Geoscience Reference
In-Depth Information
Figure 2.21 Conduction of heat through the soil matrix: a dry soil (left) and a soil
with some moisture (right).
2.3.3 Thermal Properties of Soils
The transport of heat critically depends on the thermal properties of the soil. For
some of the properties (density and heat capacities) a soil is simply a mixture of three
phases: solid particles, water and air, whereas for the conductivity (and hence the dif-
fusivity) the structure of the soil, as well as the water content, is important. Note that
the phase 'solid particles' will generally be made up of a variety of materials (quartz,
clay minerals, organic material).
We indicate the properties of the three phases with a subscript 'p' (particles), 'w' (water)
and 'a' (air), and the respective volumetric fractions of the phases by f p , θ and f a . Then
density, volumetric heat capacity and speciic heat capacity of the soil are given by:
ρρρ ρ
= + +
=
f
θ
θ
f
s
p
p
w
aa
CC
c
f
+
C
+
fC
(2.33)
s
p
p
w
aa
=
C
/
ρ
s
s
s
Thus, both the density and the volumetric heat capacity of the soil depend linearly on
the soil moisture content (see Figure 2.22). Table 2.2 lists the thermal properties of
the soil constituent used in ( 2.33 ), as well as those of a number of typical soils.
For the conductivity λ s the situation is more complex (see Figure 2.21 ). The con-
ductivity of the material of the particles is rather large (e.g., for quartz λ s = 8.8
W m -1 K -1 ; see Table 2.2 ) whereas air is nearly an insulator ( λ a = 0.025 W m -1 K -1 ).
As a result, the transport of heat in a dry soil is limited by the surface area of the
points of contact between individual soil particles. The addition of a little water then
 
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