•
Water molecules
adhere to glass
•
Water molecules
do not adhere
to a waxy surface
(hydrophobic)
•
H2O molecules
are more strongly
attracted to
themselves
Attraction of water molecules to solid surfaces = adhesion
Adhesion = adsorption
These two forces
help soil retain water
Yields clay plasticity
CAPILLARY MECHANISM
MOVEMENT BY CAPILLARITY
Forces causing capillarity:
(1) attraction of water for the solid (adhesion)
(2) surface tension of the water (cohesion)
•Capillary movement occurs in all directions
•Determined by pore size and pore size distribution
Sandy soils: rapid rise, but does not rise as far
(larger pores)
Clay soils: slow rise, but water rises farther
SOIL WATER ENERGY
•
Potential energy important in determining water
movement within soils
•
Water movement is controlled by differences in energy
levels
•
Moves from high energy to low energy state
SOIL WATER ENERGY
Wet soil
• Water not held very tightly because many particles
far from surfaces in larger pores
• Higher energy level
Dry soil
Forces
(1) Matric force (attraction of water to solids)
(2) Osmotic force (attraction of water to ions and other
solutes)
(3) Gravity (downward)
• Remaining water has little freedom of movement,
present in small pores, adhering strongly to surfaces
• Lower energy level (little freedom of movement)
2
SOIL WATER POTENTIAL
Soils water potential = Difference in energy level
between soil water and free water
Total soil water potential (ψt) (=psi_total)
1. Gravitational potential (ψg)
Æ height relative to some reference point, an object’s mass, and
the strength of the gravitational field (gravitational accelleration)
ψg = gh
SOIL WATER POTENTIAL
2. Matric potential (ψm)
• Attraction of water to solid surfaces (adhesion or capillarity)
• Synonymous with suction or tension
• Negative because water attracted to soil has a lower energy
state
• Above the water table
• Influences soil water retention and movement
• Very important in supplying water to drier regions around plant
roots!
3. Submergence or hydrostatic potential (ψs)
•Plays an important role in removing excess water
after heavy precipitation or irrigation
SOIL WATER POTENTIAL
• Positive hydrostatic pressure due to weight of water above
• Used for water below the water table
SOIL WATER POTENTIAL
4. Osmotic potential (ψo)
• The presence of solutes reduces the potential energy of water
• Reduced freedom of movement of H2O around each ion or
molecule
• Solutes tend to redistribute themselves to equalize concentrations
• Has little effect on soil water movement (no membranes)
• Important effect on uptake of water by plant roots
• If soil water is salty, ψo is more negative and it is more difficult
for plants to uptake H2O
• If soil water is very salty, water leaves the root cells and wilting or
even plasmolysis occurs
WATER CHARACTERISTIC CURVES
WATER CHARACTERISTIC CURVES
• At a given moisture content, water is held much more tightly by
clays than by loamy or sandy soils
• Clay soils hold much more water at a given potential than
loams or sandy soils
• The amount of clay determines the proportion of micropores
• Tightly held water cannot be used by plants
• Water content does not deplete as quickly if it is tightly held
• Well-structured soils have a greater water holding capacity
(higher porosity)
• A compacted soil has lower porosity (lower water holding
capacity) and water is held more tightly because macropores
are removed
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UNITS FOR WATER POTENTIAL
FIELD CAPACITY AND WILTING POINT
HYSTERESIS
HYSTERESIS
• As a soil is wetted, some of the smallest pores are bypassed
and water penetration is prevented
• During drying, some macropores cannot lose their water until
the matric potential is low enough to remove water from the
smaller pores surrounding them
• The large pore then does not lose its water until matric suction
is strong enough to remove water from the smaller pores
Matric potential (cm water)
Æ A system with hysteresis has memory, so that you can only predict the
outcome of a situation if you know the initial state
• Shrinkage and swelling also affect soil-water relationships
Volumetric Water Content, θ
TECHNIQUES MEASURING WATER CONTENT
VOLUMETRIC WATER CONTENT, θ
Volume of water associated with a volume of soil
¾
¾
¾
¾
¾
¾
¾
¾
Gravimetric
Neutron Scattering
TDR probe
Capacitance method
Tensiometer
Thermocouple psychrometer
Pressure membrane apparatus
Gypsum blocks
Gravimetric Method
1. Weigh soil
2. Dry (105°C for 24 h)
3. Weigh again (1kg loss = 1L)
Neutron Scattering
• Useful for mineral soils only
• Fast neutrons emitted
• Collisions with H2O slow them down
• Slow neutrons detected
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THE NEUTRON PROBE
TDR probes (Time-Domain Reflectometry)
Reflection by soil water of electromagnetic signals travelling in
transmission cables
LAST LECTURE
CAPACITANCE METHODS
TDR determines moisture content and salinity by measuring:
Electrical capacitance of
two electrodes thrust into
soil varies with water level
1. the time it takes for an electromagnetic impulse to travel down
parallel metal transmission rods buried in the soil
2. the degree of dissipation of the impulse at the end of the line.
