capacitance or frequency domain
The most widely employed method to measure soil water content is capacitance or sometimes referred to as frequency domain. Capacitance determines Ka by measuring the charge time of a capacitor, which uses soil as a dielectric medium. The capacitance sensor forms a pair of electrodes and the soil acts as a dielectric. The capacitor charge time is a linear function of the dielectric permittivity of the soil.
Capacitance sensors are typically manufactured as a pair of parallel pronged probes or as parallel pairs of rings along a probe or rod. The latter type of sensor are typically installed in plastic or PVC access tubes where the electric field between the sensor and the soil must pass through the plastic or PVC tube barrier. Such a design imposes uncertainties as to whether the access tube, soil, or meniscus that can form on the outside of the access tube following rainfall or irrigation, is being measured. The volume of measurement and its geometry are also uncertain (Topp 2003).
Capacitance and frequency domain sensors need to have good contact with the medium they are measuring with no air gaps. The volume of measurement is dependent on sensor size with most sensors in the order of 5cm to 10cm in length but one sensor is 3m in length. The field of influence is greatest at the sensor to substrate interface and declines rapidly from there. Generally, the field of influence is approximately 1cm distance from the sensor. Given a 3m length sensor with 1cm distance into the surrounding medium, extreme caution needs to be taken with installation to ensure there are no air gaps. Sensors of 5cm to 10cm in size are easier to install in terms of avoiding air gaps.
Topp, GC. 2003. State of the art of measuring soil water content. Hydrological Processes, 17, 2993-2996.
- Time domain reflectometry (TDR)
- Time domain transmissometry (TDT)
- Cosmic-Ray (COSMOS)
- Ground penetrating radar (GPR)