Water & Nutrient Management:
Soil Moisture-Based Irrigation Systems

Time Domain Reflectometry

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Working Principle

The soil bulk dielectric constant (Ka b) is determined by measuring the time it takes for an electromagnetic pulse (wave) to propagate along a transmission line (TL) that is surrounded by the soil. Since the propagation velocity (v) is a function of Ka b, the latter is therefore proportional to the square of the transit time (t, in seconds) down and back along the TL:

Ka b = ( c/v ) 2 = (( c.t )/( 2.L )) 2 (2)>

where c is the velocity of electromagnetic waves in a vacuum (3.10 8 m/s or 186,282 mile/s) and L is the length of the TL embedded in the soil (in m or ft).

Description

A TDR instrument requires a device capable of producing a series of precisely timed electrical pulses with a wide range of high frequencies used by different devices (e.g., 0.02-3 GHz), which travel along a TL that is built with a coaxial cable and a probe. This high frequency provides a response less dependent on soil specific properties like texture, salinity or temperature.

The TDR probe usually consists of 2-3 parallel metal rods that are inserted into the soil acting as waveguides in a similar way as an antenna used for television reception. At the same time, the TDR instrument uses a device for measuring and digitizing the energy (voltage) level of the TL at intervals down to around 100 picoseconds. When the electromagnetic pulse traveling along the TL finds a discontinuity (i.e., probe-waveguides surrounded by soil) part of the pulse is reflected. This produces a change in the energy level of the TL. Thereby the travel time (t) is determined by analyzing the digitized energy levels.

Soil salinity or highly conductive heavy clay contents may affect TDR, since it contributes to attenuation of the reflected pulses. In other words, TDR is relatively insensitive to salinity as long as a useful pulse is reflected (i.e., as long as it can be analyzed). In soils with highly saline conditions, using epoxy-coated probe rods should solve the problem. However, this implies loss of sensitivity and change in calibration. It is interesting to notice that in addition to time of travel another characteristic of the pulse traveling through the soil (i.e., change in size or attenuation of the pulse) can be related to the soil electrical conductivity. Based on this some commercial devices incorporate the possibility of measuring water content and soil salinity simultaneously.

Advantages

• Accurate (±0.01 ft3ft-3)
• Soil specific-calibration is usually not required
• Easily expanded by multiplexing
• Wide variety of probe configurations
• Minimal soil disturbance
• Relatively insensitive to normal salinity levels
• Can provide simultaneous measurements of soil electrical conductivity

Drawbacks

• Relatively expensive equipment due to complex electronics
• Potentially limited applicability under highly saline conditions or in highly conductive heavy clay soils
• Soil-specific calibration required for soils having large amounts of bound water (i.e., those with high organic matter content, volcanic soils, etc.)
• Relatively small sensing volume (about 1.2 inch radius around length of waveguides)

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This page was last updated on June 15, 2010.