Geoscience Reference
In-Depth Information
Fig. 8.3
Polarization of water molecules due to the applied electric fi eld changes the dielectric
permittivity
ʵ
of the porous material where the pores are partly or fully fi lled by water. The value
of
ʵ
depends then upon the extent of fi lling the soil pores by water and this is the principle of TDR
(time domain refl ectivity) method
Classical TDR instrumentation consists of four basic units: a timing circuit,
pulse generator, sampler, and display. The pulse generator transmits electromag-
netic waves along a transmission line leading into the soil. One of the commercially
available instruments has three rods at mutually fi xed positions that are usually
permanently inserted into the soil or, in some instances, pushed into the soil at the
time of measurement. Other instruments rely on access tubes for their insertion into
the soil at a desired depth. Changes in the soil impedance generated by the electro-
magnetic waves are refl ected, signaled back, and recorded. By measuring the time
required for a sequence of pulses to travel along the known probe length, the dielec-
tric constant of the soil can be computed and used to obtain the volumetric soil
water content. Calibration curves must be made for each soil taking into account its
specifi c porosity or degree of compaction.
Numerous other methods have been developed to measure the amount of water
in soil. Here we mention only a couple of examples. One is based on the adsorption
and scattering of gamma radiation. Because of strict, necessary geometric condi-
tions imposed on the radiation and its measurement, the method is more applicable
to soil columns measured in a laboratory than to natural soil profi les. In the fi eld, the
soil water content is adequately estimated between two access tubes only when they
have been installed exactly parallel to each other. The other method utilizes spectral
analysis of Earth's visible and infrared radiation for estimating soil water content
