Geoscience Reference
In-Depth Information
Sandy soil
15
3
Clay soil
10
2
Figure 6.5
Change of
damping depth volumetric
water content for typical
sandy, clay, and peat soils for
the daily and yearly surface
temperature cycles.
Peat soil
5
1
0
0
0
0.2 0.4
Volumetric water content
0.6
0.8
For a layer of soil with thickness equal to the effective depth, the net flow of
heat into the soil during one half cycle (i.e., the integral of
G
z=0
from −
●
π
/4
το
3
π
/4) would raise the temperature by 1 K.
The values of
D
for three types of soil are shown in Fig. 6.5 as a function of
volumetric water content for the daily and yearly cycles.
D
increases quickly as the
volumetric water content changes from 0 to 0.1 for sandy and clay soils, and
rapidly reaches values of 120-150 mm, these being typical of values often found
in the field. However, for peat soils
D
remains in the range 3-5 cm regardless of
volumetric water content, again consistent with observations that indicate organic
soils heat and cool slowly.
Important points in this chapter
Soil surface temperature
: spatial variability makes measurement difficult;
the amplitude of daily cycle is large (
●
30°C) for bare, dry soil in calm, clear
sky conditions (more than air temperature) with timing linked to radiation,
but less for wet soil and also less and linked to air temperature for vegetation-
covered soil.
∼
Subsurface soil temperature
: is easier to measure with carefully inserted
thermometers, and is driven by soil surface temperature and so differs for
dry, wet, and vegetation-covered soil, with magnitude of daily cycle reducing
in size and phase progressively delayed with depth.
●
Thermal properties of soil
: The
density
(
r
s
),
specific heat
(
c
s
), and
thermal
diffusivity
(
a
s
), and especially the
heat capacity per unit volume
(
C
s
), and
ther-
mal conductivity
(
k
s
) of soil (defined in the text) are all strongly dependent on
moisture content.
●
Thermal conduction
: the soil heat flux (
G
s
) into soil away from the surface is
described by a simple diffusion equation as
k
s
(or
a
s
C
s
) times the
negative
gradient of soil temperature with depth.
●
