13. Measuring Water Availability and Uptake in Ecosystem Studies
203
to measure'll in ecosystem studies are thermocouple psychrometry and the plant pressure chamber;
both methods are widely used and have been reviewed elsewhere (e.g., Rundel and Jarrell 1989;
Boyer 1995). We provide an overview of each approach and direct the reader to more detailed
sources. We also outline a filter paper method that
is a low-cost alternative for measuring soil 'V in
some conditions.
Thermocouple Psychrometry
Water potential (expressed in units of pressure) may
be described based on the relative vapor pressure
of water in the experimental system compared with
that of free, pure water at an equivalent temperature
and pressure:
'V = ~T In(!...)
Vw
eo
(13.5)
where R is the universal gas constant, T is absolute
temperature in OK, if w is the partial molal volume
of water, and e and eo are the vapor pressures of
water in the experimental system and in the free,
pure state, respectively. The ability to measure the
vapor pressure of water in equilibrium with an in
situ volume of soil or quantity of leaf tissue provides a convenient method to estimate'll in thermocouple psychrometry.
Thermocouple psychrometry is based on the Seebeck effect: If a circuit consists of two metals and
if the two metal junctions are at different temperatures, then a current flows around the circuit in a
direction dependent on the metals chosen and the
junction temperatures. A microvoltmeter may be
used to measure the voltage in the circuit and, consequently, the temperature difference between the
two junctions. The thermocouple junction used to
measure relative humidity typically consists of fine
chromel and constantan wires <25 /lm in diameter
(Brown 1970). The reference junction is typically
a series of gold or copper posts to which the wires
are connected. Ceramic or screen-cage covers allow
the measurement junction to come into eqUilibrium
with 'V in the surrounding soil solution.
The most common method for measuring in situ
soil'll is Peltier thermocouple psychrometry. By applying a current to the circuit, the measurement
junction in equilibrium with the soil solution is
cooled below the dew point of the surrounding air,
causing droplets of water to form on the junction.
When the Peltier current is turned off, the water on
the junction begins to evaporate at a rate dependent
on e in Equation 13.5. Isopiestic psychrometry
("iso" = equal, "piestic" = pressure) is a second
type of psychrometry used to measure leaf'll and
ex situ soil'll (Boyer and Knipling 1965). Drops of
a series of solutions of known'll, created using the
Van't Hoff relationship discussed earlier, are placed
on looped thermocouple wire and allowed to come
to eqUilibrium with a leaf or soil sample. The solution that provides the same output as a dry thermocouple, neither warming nor cooling the junction, yields the sample'll. Although no calibration
is required and the technique is very accurate
(± 0.01 MPa) (Boyer 1995), it is also time consuming and can not be automated or used for in situ
soil analysis.
Thermocouple psychrometry is a powerful, accurate technique for estimating 'V in a small soil
volume, but it has a number of difficulties. Except
for the isopiestic technique, each psychrometer
must be calibrated individually, making the method
laborious and expensive. Contaminants, particularly salts, present on the measurement junction can
alter the rate of evaporation and lead to inaccurate
estimates of e (Rundel and Jarrell 1989). Because
of problems with thermal gradients (Wiebe et al.
1977), in situ thermocouple psychrometers are usually positioned horizontally (making sure that the
wire leading to them cannot act as a conduit for
flowing water) and are generally only used below
30-cm soil depth. Thermocouple psychrometers are
also not especially accurate in very wet soils. Boyer
(1995) provides a much more detailed description
of the theory of thermocouple psychrometry and its
use in measuring'll in plant tissue; Rundel and Jarrell (1989) provide an overview of thermocouple
psychrometry for soil measurements. Commercial
manufacturers include JRD Merrill Specialty
Equipment (Logan, UT), Wescor, Inc. (Logan, UT),
and Isopiestics Co. (Lewes, DE).
