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Time Domain Reflectometry (TDR) for soil moisture measurement

Time Domain Reflectometry (TDR) calculates volumetric soil moisture content by measuring dielectric properties. This short piece sets out to explain the fundamental science behind the measurement.

TDR is still used in the Soil Moisture Equipment Handi-Trase range of soil moisture sensors.

This piece is an excerpt from ICT227 Soil Moisture Measurement Instrumentation, which was published in the 1990s by ICT International as an application note.

TDR determines the apparent dielectric (Ka) of the soil matrix and this is empirically related to the volumetric soil moisture content. The method is quick, independent of soil type, non-destructive, suited for surface and profile measurements, and allows repeatable in situ measurement. The TDR is a portable unit that can be carried allowing point soil moisture measurements or linked to a multiplexer to measure an array of buried waveguides. The moisture content determined by the TDR is the average moisture along the length of the waveguides. Therefore, to measure at depth of 20 cm, waveguides are placed in the soil horizontally at that depth. If 30 cm waveguides are placed vertically into the soil, the moisture content determined by the TDR will be the integrated moisture content from the soil surface to a depth of 30 cm.

Measuring the electromagnetic wave

The technique is based upon cable testing technology, with a broad-band Electromagnetic step pulse generated and propagated along a coaxial cable. At the end of the cable stainless steel rods (waveguides) are inserted into the ground. The time of travel of the EM wave is determined by the apparent dielectric (Ka) of the medium (in this case soil).

Travel time along the Waveguide

Water with a high dielectric (Ka ≈ 80), compared to soil (Ka ≈ 3 to 5) and air (Ka = 1), dominates the measured Ka. Thus, if the soil is saturated the Ka is high (due to the presence of increased water) and the travel time of the EM wave along the waveguides is long. If the soil is dry the travel time along the waveguides is short and the Ka is therefore low. The relationship of Ka to travel time (∆t) can be seen in the graph to the right.

Trase measures dielectric constants

Every material has a “dielectric constant”; its ability to delay an electromagnetic field travelling through it. The typical range for these constants is from 1-80, with air = 1, a minimum delay, and water = 80, a maximum delay. “Dielectric constants (K)” are derived for specific materials, while “apparent dielectric constants (Ka)” are derived for a mixture of materials. Typical dielectric constants are Air K = 1, Minerals K = 3-5, and Water K = 80. In a complex mixture like soil, the apparent dielectric value will have a wide range. For example, dry soil, Ka = 3-6, has a high volumetric content of air and minerals, but little water. On the other hand, mud, Ka = 40-50, has a large volume of water and minerals, but very little air.

In all types of materials where one component has a substantially higher dielectric constant compared to the other components, the apparent dielectric constant measured in conjunction with a conversion table, can give the volumetric content of the component which has the higher constant, as defined in Topp et al (1980).