Transcript Soil Atmosphere

Soil Water
Chapter 5
Chapter 5 Outline
I. General Properties of Water
II. Capillary Action
III. Energy Concepts
IV. Flow of Water
V. Specific Examples
Michael J. Singer and Donald N. Munns
Soils: An Introduction, 6e
What makes H2O so amazing?
I. General Properties of Water
A. Polarity
B. Hydrogen Bonding
C. Cohesion/Adhesion
D. Surface Tension
I. General Properties of Water
II. Capillary Action
A. Equation
B. Comparison of textures
III. Energy Concepts
IV. Flow of Water
V. Specific Examples
Capillary Action
h=0.15/r
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In moist soils, the pores act like
thin tubes, and water is held by
capillarity. Pores are not straight
and smooth like the tubes in the
figure, and so water does not
reach the same height in pores as
it would in tubes of the same
diameter. A soil with many small
pores holds more water by
capillarity than does a soil with
few large pores. In addition,
because of capillarity, water rises
higher from a water table in a clay
soil than in a sandy soil.
Figure 5.6
I. General Properties of Water
II. Capillary Action
III. Energy Concepts
A. Energy review
B. Potential
C. General Equation and terms
1. Gravitational potential
2. Osmotic potential
3. Pressure potential (matric and hydrostatic)
D. Measuring water content (Ө) and potential (Ψ)
IV. Flow of Water
V. Specific Examples
A. Energy Review
1. kinetic vs potential energy
2. movement from high to low energy
B. Potential
1. difference in energy level of water from one site to another
2. wet soil: most water far from soil particle surface (loosely held, high Ψ)
3. dry soil: most water located in small pores (tightly held, low Ψ)
Water potential is a measure of the water's availability, or escaping tendency,
or tendency to move. It is measured relative to water at a standard
pressure and temperature at some reference elevation.

High water potential means that water can easily move. It is loosely held.
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Water potential is high when it is zero and low when it is -1500 kPa.

More negative is lower potential.
Ψt = Ψg + Ψm + Ψo + Ψs + ….
Ψg=gravitational=pulls water downward
always positive
Ψm=matric=attraction to solids (suction, tension)
adhesion and capillary (movement moist [high] to dry [low])
Ψo=osmotic= attraction to ions (lower potential energy)
>concentration = < potential
(impt for water uptake by plant)
Ψs=submergence=hydrostatic (saturated zones only)
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What are the major components of water potential? What factors
most influence water potential in soil?
The major components of water potential are the gravitational,
osmotic, and matric potentials.
Dry Soils
 In unsaturated soils, matric and osmostic potentials are dominant
and decline respectively with decreasing water content or increasing
salinity.
Wet Soils
 In saturated soils and aquifers, gravitational potentials are
dominant.
Table 5.1
D. Measuring water content (Ө) and potential (Ψ)
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How would you measure the water content and the water potential
of a soil?
Water content (Ө) of soil is most easily measured by weighing a
sample wet and again after drying (oven to constant weight).
Water content = weight loss/dry weight.
Water potentials (Ψ) in soil materials can be measured with
tensiometers.
Water content calculation