Advantages for Soil Moisture Measurement
- Superior accuracy to within 2 or 3% of the measured soil moisture value with standard calibration.
- Calibration requirements are minimal - in many cases soil specific calibration is not needed.
- In most cases, soil salinity influence is negligible.
- Lack of radiation hazard associated with neutron probe technique.
- TDR has excellent spatial and temporal resolution.
- Installation of the probes is non-destructive to the measured object.
- Measurement is fast and easy.
- The method is capable of providing continuous measurements through automation and multiplexing.
Technical Background
The time-domain reflectometry (TDR) was primarily used for detection and localisation of electrical cables damages. Nowadays, this technique is used for detection of damages in signal transmission lines. The TDR principle of operation is generation of electrical pulses and analysis of the signal reflected in transmission lines in the time domain. The obtained reflections indicate discontinuities, impedance changes or damages of cables under test. Since the travel time of the reflected pulse can be measured and its propagation velocity is known, it is possible to calculate the distance to the discontinuity along the transmission line. The type of the damage (short or open circuit) can be also discerned.
Nowadays, the TDR technique is commonly used for in situ determination of volumetric water content θ of porous materials, especially soil [1]. Propagation velocity in a given medium of a needle pulse of 100-300 ps rise/fall time is measured. If the tested medium is non-magnetic and its electrical conductivity is not higher than 0.4 S/m [2], the propagation velocity depends on the dielectric permittivity ε of the medium according to a simplified relation
Nowadays, the TDR technique is commonly used for in situ determination of volumetric water content θ of porous materials, especially soil [1]. Propagation velocity in a given medium of a needle pulse of 100-300 ps rise/fall time is measured. If the tested medium is non-magnetic and its electrical conductivity is not higher than 0.4 S/m [2], the propagation velocity depends on the dielectric permittivity ε of the medium according to a simplified relation
where c is the velocity of light in vacuum (300 000 km/s), L is the length of the transmission line of the sensor and Δt is the time between the reflections from the beginning and the end of the open transmission line.
The factor 2 in the numerator of the equation above indicates that this is a reflection method and the pulse travels along the line and back again. The sensor used in the TDR technique is a parallel transmission line, usually in the form of two or three steel rods placed in a tested porous medium. Homogeneity of the tested medium is required for proper operation of TDR devices.
Calibration of the TDR moisture measurement method is a two-step procedure [3]. First, the probe needs to be calibrated in order to minimise the measurement uncertainty of dielectric permittivity of a tested medium by determining the ε = f(v) relation. Next, the final calibration is performed using the reference oven-drying measurements of soil volumetric water content, which provides the θ = f (ε) relation.
Literature
[1] Robinson D.A., Campbell C.S., Hopmans J.W., Hornbuckle B.K., Jones S.B., Knight R., Ogden F., Selker J., Wendroth O. Soil moisture measurement for ecological and hydrological water-scale observatories: a review. Vadose Zone Journal, 7(1), 358-389, 2008.
[2] Topp G.C., Zegelin S., White I. Impacts of the real and imaginary components of relative permittivity on time domain reflectometry measurements in soils. Soil Science Society of American Journal, 64, 1244-1252, 2000.
[3] Skierucha, W., Wilczek, A., Alokhina, O. Calibration of a TDR probe for low soil water content measurements. Sensors and Actuators A: Physical. 147, 544–552, 2010.
The factor 2 in the numerator of the equation above indicates that this is a reflection method and the pulse travels along the line and back again. The sensor used in the TDR technique is a parallel transmission line, usually in the form of two or three steel rods placed in a tested porous medium. Homogeneity of the tested medium is required for proper operation of TDR devices.
Calibration of the TDR moisture measurement method is a two-step procedure [3]. First, the probe needs to be calibrated in order to minimise the measurement uncertainty of dielectric permittivity of a tested medium by determining the ε = f(v) relation. Next, the final calibration is performed using the reference oven-drying measurements of soil volumetric water content, which provides the θ = f (ε) relation.
Literature
[1] Robinson D.A., Campbell C.S., Hopmans J.W., Hornbuckle B.K., Jones S.B., Knight R., Ogden F., Selker J., Wendroth O. Soil moisture measurement for ecological and hydrological water-scale observatories: a review. Vadose Zone Journal, 7(1), 358-389, 2008.
[2] Topp G.C., Zegelin S., White I. Impacts of the real and imaginary components of relative permittivity on time domain reflectometry measurements in soils. Soil Science Society of American Journal, 64, 1244-1252, 2000.
[3] Skierucha, W., Wilczek, A., Alokhina, O. Calibration of a TDR probe for low soil water content measurements. Sensors and Actuators A: Physical. 147, 544–552, 2010.