Air gaps cause
measurement errors
Æ Transit time is related to the amount of water in the soil.
Æ The dissipation is related to the level of salts.
TENSIOMETER
Measures the strength with
which water is held in soils
Water-filled tube: Vacuum
at top, porous ceramic at
bottom
2 MORE METHODS
Thermocouple psychrometer
Best in relatively dry soils (± 50 kPa error)
Relative humidity of soil air affected by ψo+ ψm
Soil moisture potential inversely related to the evaporation rate
Pressure membrane apparatus
Used to make soil water characteristic curves
Pressure applied to force water out (see diagram)
Pressure when downward flow stops gives water potential
Water leaves until potential in
the tensiometer is the same as
the soil matric potential
Vacuum gauge measures
negative tension at top
See: http://hydra.unine.ch/doityoursoil/demo/e/module1/sequence_20/1210_30_method_e.html
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GYPSUM BLOCKS
Porous block of gypsum
embedded with electrodes
Water absorbed in proportion to
soil water content
Resistance to flow of electricity
decreases with water content
Gypsum block data for a soil in the tropical cloud
forest of Tambito, Cauca, Colombia (Letts, 2003)
rain
0 .7
s o il mo is t ure
0 .6 5
10
0 .6
8
0 .55
6
0 .5
0 .4 5
4
Saturation ratio
Hourly Rainfall (mm)
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COMPARISON OF METHODS MEASURING SOIL WATER
0 .4
2
0
20-Sep
0 .3 5
4-Oct
16-Oct
28-Oct
0 .3
9-Nov
Date (2000)
DARCY’S LAW
WATER FLOW IN SOILS
(i) Saturated flow
(ii) Unsaturated flow
(iii) Vapour movement
Q = A ⋅ Ksat ⋅ Δψ/L
Rate of flow is determined by ease of water
transmission & force driving the water
1. Saturated Flow
Soil pores are completely filled with water
•lower portion of poorly drained soils
•above clay layers in well-drained soils
•upper soil zone after heavy downpours
DARCY’S LAW:
Q = A ⋅ Ksat ⋅ Δψ/L
Q = Discharge (Volume H2O per time)
A = cross-sectional area of column of water
through which water flows
Ksat = saturated hydraulic conductivity
Δψ = change in water potential between ends
of the column
L = length of the column
[hydraulic gradient = (ψ1-ψ2)/L]
Rearrange:
Ksat = (Q ⋅ L)/(A ⋅ Δψ ⋅ t)
Saturated hydraulic conductivity
is measured in units of distance
divided by time (eg. cm/s or cm/hr)
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SATURATED HYDRAULIC CONDUCTIVITY
PREFERENTIAL FLOW
LEACHING BY PREFERENTIAL FLOW
Normally, flow is proportional
to the fourth power of the
pore radius (macropores are
important)
Biopores, including earthworm
channels, root channels etc.
lead to preferential flow
Ped edges and shrinkage cracks
serve a similar function
Also may allow pesticides to
reach groundwater before
decomposition!
2. UNSATURATED FLOW IN SOILS
Micropores dominate in
clays.
Macropores filled with air, so water movement occurs
in micropores
Water content and potential can be highly variable, causing
complex patterns in the rate and direction of water movement
Many are still waterfilled at rel. high suction,
but macropores
(dominant in sands) are
dry.
Differences in matric potential ψm rather than gravity dominate
Movement from moist areas to dry areas along matric
potential gradient (eg. from –1kPa to –100kPa)
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LAST LECTURE
Infiltration
(gravity dominates
percolation near
surface after heavy
rain)
Infiltration rate is highest
early in a rainfall event,
before the macropores
fill up
Air fills macropores:
-10 to -30 kPa
Percolation
(matric potential
gradients most
important)
Plant needs not met:
-1500 to -2000 kPa
Evaporation
beyond W.P.
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θ AT FIELD CAPACITY AND AT HYGROSCOPIC COEFF.
READING FOR THURSDAY
CHAPTER 6: SOIL HYDROLOGY
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