Geoscience Reference
In-Depth Information
Air
Soil surface
G
z
=
0
Soil
~5 cm
Figure 7.5
Arrangement of a
soil heat flux plate and soil
thermometers when used to
estimate the heat flux at the
soil surface.
G
z
=
0
Soil heat
flux plate
Soil
thermometers
junctions alternately near the top and bottom of the soil heat plate measures the
temperature difference across the soil heat flux plate. Because the thermal
conductivity and thickness of the soil heat flux plate are known, the heat flow per
unit area through the plate can be calculated and this is assumed to be the same as
that which would be flowing through the volume of soil that the plate replaces.
Soil heat flux plates cannot realistically be used on the surface of the soil so they
are normally installed at some depth (typically 5 cm) below the soil surface, see
Fig. 7.5. Because the damping depth of heat flow in soils is typically on the order
of 5-15 cm, this means the soil heat flux measured by the plate is not a good
estimate of the surface soil heat flux. Therefore, an attempt is made to estimate the
loss or gain of heat in the layer of soil between the plate and the surface. To do this
at least one and usually more thermometers are inserted into the soil to measure
the rate of change of soil temperature between the soil flux plate and the surface.
Often a pit with a vertical edge is dug and these thermometers and the soil heat
flux plate are inserted sideways into the soil through the edge of the pit to minimize
soil disturbance.
If the heat capacity per unit volume of the soil,
C
s
, is known, the corrected soil heat
flux at the surface is estimated from the soil heat flux measure by the plate using:
T
GGC
t
=
δ
=+
δ
soil
(7.1)
z
=
0
z d
s
where
G
z=0
and
G
z=d
are respectively the soil heat fluxes at the surface and at depth
d
at which the soil heat flux plate is inserted, and
d
T
soil
is the average change in the
temperature of the soil layer above the plate over the period
d
t
for which estima-
tion is required.
Measuring latent and sensible heat
There are two general ways in which the fluxes of latent and sensible heat are
measured. One is to determine one or both of these energy fluxes by making
meteorological measurements in the turbulent air just above the surface through
