Standing Wave, or Amplitude Domain Reflectometry, uses an oscillator to generate an electromagnetic wave at a consistent frequency, which is transmitted through a central signal rod, using outer rods as an electrical shield. The electromagnetic wave is partially reflected by areas of the medium with different dielectric constants (water content), producing a measurable voltage standing wave. ADR measures volumetric soil water (VSW%) independently of all other soil variables, including density, texture, temperature and electrical conductivity. ADR does not require in-situ calibration to accurately measure VSW%.
Measures the time taken (in nanoseconds) for an electromagnetic pulse to propagate along a waveguide surrounded by soil. Time of travel, or velocity, of this pulse is effected by the dielectric constant (Ka) of the soil. Wetter soil with a higher dielectric constant, produces a slower velocity pulse. TDR measures volumetric soil water (VSW%) independently of all other soil variables, including density, texture, temperature and electrical conductivity. TDR does not require in-situ calibration to accurately measure VSW%.
Capacitance sensors operate by rapidly charging and discharging a positive and ground electrode (capacitor) in the soil, generating an electromagnetic field whose charge time is related to the capacitance of the soil. Further, the capacitance is related to the dielectric permittivity of the medium between the capacitor electrodes.
Capacitance sensors are affected by soil moisture, temperature and electrical conductivity. The frequency is chosen to measure either predominately soil moisture or bulk electrical conductivity. Capacitance sensors require in-situ calibration for accurate measurement of volumetric soil water content (VSW%).
Neutron Moderation: Fast neutrons are emitted from a radioactive source, when these neutrons collide with Hydrogen, they are thermalised (slowed dramatically). As water is the only source of hydrogen that changes from day to day in soil, increase/decrease in water content results in a proportional increase/decrease in slow neutron counts when compared to the previous measurements. Changes in volumetric soil moisture are measured very accurately. The volume of soil measured is typically 30cm in diameter, which is much larger than electrical methods, and hence, special variability is best averaged.