Water Potential and Water Content
55
In this book we use Jlkg, but the reader can consider those equivalent to
kPa if pressure units are more familiar.
The water potential is made up of several components. The total
potential is usually written as the sum of the components:
where the subscripts, g, m, p, and o are for gravitational, matric, pressure,
and osmotic components. While each of these components (and others
that could be defined) can contribute to the total potential, there are many
situations where only one or two of the component potentials are active.
The gravitational potential is the potential energy of water as a result
of its position in a gravitational field. A reference height must be specified
in order to compute a gravitational potential. The gravitational potential
is then:
where g is the gravitational constant (9.8 m sV2) and h is the vertical distance fromthe reference height to the location where potential is specified.
Above the reference h is positive and below the reference it is negative.
The matric potential arises from the attraction between water and soil
particles, proteins, cellulose, etc. Adhesive and cohesive forces bind the
water and reduce its potential energy compared to that of free water.
For any substance that imbibes water there exists a relationship between
water content and matric potential. This relationship is called the moisture
characteristic. Figure 4.1 shows moisture characteristics for soils with
three different textures. Most of the water in the clay is held very tightly
(at low potential) because the large surface area of the clay is able to bind
the water. Most of the water in the sand is held loosely (at high potential)
because the sand matrix is ineffective in binding water. Similar curves
could be made for cellulose, protein, etc., and they would have similar
shapes. Tracy (1976) obtained a moisture characteristic for a whole frog.
Note that the matric potential is always negative or zero. An empirical
equation that closely approximates most moisture characteristics over a
wide range of matric potentials is:
where w is the water content and a and b are constants determined from
data.
The pressure potential arises as a result of an applied hydrostatic or
pneumatic pressure. Examples of this potential are the blood pressure in
an animal, the water pressure under a water table in the soil, the turgor
pressure inside plant cells, or the air pressure inside a pressure vessel
which measures water potential in leaves or matric potential in soil. In
many cases the pressure potential is hard to distinguish from the matric
potential. For example, in soil a positive hydrostatic pressure is called a
pressure potential and a negative pressure a matric potential. In the xylem
of plants the pressures are generally negative, but the potential is referred
55
In this book we use Jlkg, but the reader can consider those equivalent to
kPa if pressure units are more familiar.
The water potential is made up of several components. The total
potential is usually written as the sum of the components:
where the subscripts, g, m, p, and o are for gravitational, matric, pressure,
and osmotic components. While each of these components (and others
that could be defined) can contribute to the total potential, there are many
situations where only one or two of the component potentials are active.
The gravitational potential is the potential energy of water as a result
of its position in a gravitational field. A reference height must be specified
in order to compute a gravitational potential. The gravitational potential
is then:
where g is the gravitational constant (9.8 m sV2) and h is the vertical distance fromthe reference height to the location where potential is specified.
Above the reference h is positive and below the reference it is negative.
The matric potential arises from the attraction between water and soil
particles, proteins, cellulose, etc. Adhesive and cohesive forces bind the
water and reduce its potential energy compared to that of free water.
For any substance that imbibes water there exists a relationship between
water content and matric potential. This relationship is called the moisture
characteristic. Figure 4.1 shows moisture characteristics for soils with
three different textures. Most of the water in the clay is held very tightly
(at low potential) because the large surface area of the clay is able to bind
the water. Most of the water in the sand is held loosely (at high potential)
because the sand matrix is ineffective in binding water. Similar curves
could be made for cellulose, protein, etc., and they would have similar
shapes. Tracy (1976) obtained a moisture characteristic for a whole frog.
Note that the matric potential is always negative or zero. An empirical
equation that closely approximates most moisture characteristics over a
wide range of matric potentials is:
where w is the water content and a and b are constants determined from
data.
The pressure potential arises as a result of an applied hydrostatic or
pneumatic pressure. Examples of this potential are the blood pressure in
an animal, the water pressure under a water table in the soil, the turgor
pressure inside plant cells, or the air pressure inside a pressure vessel
which measures water potential in leaves or matric potential in soil. In
many cases the pressure potential is hard to distinguish from the matric
potential. For example, in soil a positive hydrostatic pressure is called a
pressure potential and a negative pressure a matric potential. In the xylem
of plants the pressures are generally negative, but the potential is referred