Water Potential Measurements In Trees, Stems and Roots

A. Water Potential Measurements in Trees

Wiebe et al. (1970) used minature (20 mm long) porous cup
psychrometer to measure water potentials of trees under field
conditions that included variable temperatures. All
psychrometers were installed at heights of 1 to 2 m above
ground in juniper, elm, Russian olive, and maple trees. For
trunk installations, holes were drilled through the bark to a
depth of about 10 mm under the cambium. Branch installations
were made in the angle betweeen two branches at (least 20 mm
in diameter). Each hole containing a psychrometer was
immediately covered with an asphalt compound for grafting wax
to prevent drying. Theses workers used polyure-thane foam in
sheets and as a spray to insulate the stem in the vicinity of
the implanted psychrometer and thus reduce temperature changes
caused by intermittent dorect sunlight. To test the
reliability of the tree water potential measurements, Wiebe et
al. (1970) simultaneously determined twig and leaf water
potentials using other methods and psychrometer soil water
potential measurements. The highest water potentials were
recorded in the soil installations and in each case decreased
progressively up the tree trunk to the branches and leaves.
When transpiration was reduced ( at night or on a rainy day) ,
the water potential of the twigs and branches increased and
the overall gradient decreased. The data obtained from the
implanted stem psychrometers were always in good agreement
with data obtained from pressure chamber and laboratory
psychrometer chamber measurements when adjusted for the
gradient through the trees.
Leaf water potential determinations never gave higher values
than the twig water potential determinations with pressure
chamber.

B. Water Potential Measurement In Stems
Michel (1977) has pointed out that knowledge of root
permeability to water movement was limited by lack of
continuous records of water potential at the base of an intact
plant stem. To overcome this deficiency, Michel (1977) and
Pallas and Michel (1977) devised a technique for attaching
dewpoint hygrometers to the secondary xylem vessels at the
base of plant hypocotyls. Some difficulty was encountered
during measurements because of thermocouple contamination by
fungal growth and temperature gradients Michel (1979). In
spite of these problems, continuous monitoring of water
potential was achieved for soybean (Michel 1975, 1977) and
peanuts (Pallas and Michel, 1978; Pallas et al., 1979). In a
controlled environment experiment, Pallas and Michel (1978)
compared results from hygrometers attached to the leaves and
stem of peasant plants. Theses workers found that within 30
min after illumination their controlled environment, the stem
dewpoint hygrometers could detect the beginning of an
oscillation in stem water potential. By contrast, only one of
their four leaf dewpoint hygrometers detected any oscillation
in leaf water potential. They found that the amplitude of
cycles in stem water potential detected were several hundreds
of kilopascal greater than those detected by the leaf
hygrometer. Their study indicated that leaf hygrometers on
peanut and soybean leaves with intact cuticles were less
sensitive to dynamic changes in water potential of the plant
than embedded stem hygrometers were. When the water potential
changes of the plant were not rapid, leaf hygrometer
measurements agreed closely with stem hygrometer measurements.

C. Water Potential Measurements in Roots
Measurement techniques used in root water uptake experiments
prior to 1977 have been reviewed by Herkelrath et al. (1977).
Since then, field measurements of root water potential have
been performed (Nnyamah and Black, 1977a,b; nnymah et
al.,1978), using the dewpoint technique. Root hygrometers were
installed by exposing the root xylem tissue and extended to
form a lip. The exposed inner surface was lined with dry
gypsum powder; a porou cup soil hygrometer was placed axcially
against the xylem beneath the lip and sealed by three layers
of electrician tape and a coating of Dow Corning 781 silicone
rubber. Soil was replaced around the root and 24 hr were
allowed for equilibration (Nnyamah and Black, 1977a). As a
check on the performance of the root hygrometers, root xylem
pressure potential was measured using the pressure chamber
apparatus (Table IV). The roots were 1.5-2 mm in diameter and
150 mm long at a depth of 150-200 mm. These workers concluded
that their technique reported in the literature because
measurements were made directly reported in the literature
because measurements were made directly and continuously in
the path of water movement (Nnyamah et al., 1978). Brunini
(1979, cited by Brunini and Thurtell, 1982), also measured
root water potential directly using dewpoint hygrometers.

