13. Measuring Water Availability and Uptake in Ecosystem Studies
201
be obtained from this disequilibrium. Water potential is most commonly expressed in units of pressure such as megapascals (MPa, and 1 Pa = 1
N m- 2 = 1 (kg m S-2) m- 2 ). The total water potential of most systems is usually negative (see below) because the reference state is defined as '"
= O. When comparing two experimental states,
such as the soil and an adjacent root, water flows
from the system with a less negative", to that with
a more negative ",.
Total water potential, "' w, can be broken down
into several components:
"' w = "' g + "' 0 + "' m + "' p (13.4)
where g, 0, m, and p refer to the gravitational, osmotic, matric, and pressure components of water
potential. The gravitational component reflects the
work required to lift water and is described by '" g
= Pwgh, where g is the acceleration due to gravity
and h is the vertical change in height. The value of
"' g changes approximately 0.01 MPa per meter of
difference in height, so relatively large values of h
are required for", g to become significant. The osmotic component, "'0' is described by the Van't
Hoff relation for dilute ideal solutions: "' 0 = cRT,
where T is absolute temperature, R is the universal
gas constant (8.314 m 3 Pa mol-I K - 1), and c is the
osmolality of the solution. The matric component,
'" m is a function of the surface tension of water
among soil particles: "' m = (- 2ycosfJ)/r, where y
is the surface tension of the liquid, fJ is the contact
angle between liquid and soil, and r is the radius of
the soil pore (Hanks and Ashcroft 1986 but see Tyree and Karamanos, 1980; Sperry, 1997). The pressure, "' p, is generally taken as zero in the soil where
the system is at atmospheric pressure. In the plant,
"'p is typically negative in the apoplast and positive
in the symplast due to turgor generated by osmosis.
More detailed descriptions of water potential and
the accompanying thermodynamics are presented
in Slatyer (1967), Nobel (1991), and Kramer and
Boyer (1995).
Conversions between "', em, and ev are generally
based on correlations with soil texture (the proportion of sand [0.05 to 2 mm], silt [0.002 to 0.05 mm]
and clay [<0.002 mm] particles that make up the
soil). The amount of available soil water (the difference between field capacity and the permanent
wilting percentage) is smallest for a sandy soil,
greatest for a loam, and intermediate for a clay soil
(Fig. 13.2, bottom). Moisture tension release curves
showing the relationship between '" and em or ev
are generated by measuring", directly with thermocouple psychrometry (see below) and then destructively harvesting the soil to measure em or ev
(see Fig. 13.2, top).
Methods for Estimating Plant
and Soil Moisture
We describe four approaches for measuring water
status in the environment. The first is a brief discussion of gravimetric techniques, included because most other methods are calibrated against
gravimetric em and ev• The second approach is
methods for measuring '" directly in the environment, including the plant pressure chamber, thermocouple psychrometry, and the filter-paper
method. The third is time domain reflectometry, an
increasingly important technique described in some
detail. The fourth and final approach is microwave
radiometry for remotely sensing soil moisture, a
technique with considerable promise for ecosystem
and landscape studies.
Gravimetric Measurements of em and e v
The mass water content of soils (em) is determined
simply and accurately using gravimetric techniques. Field samples are collected in soil cores
sealed in plastic bags or in metal tins sealed with
electrical tape. The wet samples are best weighed
with a balance in the field, but transporting them to
the laboratory in a cooler for weighing is also common. After drying the soil to constant weight at
105°C, the wet soil mass and oven-dry soil mass
are used to calculate em in Equation 13.1. The volumetric water content of soils (e v ) is similarly obtained by collecting soil samples of known volume
and assuming a density of 1 g cm - 3 for water. Both
em and ev are easily converted if the soil's bulk
density is known.