Plant Pressure Chamber
A second technique for measuring'll in the environment is the plant pressure chamber. First used
in the 1800s (Dixon 1914), the modem version
dates to Scholander et al. (1965). A stem, branch,
or leaf is cut from a plant and placed upside-down
in an air-tight chamber with the point of excision
203
to measure'll in ecosystem studies are thermocouple psychrometry and the plant pressure chamber;
both methods are widely used and have been reviewed elsewhere (e.g., Rundel and Jarrell 1989;
Boyer 1995). We provide an overview of each approach and direct the reader to more detailed
sources. We also outline a filter paper method that
is a low-cost alternative for measuring soil 'V in
some conditions.
Thermocouple Psychrometry
Water potential (expressed in units of pressure) may
be described based on the relative vapor pressure
of water in the experimental system compared with
that of free, pure water at an equivalent temperature
and pressure:
'V = ~T In(!...)
Vw
eo
(13.5)
where R is the universal gas constant, T is absolute
temperature in OK, if w is the partial molal volume
of water, and e and eo are the vapor pressures of
water in the experimental system and in the free,
pure state, respectively. The ability to measure the
vapor pressure of water in equilibrium with an in
situ volume of soil or quantity of leaf tissue provides a convenient method to estimate'll in thermocouple psychrometry.
Thermocouple psychrometry is based on the Seebeck effect: If a circuit consists of two metals and
if the two metal junctions are at different temperatures, then a current flows around the circuit in a
direction dependent on the metals chosen and the
junction temperatures. A microvoltmeter may be
used to measure the voltage in the circuit and, consequently, the temperature difference between the
two junctions. The thermocouple junction used to
measure relative humidity typically consists of fine
chromel and constantan wires <25 /lm in diameter
(Brown 1970). The reference junction is typically
a series of gold or copper posts to which the wires
are connected. Ceramic or screen-cage covers allow
the measurement junction to come into eqUilibrium
with 'V in the surrounding soil solution.
The most common method for measuring in situ
soil'll is Peltier thermocouple psychrometry. By applying a current to the circuit, the measurement
junction in equilibrium with the soil solution is
cooled below the dew point of the surrounding air,
causing droplets of water to form on the junction.
When the Peltier current is turned off, the water on
the junction begins to evaporate at a rate dependent
on e in Equation 13.5. Isopiestic psychrometry
("iso" = equal, "piestic" = pressure) is a second
type of psychrometry used to measure leaf'll and
ex situ soil'll (Boyer and Knipling 1965). Drops of
a series of solutions of known'll, created using the
Van't Hoff relationship discussed earlier, are placed
on looped thermocouple wire and allowed to come
to eqUilibrium with a leaf or soil sample. The solution that provides the same output as a dry thermocouple, neither warming nor cooling the junction, yields the sample'll. Although no calibration
is required and the technique is very accurate
(± 0.01 MPa) (Boyer 1995), it is also time consuming and can not be automated or used for in situ
soil analysis.
Thermocouple psychrometry is a powerful, accurate technique for estimating 'V in a small soil
volume, but it has a number of difficulties. Except
for the isopiestic technique, each psychrometer
must be calibrated individually, making the method
laborious and expensive. Contaminants, particularly salts, present on the measurement junction can
alter the rate of evaporation and lead to inaccurate
estimates of e (Rundel and Jarrell 1989). Because
of problems with thermal gradients (Wiebe et al.
1977), in situ thermocouple psychrometers are usually positioned horizontally (making sure that the
wire leading to them cannot act as a conduit for
flowing water) and are generally only used below
30-cm soil depth. Thermocouple psychrometers are
also not especially accurate in very wet soils. Boyer
(1995) provides a much more detailed description
of the theory of thermocouple psychrometry and its
use in measuring'll in plant tissue; Rundel and Jarrell (1989) provide an overview of thermocouple
psychrometry for soil measurements. Commercial
manufacturers include JRD Merrill Specialty
Equipment (Logan, UT), Wescor, Inc. (Logan, UT),
and Isopiestics Co. (Lewes, DE).
Plant Pressure Chamber
A second technique for measuring'll in the environment is the plant pressure chamber. First used
in the 1800s (Dixon 1914), the modem version
dates to Scholander et al. (1965). A stem, branch,
or leaf is cut from a plant and placed upside-down
in an air-tight chamber with the point of excision