{{one source, date=August 2015 Many

soil moisture Soil moisture is the water content of the soil. It can be expressed in terms of volume or weight. Soil moisture measurement can be based on ''in situ'' probes (e.g., capacitance probes, neutron probes) or remote sensing methods. Water that ent ...

measuring instrument A measuring instrument is a device to measure a physical quantity. In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Es ...

s are based on the principle of Amplitude Domain Reflectometry (ADR). This method measures the

electrical impedance In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit. Quantitatively, the impedance of a two-terminal circuit element is the ratio of the comp ...

Electromagnetic waves In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) lig ...

traveling along

Transmission Line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...

s (TL) enter in soil medium whose impedance is different from TL a part of the energy is reflected back to transmitter. The reflected wave interferes with incident wave and produces a standing wave along the TL, this changes the amplitude of wave along the TL. The impedance can be measured from difference in amplitude. The impedance has two components:

electrical conductivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...

and dielectric constant. The effect of conductivity can be minimized by selecting an appropriate frequency.

References

Robinson, D. A., C. S. Campbell, J. W. Hopmans, B. K. Hornbuckle, S. B. Jones, R. Knight, F. Ogden, J. Selker and O. Wendroth (2008), Soil moisture measurement for ecological and hydrological watershed-scale observatories: A review, Vadose Zone Journal, 7(1), 358–389. Soil science Impedance measurements