Liquid Water in Organisms and their Environment
things, is not like the water in a glass. It is bound by the tissue or soil
matrix, diluted by solutes, and sometimes is under pressure or tension.
Its energy state is therefore quite different from that of water in a glass.
The water content is simply the ratio of the volume of water in a
material to its total volume, or the ratio of mass of water to dry or wet
mass of the material. Different bases are used as the standard in different
disciplines, and all are called water content, so it is easy to make mistakes
if one is not careful. In this book water content is defined as:
where V is the volume, m is the mass, and subscripts w , t, and d refer to
water, total, and dry volume or mass. We call 9 volumetric water content
and w the mass water content. These are related by
where pb is the bulk density
Water potential is defined as the potential energy per mole, per unit mass,
per unit volume, or per unit weight, of water, with reference to pure
water at zero potential. In thermodynamic terms, the energy per mole is
the molar Gibbs free energy of the water in the system. A gradient of
the water potential is the driving force for liquid water movement in a
system.
As indicated, several sets of units are in use to describe water potential.
For consistency with the rest of this book, we should use energy per mole,
but this has not been used elsewhere, and may be completely unfamiliar to
readers. Our preference is for energy per unit mass (Jkg). The units clearly
show energy and mass, and, unlike volume, the mass does not vary with
the density of the water. Energy per unit volume (Urn3) is dimensionally
equivalent to pressure (kPa or MPa). These units are frequently used for
water potential, but fail to indicate a relationship to specific energy and
have a less sound basis for the computation (the specific volume of water
varies with density and is therefore dependent on temperature and binding
energy). While these are minor objections to the use of pressure for water
potential, it should be pointed out that there certainly are no advantages to
the use of pressure units, and the mass-based units have historical priority.
Energy per unit weight (J/N) is dimensionally equivalent to the height of
a water column (m) in a gravitational field. It is used mainly in soil water
flow problems where height of a physical water column is a convenient
reference for other potentials. If the density of water is assumed to be 1
~ g / m ~ ,
and the gravitational constant is 9.8 m s - ~ ,
then
things, is not like the water in a glass. It is bound by the tissue or soil
matrix, diluted by solutes, and sometimes is under pressure or tension.
Its energy state is therefore quite different from that of water in a glass.
The water content is simply the ratio of the volume of water in a
material to its total volume, or the ratio of mass of water to dry or wet
mass of the material. Different bases are used as the standard in different
disciplines, and all are called water content, so it is easy to make mistakes
if one is not careful. In this book water content is defined as:
where V is the volume, m is the mass, and subscripts w , t, and d refer to
water, total, and dry volume or mass. We call 9 volumetric water content
and w the mass water content. These are related by
where pb is the bulk density
Water potential is defined as the potential energy per mole, per unit mass,
per unit volume, or per unit weight, of water, with reference to pure
water at zero potential. In thermodynamic terms, the energy per mole is
the molar Gibbs free energy of the water in the system. A gradient of
the water potential is the driving force for liquid water movement in a
system.
As indicated, several sets of units are in use to describe water potential.
For consistency with the rest of this book, we should use energy per mole,
but this has not been used elsewhere, and may be completely unfamiliar to
readers. Our preference is for energy per unit mass (Jkg). The units clearly
show energy and mass, and, unlike volume, the mass does not vary with
the density of the water. Energy per unit volume (Urn3) is dimensionally
equivalent to pressure (kPa or MPa). These units are frequently used for
water potential, but fail to indicate a relationship to specific energy and
have a less sound basis for the computation (the specific volume of water
varies with density and is therefore dependent on temperature and binding
energy). While these are minor objections to the use of pressure for water
potential, it should be pointed out that there certainly are no advantages to
the use of pressure units, and the mass-based units have historical priority.
Energy per unit weight (J/N) is dimensionally equivalent to the height of
a water column (m) in a gravitational field. It is used mainly in soil water
flow problems where height of a physical water column is a convenient
reference for other potentials. If the density of water is assumed to be 1
~ g / m ~ ,
and the gravitational constant is 9.8 m s - ~ ,
then