Umwelt-Geräte-Technik GmbH
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3.
Volumetric water content of the soil
The water content is one of the most important characteristic variables in soil science. It is
used by agriculture as limit value and control variable for irrigation, by construction as limit
value for the ability to drive on ground, and in environmental and landscape protection both
as a decision criterion for planned measures and as an important indicator.
Since a direct determination of the water content by weighing is not possible in the field,
indirect procedures are deployed to determine the volumetric water content to obtain non-
destructive and continuous measurements in the field.
The volumetric water content is the ratio of water volume in the soil to total volume and is
calculated according to (1):
𝜃
=
𝑉
𝜔
𝑉
𝑇
∙
100%
(1)
where
𝜃
…
Volumetric water content
𝑉
𝜔
... Volume of water
𝑉
𝑇
…
Total volume
4.
Mode of operation
4.1.
Water content measurement
The water content measurement procedure on the UMP-1 sensor is assigned neither to
the FDR sensors nor the TDR sensors.
The basic mode of operation however corresponds to the FDR process, because the
sensor operates with a continuous signal at constant frequency (60 MHz) and not with a
pulse. However the phase shift rather than the frequency change is taken into
consideration to calculate the measurement reading.
The measurement pipe consists of two stainless steel rods (antennae) 10 cm in length
and with a diameter of 3 mm. These dimensions and the measurement frequency
produce a measurement volume of approx. 1 litre. Faults within the measurement
volume such as vessel walls, large air pockets or cables can distort the measurement
result. The continuous signal runs along the antennae in the ground. The waves of the
signal running back and forth are compared here. The dielectric constant
ε
of the
surrounding soil/water mix is determined from the phase difference of the signals
running back and forth. The dielectric constant for pure water at 20°C is
ε
=80. Dry soil
as a
ε
of 2…5. The dielectric constant of the soil/water mix lies between these two
levels, and is higher the more water there is in the soil.
Using the calculated dielectric constant, the water content of the soil is then determined
in the unit ([m³])/([m³]) according to the Topp/Davis equation.