The advantages of the gravimetric method are its
simplicity and low cost, requiring only a drying
oven and a balance. This technique is used to calibrate other soil moisture instruments and can be
applied under a range of soil depths and moisture
conditions; it is one of the only techniques accurate
201
be obtained from this disequilibrium. Water potential is most commonly expressed in units of pressure such as megapascals (MPa, and 1 Pa = 1
N m- 2 = 1 (kg m S-2) m- 2 ). The total water potential of most systems is usually negative (see below) because the reference state is defined as '"
= O. When comparing two experimental states,
such as the soil and an adjacent root, water flows
from the system with a less negative", to that with
a more negative ",.
Total water potential, "' w, can be broken down
into several components:
"' w = "' g + "' 0 + "' m + "' p (13.4)
where g, 0, m, and p refer to the gravitational, osmotic, matric, and pressure components of water
potential. The gravitational component reflects the
work required to lift water and is described by '" g
= Pwgh, where g is the acceleration due to gravity
and h is the vertical change in height. The value of
"' g changes approximately 0.01 MPa per meter of
difference in height, so relatively large values of h
are required for", g to become significant. The osmotic component, "'0' is described by the Van't
Hoff relation for dilute ideal solutions: "' 0 = cRT,
where T is absolute temperature, R is the universal
gas constant (8.314 m 3 Pa mol-I K - 1), and c is the
osmolality of the solution. The matric component,
'" m is a function of the surface tension of water
among soil particles: "' m = (- 2ycosfJ)/r, where y
is the surface tension of the liquid, fJ is the contact
angle between liquid and soil, and r is the radius of
the soil pore (Hanks and Ashcroft 1986 but see Tyree and Karamanos, 1980; Sperry, 1997). The pressure, "' p, is generally taken as zero in the soil where
the system is at atmospheric pressure. In the plant,
"'p is typically negative in the apoplast and positive
in the symplast due to turgor generated by osmosis.
More detailed descriptions of water potential and
the accompanying thermodynamics are presented
in Slatyer (1967), Nobel (1991), and Kramer and
Boyer (1995).
Conversions between "', em, and ev are generally
based on correlations with soil texture (the proportion of sand [0.05 to 2 mm], silt [0.002 to 0.05 mm]
and clay [<0.002 mm] particles that make up the
soil). The amount of available soil water (the difference between field capacity and the permanent
wilting percentage) is smallest for a sandy soil,
greatest for a loam, and intermediate for a clay soil
(Fig. 13.2, bottom). Moisture tension release curves
showing the relationship between '" and em or ev
are generated by measuring", directly with thermocouple psychrometry (see below) and then destructively harvesting the soil to measure em or ev
(see Fig. 13.2, top).
Methods for Estimating Plant
and Soil Moisture
We describe four approaches for measuring water
status in the environment. The first is a brief discussion of gravimetric techniques, included because most other methods are calibrated against
gravimetric em and ev• The second approach is
methods for measuring '" directly in the environment, including the plant pressure chamber, thermocouple psychrometry, and the filter-paper
method. The third is time domain reflectometry, an
increasingly important technique described in some
detail. The fourth and final approach is microwave
radiometry for remotely sensing soil moisture, a
technique with considerable promise for ecosystem
and landscape studies.
Gravimetric Measurements of em and e v
The mass water content of soils (em) is determined
simply and accurately using gravimetric techniques. Field samples are collected in soil cores
sealed in plastic bags or in metal tins sealed with
electrical tape. The wet samples are best weighed
with a balance in the field, but transporting them to
the laboratory in a cooler for weighing is also common. After drying the soil to constant weight at
105°C, the wet soil mass and oven-dry soil mass
are used to calculate em in Equation 13.1. The volumetric water content of soils (e v ) is similarly obtained by collecting soil samples of known volume
and assuming a density of 1 g cm - 3 for water. Both
em and ev are easily converted if the soil's bulk
density is known.
The advantages of the gravimetric method are its
simplicity and low cost, requiring only a drying
oven and a balance. This technique is used to calibrate other soil moisture instruments and can be
applied under a range of soil depths and moisture
conditions; it is one of the only techniques accurate