Instruction manual

SECTION 2. STATION INSTALLATION
2-4
TABLE 2.4-1 Polynomial Coefficients
a
0
= 6984.505294
a
1
= -188.9039310
a
2
= 2.133357675
a
3
= -1.288580973 x 10
-2
a
4
= 4.393587233 x 10
-5
a
5
= -8.023923082 x 10
-8
a
6
= 6.136820929 x 10
-11
Similar algorithms can be found in Goff and
Gratch (1946) and Weiss (1977).
The vapor pressure is then given by the
equation below.
eRHe
s
=××001. (5)
The vapor pressure (e) is in kPa and RH is the
relative percent humidity. Finally, the water
vapor density is found with the following:
ρ
v
v
e
TR
=
∗10
6
(6)
where
ρ
v
is the vapor density (g m
-3
), R
v
is the
gas constant for water vapor (461.5 J K
-1
kg
-1
)
(Stull, 1988).
2.5 SOIL THERMOCOUPLES, HEAT
FLUX PLATES, AND CS615
The soil thermocouples, heat flux plates, and
the water content reflectometer are typically
installed as in Figure 2.5-1. The TCAV parallels
four thermocouples together to provide the
average temperature (see Figure 2.5-2). It is
constructed so that two thermocouples can be
used to obtain the average temperature of the
soil layer above one heat flux plate and the
other two above a second plate. The
thermocouple pairs may be up to two meters
apart.
The location of the two heat flux plates and
thermocouples should be chosen to be
representative of the area under study. If the
ground cover is extremely varied, it may be
necessary to have additional sensors to provide
a valid average.
Use a small shovel to make a vertical slice in
the soil and excavate the soil to one side of the
slice. Keep this soil intact so that it may be
replaced with minimal disruption.
2 cm
6 cm
8 cm
Ground Surface
Up to 1 m
2.5 cm
FIGURE 2.5-1. Placement of Thermocouples and Heat Flux Plates
Partial emplacement of the HFT3 and TCAV
sensors is shown for illustration purposes. All
sensors must be completely inserted into the soil
face before the hole is backfilled.