22
Volumetric Water
Content
Measurements
Volumetric Water Content (VWC)
The soil can be thought of as being composed of soil,
water and air. The volumetric water content (VWC) is the
ratio of the volume of water in a given volume of soil to
the total soil volume. This can be expressed as either a
decimal or a percent. Three soil moisture levels of
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 underlying principal of TDR involves measuring the
travel time of an electromagnetic wave along a wave-
guide. The speed of the wave 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 de-
termine the VWC of the soil. The VWC measured by
TDR is an average over the length of the waveguide.