VII Soil Hygrometer
A. Construction

The materials used for construction of the soil hygrometer and
the shape, arrangement of components, and lead wire gauge
diameter affect the performance of the instrument. At present,
these factors are subject of considerable research and no
clear optimal specifications have yet emerged. The
consequences that arise from hygrometer construction are
discussed in this section. Soil hygrometer sensors for in situ
measurement of water potential have been constructed from a
variety of materials. The basic elements of a soil hygrometer
consist of a body into which the lead wire passes and, in
turn, attaches to the thermocouple sensing junction. This
junction is enclosed and protected by a porous barrier which
maintains the cavity in the soil (Rawlins and Dalton, 1976)
and which allows water vapor equilibration between the
thermocouple cavity and soil.
The body of the hygrometer is usually constructed from a
Teflon plug and coated with epoxy (Hoffman and Splinter,
1968a; Brown, 1970; Wiebe et al., 1971) or inserted into an
acrylic (Rawlins and Dalton, 1967 or metal body. Various
metals have been used for this purpose, including brass
(Campbell, 1979), copper (McAneney et al., 1979); Brunni and
Thurtell, 1982), and stainless steel (some commercial models).
Copper heat sinks have been employed in the body for thermal
stability (Rawlins and Dalton, 1967).
The interior of the thermocouple chamber should be constructed
from material that does not absorb large quantities of water.
Campbell (1972) investigated the adsorption properties of a
range of materials and found Vaseline to absorb the least
water followed by brass, stainless steel, nickel,
polyethylene, Teflon and paraffin wax. Tygon, axle grease and
rubber cement were found to be unsuitable. Some users coated
the interior of the hygrometer chamber with resolidated wax
just prior to use (Baughn, 1974; McAneney et al., 1979).
The porous barrier between the thermocouple cavity and the
soil serves to protect the sensor from contamination and
provides an equilibration path for water vapor. Materials used
for construction include ceramic (Rawlins and Dalton 1967),
which usually has an air entry pressure of about 100 kPa, but
McAneney et al. (1979) used a ceramic plug with an air entry
pressure of 1500-kPa. Ingvalson et al. (1970) incorporated a
1500 kPa plug in a 100-kPa ceramic bulb for measuring osmotic
and water potential. Lang (1968) was the first to replace the
ceramic bulb with a cylindrical steel mesh number of 200 with
openings of 74 <$Emu>m. Brown and Collins (1980) designed a
double-screen cage (inner 400 mesh, outer 200 mesh) to improve
protection against contamination of the thermojunction by
soil. The thermocouple was also provided with an additional
copper lead to measure temperature at the junction. Brunni and
Thurtell (1982) introduced the use of a porous silver membrane
with an air entry pressure of 1500 kPa. the shape of soil
hygrometer varies from porous spherical ceramic bulbs to
cylindrical ceramic or stainless steel mesh cups or solid
cylindrical bodies with porous end windows (Wiebe et al.,
1977). Porous, disk-shaped units have also been designed
(Cammpbell, 1979; Brunni and Thurtell, 1982).
The diffusion resistance of the ceramic barrier 9wall) will
affect the response time of the hygrometer. Rawlins and Dalton
(1976) calculated the wall conductivity required to maintain a
potential difference of less than 10 kPa between the
thermocouple cavity and the soil. They concluded that the wall
conductivity was about one-sixth the saturated conductivity of
porous ceramic (with an air entry pressure of 100 kPa) used in
tensiometers. Brown (1970) investigated the water vapor
equilibrium time for junctions that were enclosed in ceramic
cups, stainless steel mesh, and a bare junction. The bare
junction reached vapor equilibrium in about the same time (20
min) as temperature but the screen-caged junction required 33
min, and junction in the ceramic cup 170 min. Extended periods
of time in soil would tend to reduce the conductivity of the
ceramic cup more than steel mesh owing to the coarser mesh
apertures (about 74 <$Emu>m). In a laboratory study, Riggle
and Slack (1980) abandoned use of their screen mesh
psychrometer as corrosion (rust) occurred along the seams of
the protecting cover during calibration by complete immersion
in salt solution.
McAneney et al (1979), using a 1500-kPa ceramic plug, did not
identify vapor diffusion resistance of their instruments as a
major limitation. However, they did conclude that their sensor
was unsuitable for monitoring rapid changes in osmotic
potential because of the high diffusion resistance of the
ceramic to solutes. A similar limitation may apply to
diffusion of water vapor. Brunni and Thurtell (1982) replaced
the ceramic cup with a silver membrane with an air entry
pressure of 1500 kPa and determined that the solute
equilibrium time of their device was 2 hr as opposed to the
equilibration time of 23 hr required by the device of McAneney
et al. (1979). Poor contact between the soil and the
hygrometer porous barrier may contribute to diffusion
resistance problems for a rapidly changing soil water
potential. Coarse-grained soils, which have a preponderance of
particles in the 200- to 2000 <$Emu>m diameter class,
generally after a large limited number of liquid films that
actually touch the ceramic surface. This contact resistance
has been identified with tensiometer response (Towner, 1980),
but it could be greater in hydrometry where measurements are
made at lower potentials than in tensiometry. Merrill and
Rawlins drew attention to the possibility of a high contact
resistance developing in swelling soil as shrinkage causes
soil to be drawn away from the hygrometer. Ceramic cups are
likely to be more adversely affected than steel mesh cylinders
because of the difference in the mode of operation between
these materials. Ceramic cups are in liquid contact with soil
water and the surface where measurements are made in the
interior of the cup. Screen cages, however, probably have a
lesser liquid conducting role and the recessing surface is
probably the soil-water interface against the mesh. The high
contact resistance problem would cause measurement errors if
the soil water were rapidly changing; if the soil water
potential changes slowly, vapor flow alone would cause the
enclosed air and soil water potential to equilibrate (Merrill
and Rawlins, 1972).

