TCAV – Averaging Soil Thermocouple Probe
3
2.2 Calculating Soil Heat Flux
The soil heat flux at the surface is calculated by adding the measured flux at a
fixed depth (in this case 8 cm) to the energy stored in the layer above the heat
flux plates. The heat capacity of the soil and the change in temperature over the
output interval are required to calculate the average stored energy, S (W m
–2
):
S = (T
i
– T
i–1
) • D • Cs / t
Where:
T
i
is the current temperature (°C),
T
i–1
is previous temperature (°C),
D is depth to heat flux plate (m),
C
s
is the soil heat capacity (J m
–3
°C
–1
), and
T is the time interval (s).
One method of calculating Cs is to add the specific heat of the dry mineral soil,
Csd, to that of the soil water, Cw (4190 J kg
–1
°C
–1
). The values used for Csd
and Cw are on a mass basis (J kg
–1
°C
–1
). Cs, the heat capacity of the moist soil
on a volume basis (J m
–3
°C
–1
) is:
Cs = BD • (Csd + W • Cw)
This calculation requires site specific inputs for bulk density (BD, kg m
–3
),
mass basis water content (W, kg water/kg soil), and Csd, the specific heat of
the dry soil. 840 J kg
–1
°C
–1
is a reasonable value for Csd for most mineral soils
(Hanks and Ashcroft, Applied Soil Physics, Published by Springer-Verlag,
New York, 1980).
