Lecture 14 Soil Water (1)

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Lecture 14 Soil Water (1)
Soil Properties

• Basic Soil Properties
• Soil Water Storage
• Soil Water Forces (Potential)

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General Comments

• Only 0.17 of all fresh water in the world, but
  very important for plant growth and food
  production.
•  
• Short residence time of about 3 months, that
  effectively quadruples its annual contribution to
  water cycle.

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Hydrological Processes in Soil

• Capillary Rise
• Infiltration
• Percolation
• Runoff

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Basic Soil Physical Properties
Soil system consists of three phases solid
phase comprising the mineral and organic
particles of the porous medium gaseous phase of
soil air in the pores liquid phase of soil water
A. Wild, 1995. Soils and the Environment An
Introduction. Cambridge University Press
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Soil Texture
Texture size distribution of the mineral
particles in the soil. Three basic soil textural
types are clay (small particles), silt (medium
particles), sand (large particles) (Figure 2.1
AW)
Basic Soil Physical Properties
A. Wild, 1995. Soils and the Environment An
Introduction. Cambridge University Press
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Terms to Remember
Texture size distribution of the mineral
particles in the soil. Three basic soil textural
types are clay (small particles), silt (medium
particles) and sand (large particles) (Figure 2.1
AW) Structure the aggregation of these
particles Porosity the percentage of pore space
occupied by either air of water in the soil. It
is determined mostly by soil texture and by
structure to some extent. So it can be separated
into textural porosity and structural
porosity Permeability connectedness of
pores Specific surface particle area per unit
volume. Clays have the largest, and sands have
the smallest specific area (Table 6.1 WR)
Basic Soil Physical Properties
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Specific Surface Area
Basic Soil Physical Properties
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Soil Water StorageTerms to Remember
Soil water content percentage of volume occupied
by water in the soil, usually in units of
cm3/cm3 Field capacity the capacity of soil to
hold water under gravity Wilting point minimum
water content of the soil at which plants can
extract water. It depends on texture. Clays have
the highest wilting point while sands have the
lowest. Available water capacity field capacity
less wilting point Water table depth of soil at
which the soil is saturated (Figure 3.1 Jones)
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Water Table in a Hydrological System
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Soil Water Forces
Capillarity surface tension at the interface
between soil air and soil water Adsorption
electrostatic forces in which the polar water
molecules are attached to charged faces of the
solids. They occur only very close to the surface
of solids. Osmosis forces due to solutes in
soil water. Saline water, for example, can have
large osmosis that prevents plant roots from
extracting the water. Gravity vertical forces
exerted to water by gravity of the
Earth. Pressure forces that increase below
water table
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Soil water potential
The potential energy of soil water relative to
that of water in a standard state (elevation,
purity, atmospheric pressure, temperature)
? is the total water potential ?g is
gravitational potential ?p is pressure
potential ?o is osmotic potential
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Water Potential
What is the physical meaning of water
potential? It is energy per unit volume. It has
the same unit as pressure (bar, mb, Pascal) Unit
of mass kg (or g) Units of forces kg m/s2 Units
of pressure, i.e., force per unit area (kg
m/s2)/m2 kg/(ms2)Pascal Units of energy
(equivalent to force times distance) Jkg m/s2
? m kg m2/s2 Units of energy per unit volume
(pressure) (kg m2/s2)/m3kg/(ms2)