Acta agriculturae Slovenica, 85 - 2, november 2005
360
useful method for nondestructive, continuous measurements of soil water content and
bulk electrical conductivity in the 1980s (Evett, 2000, Chandler et al, 2004). After
being used for telecommunications industry, Topp et al (1980) applied the TDR
method for measurement of the apparent dielectric constant of soil K
a
, which is
strongly dependent on water content. With TDR the apparent soil dielectric constant
K
a
is measured and related to soil volumetric water content (VWC) using a calibration
equation (Chandler et al, 2004). Topp et al (1980) demonstrated that TDR could
measure water content with an accuracy of better that 2% VWC and that a single
calibration equation could be applied to nearly all soils (Baker and Allmaras, 1990).
Automated TDR systems for water content measurement were described by Baker and
Allmaras (1990), Heimovaara and Bouten (1990), Herkelrath et al (1991) and
Noborio (2001).
TDR is an electromagnetic method in which the applied signal is guided along a
transmission line though a soil sample. Most TDR systems currently used for soil
measurements apply a fast rise time electromagnetic pulse to the soil transmission
line. The time delay between the reflections of the pulse from the beginning and end
of the soil transmission line is used to determine the velocity of propagation through
the soil along the transmission line (Topp and Ferré, 2002).
In spite of the TDR method’s long-standing presence on the market and application in
practice, the equipment is still costly and the shipment time consuming. The aim of this
study was to compare the measurements of the TDR 100 Time Domain Reflectometer
using original and homemade parts.
2 MATERIALS AND METHODS
2.1 Materials
The TDR 100 Time Domain Reflectometer is a connector-type TDR sensor, which generates
a very short rise time electromagnetic pulse that is applied to a coaxial system. The coaxial
system includes a TDR probe for soil water measurements and samples and digitises the
resulting reflection waveform for analysis or storage (Figure 1). The elapse travel time and
pulse reflection amplitude contain information used by the on-board processor to quickly and
accurately determine soil volumetric water contents (VWC) (Campbell Scientific, 2004).
These sensors are reported to have the same area of influence and accuracy specifications
as the buriable-type sensors, which are directly attached to a cable. The connector-type TDR
sensors consist of two 6 mm diameter waveguides – steel rods - that are inserted from the
soil surface. The rods are connected to the cable with clamps. Given the design and diameter
of these waveguides, they could be inserted from the surface into undisturbed soil without
pre-forming holes, even under relatively dry soil conditions.
The vertically inserted connector-type TDR sensors provide average soil moisture
measurement over range of 0–10 and 0–20 cm, being the length of the waveguides used.
While this technique can be automated, in this application measurements were made on
experiment basis, simultaneously as measurements with other sensors.
Also, original steel rods of 10 and 20 cm length (TDR 100 10a and 20a), provided by the TDR
100 producer Soil Moisture were reproduced from the same material in exact same
dimensions (TDR 100 10b and 20b).
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