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WP
PotentiaMeter
4C
9. Theory: Water Potential
Another effect of temperature on water potential occurs
with samples that are near saturation (like many soil sam-
ples). A sample that is close to 0.00 MPa and is only
slightly warmer than the sensor block will condense water
within the block. This will cause errors in the measure-
ment, and in subsequent measurements until the conden-
sation disappears. The Ts-Tb function helps the user
ensure that the sample won't condense on the sensor
block.
Estimating Osmotic Potential
The WP4C measures the sum of osmotic and matric
potential. An approximate value for the osmotic potential
can be found by measuring the electrical conductivity
(EC) of the saturation extract of the soil. The osmotic
potential of the saturation extract is computed from:
Ψ
os
The osmotic component of the water potential is then
computed from:
where θ is the volumetric water content of the sample and
θ
is the volumetric water content at saturation. The
s
matric potential is the total potential minus the osmotic.
(MPa) = -0.036EC (dS/m)
Ψ
Ψ
=
os
38
θ
s
---- -
θ
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