Geoscience Reference
In-Depth Information
as the air in the pores of the soil is replaced by water which is much denser,
see Table 6.1.
Specific heat of soil,
c
s
The specific heat of soil in J kg
−1
K
−1
(1 J kg
−1
K
−1
is the same as 1 J kg
−1
°C
−1
) is the
amount of heat absorbed or released in raising or lowering the unit mass of soil by
1 K. Because dry soil is porous, its specific heat is only about 25-40% of the specific
heat of water, but the specific heat of soil typically almost doubles when the soil
becomes saturated, see Table 6.1.
C
s
Heat capacity per unit volume,
The heat capacity per unit volume in J m
−3
K
−1
(1 J m
−3
K
−1
is the same as 1 J m
−3
°C
−1
)
is the amount of heat absorbed or released in raising or lowering unit volume
of soil by 1 K. It is the product of the density of the soil with its specific heat,
thus:
C
=
r
c
(6.1)
s
s
s
Because both
r
s
and
c
s
separately increase with soil moisture content, there is an
even greater proportional increase in the value of
C
s
as the moisture content of the
soil increases, see Table 6.1.
Thermal conductivity,
k
s
Thermal conduction of heat in soil is described by a simple diffusion equation
with the form:
∂
T
soil
G
=−
k
(6.2)
z
s
z
∂
where
G
z
(in W m
−2
) is the local vertical soil heat flux at depth
z
below the surface
of the soil (
z
is measured downward), ∂
T
soil
/∂
z
is the vertical temperature gradient
at depth
z
in K m
−1
, and
k
s
is the local thermal conductivity of the soil in W m
−1
K
−1
(1 W m
−1
K
−1
is the same as 1 W m
−1
°C
−1
). The negative sign is required on the
right hand side of this equation because soil heat flux is defined to be positive
when directed away from the surface (see Chapter 4), and this occurs when soil
temperature
decreases
with depth below ground. As is the case for other soil
properties, there is a marked change in thermal conductivity when soil moisture
content increases, see Table 6.1.
