20
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 is allowed
to 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 de-
pendent 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 approximately
3 cm out from the rods. The high frequency signal information 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.
