Water Potential Theory
Water Potential
Water Potential is defined as the potential energy of water per unit mass of water in the system. The
total water potential of a sample is the sum of four component potentials: gravitational, matric,
osmotic, and pressure. Gravitational potential depends on the position of the water in a
gravitational field. Matric potential depends on the adsorptive forces binding water to a matrix.
Osmotic potential depends on the concentration of dissolved substance in the water. Pressure
potential depends on the hydrostatic or pneumatic pressure on the water.
The WP4 measures the sum of the osmotic and matric potentials in a sample. Often one or the other of
these potentials will be the dominant factor in determining the total potential. For example,
solutions like the KCl calibration standard have only an osmotic component. Soils bind water mainly
through matric forces, and
therefore have mainly a matric component (though salt-affected
soils can
have a significant osmotic component).
Measuring Water Potential with the WP4
The water potential of a solid or liquid sample can be found by relating the sample water potential
reading to the vapor pressure of air in equilibrium with the sample. The relationship between the
sample's water potential (Y) and the vapor pressure of the air is:
where p is the vapor pressure of the air, p0 is the saturation vapor
pressure at sample temperature, R is the gas constant (8.31 J/mol K), T is the Kelvin
temperature of the sample, and M is the molecular mass of water. The vapor pressure of the air
can be measured using a chilled mirror, and po is computed from sample temperature.
The WP4 measures water potential by equilibrating the liquid phase water of the sample with the vapor
phase water in the headspace of a closed chamber, then measuring the vapor pressure of that
headspace. In the WP4, a sample is placed in a sample cup, which is sealed against a sensor block.
Inside the sensor block is a fan, a dew point sensor, a temperature sensor, and an infrared
thermometer. The dew point sensor measures the dew point temperature of the air, and the infrared
thermometer measures the sample temperature. The purpose of the fan is to speed equilibrium and to
control the boundary layer conductance of the dew point sensor.
From these measurements, the vapor pressure of the air in the headspace is computed as the saturation
vapor pressure at dewpoint temperature. When the water potential of the sample and the headspace air
are in equilibrium, the measurement of the headspace vapor pressure and sample temperature (from
which saturation vapor pressure is calculated) gives the water potential of the sample.
In addition to equilibrium between the liquid phase water in the sample and the vapor phase, the
internal equilibrium of the sample itself is important. If a system is not at internal equilibrium,
one might measure a steady vapor pressure (over the period of measurement) which is not the true
water potential of the system.
Effect of Temperature on Water Potential
Temperature plays a critical role in water potential determinations. Most critical is the measurement
of the difference between sample and dew point temperature. If this temperature difference were in
error by 1°C, an error of 8 MPa would result. In order for water potential measurements to be
accurate to 0.1 MPa, temperature difference measurements need to be accurate to 0.005°C.
WP4's infrared thermometer measures the difference in temperature between the sample and the block.
It is carefully calibrated to minimize temperature errors, but achieving 0.005°C accuracy is
difficult when temperature differences are large. Best accuracy is therefore obtained when the sample
is near chamber temperature.
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