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5. EVALUATION OF DIELECTRIC MIXING MODELS OF MINERAL
SOILS
The increasing popularity of Time Domain Reflectometry (TDR) technique in
the dielectric determination of soil water content (Or and Wraith [72], Topp et al.
[90] implies the necessity of verification of the dielectric mixing models. The
commonly used models describing the soil dielectric constant as the mixture of
solid matter, water and air are: α model (Birtchak [9], Bohl et al. [11], Steru
[86]), the model proposed by de Loor [25] and empirical models (Malicki and
Skierucha [59], Malicki et al. [58], Skierucha [78], Topp et al. [90]).
The soil water content sensor used in TDR method and presented
schematically in Fig. 10, consists of two parallel metal rods (or three metal rods
applied by Heimovaara [39], Wright and Or [105], Zegelin et al. [106]) of the
length L, inserted into the tested soil [32,38,59]. The time, t, for the
electromagnetic (EM) wave to cover the distance L may be calculated as the sum
of propagation times along the individual soil phases (Kraszewski [48]):
t = t + t + t
(23)
where the indexes s, a and w stand for solid, gas and water phases, respectively.
s
a
w
Fig. 10.
TDR interpretation of soil as a tree phases medium in dielectric mixing model α
In most practical conditions the velocity of EM wave propagation, v, in the
soil may be presented as:
c 2L
v = =
(24)
ε t
where c is the velocity of light in free space, ε is the soil dielectric constant. The
total propagation time, t, of EM wave can be expressed by dividing it into
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