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VWC Measurements
Volumetric Water Content (VWC)
The ratio of the volume of water in a given volume of soil to the
total soil volume expressed as a decimal or a percentage. Three soil
moisture levels of most importance can be defined as follows:
Saturation: All soil pores are filled with water. The VWC will equal
the percent pore space of the soil.
Field Capacity: The condition that exists after a saturated soil can
drain to a point where the pull of gravity is no longer able to remove
any additional water.
Permanent Wilting Point: The highest moisture content at which a
plant can no longer extract water from the soil.
Additionally, we can define Plant Available Water as the amount of
water between Permanent Wilting Point and Field Capacity. One
rule of thumb is that irrigation should be initiated when half the
Plant Available Water has been depleted.
Time Domain Reflectometry (TDR)
The speed of an electromagnetic wave along a waveguide in soil is
dependent on the bulk dielectric permittivity (
) of the soil matrix.
The fact that water (
= 80) has a much greater dielectric constant
than air (
= 1) or soil solids (
= 3
-
7) is exploited to determine the
VWC of the soil. The VWC measured by TDR is an average over
the length of the waveguide.
The sampling volume is an elliptical cylinder that extends approxi-
mately 3 cm out from the rods. The high frequency signal infor-
mation is then converted to volumetric water content. High amounts
of clay or high electrical conductivity (EC>2 mS/cm) will attenuate
the high
-
frequency signal and affect the reading displayed by the
meter. Very high organic matter content will similarly affect the
VWC reading.