Soil water contain Measurement using Time Domain Reflectometry

Soil water contain Measurement using Time Domain Reflectometry

Soil water contain Measurement

Soil water content is a measurement of the amount of water in a known amount of soil; it can be expressed as % water by weight or volume of soil, or inches of water per foot of soil.

Time Domain Reflectometry

A time-domain reflectometer (TDR) is an electronic instrument used to determine the characteristics of electrical lines by observing reflected waveforms.
Time domain reflectometry (TDR) is an indirect measure of soil water content based on the travel time of a high frequency electromagnetic pulse through the soil; this travel time is used to calculate the permittivity (dielectric constant) of the material. 
The TDR probes are inserted directly into the soil for in situ measurement at the desired soil depth. The measurement takes only seconds, and the instrument can be attached to a data logger for ongoing measurements.
TDR applied to soil water content determinations is cable radar in which a very fast rise-time voltage pulse is propagated down and reflected back from the end of a transmission line or wave guide (the probes) in the soil. 
There is a variety of TDR probes available, which allows the measurement of soil water content to be “tailored” to the experimental objectives. The choice of soil probe type and orientation of probe installment into the soil depend on the intended use of the resulting data.
The TDR method measures the velocity of propagation of a high-frequency signal. The velocity of propagation in soil (v) can be calculated using the following equation
v = c / √K’
Where:
c = 3 x 108 m s-1 (propagation velocity of electric signals and light in a vacuum)
K’ = dielectric constant of the soil

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