B. Calibration

The procedures for calibration of soil hygrometers follow the
general method discussed in Section IV. However, certain
details of the procedure differ from leaf hygrometer
calibration because of the presence of large and persistent
temperature gradients in soil and because soil hygrometers
cannot be calibrated in situ. For this reason, meticulous
attention to the temperature dependence of the calibration
sensitivity is necessary. In addition, the chamber during
calibration must be as similar to that of the situ geometry as
possible. Soil hygrometers may be calibrated in the
psychrometric or dewpoint mode. There appear to be certain
advantages in selecting the dewpoint mode (Section V,F), but
few rigorous comparisons have been conducted to confirm this.
However, Nnyamah and Black (1977a) found that dewpoint and
psychrometer measurements of water potential in the field were
comparable to within 30 kPa over the range - 1200 to -300 kPa.
Calibration of screen-caged and ceramic hygrometers differs
because the evaporation surface is at the screen and
soil-water interface system for the former and at the interior
of the ceramic surface for the latter. The ceramic cups acts
as a continuation of the soil pore system and ideally, it
should be in liquid equilibrium with soil water. For this
reason, the wet-bulb depression can be assumed to be the
difference in temperature between the inner ceramic surface
temperature and the wet-bulb temperature (Campbell, 1979).
Brown (1970) described the calibration of screen-caged
psychrometers using small test tubes lined with filter paper,
just saturated with calibrating solution. The sensor is placed
so as to be surrounded by filter paper and the test tube is
sealed with a rubber stopper and immersed in a
constant-temperature water bath. Subsequently, Brown and
Collins (1980) introduced a stainless steel chamber that could
be sealed using rubber O-rings with a small cavity entirely
lined with filter paper. The chambers are immersed in an
isothermal water bath with about 350 mm of lead wire to
prevent heating flow along the wires from reaching the
chambers. Brown and Collins (1980) describe a double mesh,
screen-caged hygrometer (inner 400- and outer 200-mesh
stainless steel screens) that could be immersed in salt
solution for calibration purposes. Ceramic cup hygrometers are
usually calibrated by immersion of the hygrometer in a small
container of calibrating solution, which is then placed in an
isothermal water bath (Wiebe et al.,1971). Two concentrically
arranged glass test tubes have been also used to facilitate
good thermal contact between and calibrating solution (Oster
et al., 1969). Wheeler et al. (1972) found that immersion of
psychrometers in salt solutions resulted in leakage of salt
into the sensor chamber, thereby causing about 30% of the
units to malfunction. Greater success was obtained by using
alternate methods such as a vibrating bath, which splashed
calibrating solution on to the exterior of the ceramic bulb.
After calibrating, the sensors should be thoroughly washed in
several changes of deionized water. Neither of these
techniques simulate the geometry surrounding the hygrometer
when in position in the soil. The error which this introduces
the hygrometer when in position in the soil. The error which
this introduces is not known but is generally assumed to be
negligible. In some cases, only the thermocouple junction was
calibrated in ceramic cups permanently affixed in calibrating
chambers and then transferred to different ceramic cups for
placement in soil. This practice was, however, identified as a
possible source of error in psychrometric water potential
measurement because it displaced the calibration intercept
(Rawlins and Dalton, 1967).

