Water & Nutrient Management:
Soil Moisture-Based Irrigation Systems

Volumetric Sensors for Soil Moisture:
Amplitude Domain Reflectometry

VR Instructions: To turn the sensor, click and drag your cursor around the movie window in any direction. Click the + and - buttons to zoom in and out. For problems viewing the VR, see VR Help.

Working Principle

When an electromagnetic wave (energy) traveling along a transmission line (TL) reaches a section with different impedance (which has two components: electrical conductivity and dielectric constant), part of the energy transmitted is reflected back into the transmitter. The reflected wave interacts with the incident wave producing a voltage standing wave along the TL, i.e., change of wave amplitude along the length of the TL. If the soil/probe combination is the cause for the impedance change in the TL, measuring the amplitude difference will give the impedance of the probe (Gaskin and Miller, 1996; Nakashima et al., 1998). The influence of the soil electrical conductivity is minimized by choosing a signal frequency, so that the soil water content can be estimated from the soil/probe impedance.

Description

Impedance sensors use an oscillator to generate a sinusoidal signal (electromagnetic wave at a fixed frequency, e.g., 100 MHz) that is applied to a coaxial TL that extends into the soil through an array of parallel metal rods, the outer of which forms an electrical shield around the central signal rod. This rod arrangement acts as an additional section of the TL, having impedance that depends on the dielectric constant of the soil between the rods.

Advantages

• Accurate with soil-specific calibration (±0.01 ft3ft-3; ±0.05 ft3ft-3 without it)
• Allows measurements in highly saline conditions (up to 20 dS/m)
• Minimal soil disturbance
• Can be connected to conventional loggers (DC output signal)
• Inexpensive due to standard circuitry
• Not affected by temperature
• In situ estimation of soil bulk density possible (Wijaya et al., 2002)

Drawbacks

• Soil-specific calibration recommended for reliable measurements
• Measurement affected by air gaps, stones or channeling water directly onto the probe rods
• Small sensing volume (0.27 in3)

Return to top

This page was last updated on June 15, 2010.