disadvantages
Complex electronics and expensive equipment required for "pure" TDR.
Effects of salinity on TDR
The TDR technique as practised by many is indeed relatively insensitive to
salinity as long as the salinity level is low enough that a useful wave
form is returned! The big problem is that, as salinity levels increase, the
signal reflection from the ends of the rods in the TDR probe is lost. This
occurs because of conduction of the signal through the saline soil between
the rods. The amount of conduction increases as the soil wets. Thus, one
can obtain quite good wave forms in a very dry saline soil and useless wave
forms in the same soil when wet. To backtrack a little, when people say
that TDR is insensitive to salinity they mean that the travel time is
little affected by salinity. So, as long as the wave form can be
interpreted, the water content estimation will be good. That fact doesn't
help much if the wave form is not interpretable due to signal loss. There
have been many attempts to solve this problem by coating the probe rods.
However, coating the rods introduces some other problems (change in
calibration, loss of sensitivity, wear of the coating affecting results,
etc.). I think there are others out there that could address the
possibilities associated with using coated rods.
See
Can. Geotech Journal, 1985, No 22, pp 95-101, DE Pattersson & MW Smith. "Unfrozen water content in saline soils: results using time-domain reflectometry"
It is an old journal article but delas with TDR, Saline soils and
water content.
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