C. Temperature Effects
Fluctuating temperatures are a major source of error in the
measurement of the wet-bulb depression. If the dry-bulb
temperature is not equal to the sample temperature (ceramic
surface or soil water-screen interface0, this would represent
an error in measurement of the temperature difference between
the wet-bulb and the sample. This error is proportional to the
difference between internal instrument zero and the voltage
measured when both sensing and reference junctions are dry
(zero offset voltage). Because temperature gradients are an
unavoidable feature of soil hygrometry, it is necessary to
adopt precautions to reduce the associated measurement errors.
At present it is thought that this reduction may be achieved
either by using high thermal conductivity materials for
construction (Neumann and Thurtell, 1972; Campbell, 1979), or
by a careful attention to design so that vapor and heat flow
paths are identical, or by both (Rawlins and Dalton, 1967;
Wiebe et al., 1977). Other hygrometer modifications such as
double-junction thermocouples (Hsieh and Hungate, 1970) and
two thermocouples of opposite polarity (Hsieh and Hungate,
1970; Calissendorff and Gardner, 1972; McAneney et al., 1979)
have not served to solve the problem of measurement errors in
temperature gradients, although they may have served other
purposes (Wiebe et al., 1977).
Early hygrometers were very sensitive to temperature gradients
and precise temperature control was necessary (+-0.001 o
C). Introduction of a spherical ceramic cup by Rawlins and
Dalton (1967), followed by the subsequent modifications,
improved but did not eliminate temperature gradient
sensitivity. Further intensive research on the effect of soil
hygrometer design on temperature gradient errors has been
conducted by Weibe et al. (1977), Campbell (1979), and Weibe
and Brown (1979). Weibe et al. (977) used several types of
hygrometers in their investigation (spherical and cylindrical
ceramic, stainless steel mesh, and stainless steel with a mesh
end window). The design which was least sensitive to
temperature gradients was a stainless steel mesh cylinder with
the thermocouple located near the distal end of the chamber.
Campbell (1979) tested several brass hygrometers with ceramic
barriers for thermal stability in temperature gradients of
0.05 to 0.1 o C /mm. Errors in measuring the
temperature at the sensing junction were negligible compared
to measurement of wet-bulb depression. The design that gave
the lowest temperature gradient error was a symmetrical, small
brass hygrometer with a symmetrical ceramic side-window.
In order to ensure that the dry junction temperature is equal
to that of the ceramic surface, the thermocouple sensing
junction must symmetrically placed in relation to the surface
of the ceramic. In addition, heat conduction along the
thermocouple wires to the junction must be minimal. the
spherical design of Rawlins and Dalton (1967) approached this
ideal; Weibe et al. (1977) were able to demonstrate that some
of the cylindrical cup psychrometer also   approached this
ideal. Campbell 91979) showed that the junction temperature
could be maintained at chamber air temperature by ensuring
that the length of the thermocouple was at least 3 mm, if wire
diameter of 25 <$Emu>m was used. Campbell (1979) indicated
that an additional source of error from temperature gradients
is drift in the zero setting amplifier circuit. this change
can arise from changes in thermal voltages at contacts or
within the microvoltmeter and from changes in the reference
junction temperature during cooling, as may happen when there
