Campbell Scientific HFT3 REBS Soil Heat Flux Plate - Manual (Page 9)

Model HFT3 Soil Heat Flux Plate
5
Example 2 Sample CR23X Program using a Differential Measurement Instruction
;Measure the HFT3 Soil Heat Flux plate.
;
01: Volt (Diff) (P2)
1: 1 Reps
2: 21 10 mV, 60 Hz Reject, Slow Range ;CR510, CR10(X) (7.5 mV); 21X, CR7 (5 mV)
3: 9 DIFF Channel ;Black wire (9H); White wire (9L)
4: 1 Loc [ HFT3 ]
5: 1 Mult ;Enter Calibration
6: 0 Offset
6. Soil Heat Flux and Storage
The soil heat flux at the surface is calculated by adding the measured flux at a
fixed depth, d, to the energy stored in the layer above the heat flux plates. The
specific heat of the soil and the change in soil temperature,
∆T
s
, over the output
interval, t, are required to calculate the stored energy.
The heat capacity of the soil is calculated by adding the specific heat of the dry
soil to that of the soil water. The values used for specific heat of dry soil and
water are on a mass basis. The heat capacity of the moist soil is given by:
( )
C C C C C
s b d m w b d v w w
= + = + ρ θ ρ θ ρ
(1)
θ
ρ
ρ
θ
m
w
b
v
=
(2)
where C
S
is the heat capacity of moist soil,
ρ
b
is bulk density,
ρ
w
is the density
of water, C
d
is the heat capacity of a dry mineral soil,
θ
m
is soil water content
on a mass basis,
θ
v
is soil water content on a volume basis, and C
w
is the heat
capacity of water.
This calculation requires site specific inputs for bulk density, mass basis soil
water content or volume basis soil water content, and the specific heat of the
dry soil. Bulk density and mass basis soil water content can be found by
sampling (Klute, 1986). The volumetric soil water content is measured by the
CS615 water content reflectometer. A value of 840 J kg
-1
K
-1
for the heat
capacity of dry soil is a reasonable value for most mineral soils (Hanks and
Ashcroft, 1980).
The storage term is then given by Eq. (3) and the soil heat flux at the surface is
given by Eq. (4).
S
T C d
t
s s =
∆
(3)
G G S
sfc cm
= +
8
(4)