is significant flux heat energy along connecting wires. Hence
it is advisable to use as short a cooling time as possible
free soil water potential measurements (say 10 sec for values
greater than -1500 kPa and 15 sec for values less than this).
Campbell (1979) concluded that zero offset values and
temperature differences between the evaporating (ceramic)
surface and the sensing junction were minimized by
constructing the hygrometer body from materials with high a
thermal conductivity (brass, stainless steel, aluminum). The
largest errors in water potential measurement arose from the
effect of the hygrometer on the water and heat flow patterns
in a temperature gradient. This manifests itself as a change
in soil water potential adjacent to the hygrometer. With the
use of suitable materials and a symmetrical design. Campbell
(1979) showed that it was possible to minimize the
perturbation of water and heat flow and reduce zero offset
values to one-tenth and measurement variability to one-third
that for a commercial ceramic hygrometer.
Weibe and Brown (1979) showed that variability in measurements
from several types of hygrometer was minimized by reducing the
size of the hygrometer and by using nonmetallic materials for
construction. This conclusion differs from that of Campbell
91979), who identified clear advantages in using metallic
hygrometers to reduce the temperature gradient within the
hygrometer chamber. Weibe and Brown (1979) showed that while
metal hygrometers reduced temperature gradients within the
hygrometer, gradients in soil outside the hygrometer were very
much steeper. These steep gradients appear to enhance water
vapor flux in the vicinity of the hygrometer. The movement of
water vapor and accumulation or loss of liquid water at the
surface and within the chamber of the hygrometer introduces
further error in measurement. Weibe and Brown 91979) found
that, in the presence of temperature gradients, cylindrical,
metal hygrometer with a metal screen end-window trapped large
amounts of water in soil adjacent to the outer surfaces, as
well as inside the hygrometer. this condition occurs because
the pathways for heat and vapor flow are not identical.
Smaller, ceramic hygrometers, which bridge a smaller total
temperature change in the temperature gradient, cause less
perturbation to the pattern of the water movement. the
condensation of water around cylindrical porous cup soil
psychrometers was alluded to by Rawlins (1976) in his
discussion of the work by Merrill and Rawlins (1972) in which
the latter found relatively large errors for soil water
potentials close to zero. Hygrometers that provide the best
coincidence of heat and vapor pathways as well as low
temperature gradient in the hygrometer body appear to be
screen-caged units. However, neither Campbell 91979) nor Weibe
and Brown (1979) tested these units. In the presence of
temperature gradients, Campbell (1979) noticed that soil in
the vicinity of the ceramic element apparently wetted or dried
so that, when the temperature gradient decreased to zero, the
measured water potential was too high or too low. He pointed
out that this error can be more serious than the temperature
gradient error as there is no unique correction factor for it.
The hysteresis apparently results from evaporation of water
around the hot end of the hygrometer and the condensation
around the cold end. It is not known whether such a hysteresis
effect would occur in field measured water potentials, and why
such a water potential gradient should not dissipate in the
absence of thermal gradients and a finite soil resistance to
water vapor diffusion.

d> Field Measurements
Relatively few comprehensive field measurements of water
potential have been adopted to reduce the effects of
temperature gradients and water condensation on hygrometers. 
No attempts to use hygrometry for large-scale soil water
potential in the root zone of field plots irrigated with
saline water. Psychrometers were initially installed
vertically, but they observed that the diurnal variation on
the psychrometer output could be reduced by horizontal
placement of the sensors. furthermore, 50 to 100 mm of lead
wire adjacent to the psychrometer was placed horizontally.
Weibe and Brown (1979) also suggested that water potential
measured could be obtained when the surface net radiation is
low (that is, near sunset or sunrise), thereby ensuring that
zero offsets are a minimum.
Merrill and Rawlins (1972) measured soil temperatures which
were used to correct for the temperature dependence of the
thermocouple output. A well-designed and constructed automatic
data scanning system was used. A distance of over 100 m
between the field experiment and the data loggers necessitated
several special precautions to achieve the desired level of
precision in the measurements. Two types of psychrometers were
used, a laboratory-constructed ceramic type containing several
grams of brass to act as a heat sink and a commercial ceramic
type without a heat sink. The former sensors showed smaller
diurnal variation in output, possibly because the presence of
the heat sinks promoted better thermal stability. However, the
commercial psychrometers displayed greater calibration
stability. Any deterioration in calibration sensitivity could
be restored by washing the thermocouple junction in hot 10%
alcoholic KOH solution. Moore and Caldwell (1972) constructed
soil sensors by mounting psychrometers in stainless steel
tubes which were perforated at the level of the sensor. These
were installed by driving them to a required depth. Other
hygrometers were installed using the convectional technique of
the time, that is, by forming an access hole by driving a
metal rod to the required depth, inserting the psychrometer
vertically, and backfilling with soil. temperature gradients
were large, for example, about 0.13 o C/mm at the 150-
to 300-mm depth interval and fluctuations of 1000 to 2000 kPa
were observed in the water potential measured under those
conditions. Wheeler et al. (1972) used the latter technique to
install psychrometer in the field but encased the lead wires
in polyethylene tubing. Easter and Sosebee (1974) used Teflon
end-window, double junction psychrometers to monitor soil
water potential in two field plots: one irrigated to monitor a
water potential of -100 kPa, the other not irrigated. Results
of the study are obscure, but temperature gradient and water
vapor flux error were probably large. It was the experience
gained by these and other pioneering gradients. Brown and
Johnson (1976) report on the durability of end-window
psychrometers during extended periods of field use 92-40
months). Generally, the psychrometric modes, with mean
dewpoint sensitivity of -`33 kPa/<$Emu V> (at 25 o C)
and 3% coefficient of variation among hygrometers. The
hygrometers were installed in the field by excavating trenches
to depths as great as 750 mm and driving a 9-mm steel rod 300
mm horizontally into the walls of the trenches at various
intervals down the profile. Hygrometers were inserted into the
apertures thus formed, presumably soil was repacked around the
leads, and the trenches were sequentially refilled at 100-mm
intervals. the stated advantage of the method are that the
hygrometers are inserted into undisturbed soil and the lead
wires are oriented perpendicular to the temperature gradient.
Nnyamah and Black (1977a) do not mention taking any
precautions to ensure that the plane of the thermocouple was
placed horizontally. The hygrometers performed well over a
period of 3 months, with only 1 unit in 30 malfunctioning
during this time. Soil water potential values obtained from
neutron water meter measurements and direct sampling in
conjunction with a laboratory-determined water retention
curve. Over the range -1200 to -200 kPa, agreement was linear
and within 30 kPa with <$Er> = 0.99 (Table IV). Similar,
results were obtained from comparison of field measured
dewpoint and psychrometric water potential. this results
suggest that temperature gradients were not a serious problem
in their field experiments and that there does not appear to
be any particular advantage in the dewpoint mode as is
suggested by the lower temperature sensitivity of this
technique. In addition to these comparisons, these workers
were able to measure simultaneous soil water potentials and
root xylem potential of Douglas fir trees over a period of 2
months. Features of this comparison were that root xylem
potential was always lower than soil water potential and the
former responded more rapidly to rainfall. Although, Nnyamah
and Black (1977a) interpreted this as uptake of water by roots
close to the surface, before water had penetrated to the depth
of measurement, the possibility that the equilibrium time of
the ceramic hygrometer could have lagged root xylem water
cannot be excluded. Nnyamah and Black (1977b) reported the use
of field hygrometers, tensiometers, and a neutron water meter
for characterizing the unsaturated hydraulic conductivity of
the soils over a range 0.001 to 10 mm/day. Using this
information, together with computed evapotranspiration rates
(Bowen ratio/energy balance method), they were able to measure
the flux of water through their field site. This experiment
represents the most sophisticated use of soil hygrometers in
the field recorder thus far.

In summary, the essential requirements for successful field
soil hygrometry must include the following precautions to
minimize the effects of fluctuating diurnal temperatures and
temperature gradients.
1.  Hygrometer size: The size of the hygrometer must be small
so that temperature gradients are small and to facilitate flux
of water vapor through the chamber (Weibe and Brown, 1979).
2.  Hygrometer shape: The geometry must be concentric with the
thermocouple sensor centrally or dislocated in the chamber
(Rawlins and Dalton, 1967; Campbell, 1972; Weibe et al.,
1977).
3.  Thermocouple junction: Single junction, allowing
temperature measurement along the sensor (Weibe et al., 1977)
junction length to reduce at least 3 mm long and constructed
of fine wire (25 <$Emu>. diameter) to reduce flux of heat from
the body of the hygrometer to the junction (Campbell, 1972).
4.  Construction material: Controversy exists concerning the
material for construction of the body of the hygrometer
(Campbell, 1979; Weibe and Brown, 1979), but from available
evidence it appears that the optimal material is
low-conductivity material for the body and a cylindrical
stainless steel mesh for rapidly changing soil water potential
conditions.
5.  Field placement: Hygrometer should be installed in
undisturbed soil with as much lead wire buried horizontally as
,possible (Merrill and Rawlins, 1972) close to the hygrometer.
the plane of the thermocouple junction should be orientated at
right angles to the temperature gradient (Weibe et al., 1977).
6.  Measurement times: Measurements could be performed at
times of day when the soil heat energy flux is close to zero
(Weibe and Brown, 1979), with resultant zero offsets being
comparatively small.  In practice, such times are around
sunset and sunrise. With the use of microprocessor systems
(Briscoe and Tippets, 1982) such measurements are practical on
a day-to-day basis.

VII. Summary And Conclusions

The accuracy of nondestructive soil and leaf water potential
measurements using thermocouple hygrometers is dictated by the
calibration method. In the case of leaf hygrometers used in
the field, it is best to calibrate units under cloudless and
calm field conditions, thereby ensuring that there are
virtually no thermal gradients within the instrument. At
present, it is not possible to accurately calibrate soil
hygrometer in the field.
In the case of leaf water potential measurements, the
cuticular resistance may introduce measurement errors in both
psychrometric and dewpoint methods. The type of abrasion used,
if necessary, is determined by the cuticular resistances
greater than 20 sec/cm. In situ field comparisons with
pressure chamber measurements are presented for a whole range
of crops. Practical aspects associated with such hygrometers
measurements are detailed for both leaf and soil water
potential.
On of the major problems associated with the soil water
potential measurements appears to be the condensation or
drying of soil water in the vicinity of the hygrometer and in
the presence of temperature gradients. Depending on the
direction of the temperature gradient causing condensation or
drying out, the measured water potential may be too high or
too low even after the temperature gradient dissipates.