Available Energy: Net Radiation and Soil Heat Flux - Transport in the Atmosphere-Vegetation-Soil Continuum - page 48

T 1 > T 2

e 1 > e 2

1

T 1

e 1

e 2

T 2

2

Figure 2.20 Heat transport by distillation: the temperature difference T 2 - T 1 causes
a difference in vapour pressure e 2 - e 1 , which in turn induces moisture transport.
Water evaporates at location 1 and condensates at location 2, hence transporting
latent heat.

Note that we deal solely with heat transport by conduction. However, if soil mois-
ture movement occurs in a direction in which a temperature gradient exists, this trans-
port of liquid water will entail a transport of heat as well. This can be particularly
relevant in the case of iniltration of rain (see Kollet et al., 2009 ). Another mode of
heat transport is through the movement of water vapour: the transport of latent heat
(see Figure 2.20 ). The air entrapped in soil pores is generally saturated with water
vapour (except for very dry soils). Because the saturated vapour pressure depends on
temperature, a gradient in temperature will also entail a gradient in vapour pressure.
The latter will induce the transport of water vapour down the vapour pressure gradient
(which has the same direction as the temperature gradient). At the low-temperature
end of the soil pore the water vapour will condensate, releasing its latent heat (distil-
lation). Van Wijk ( 1963 ) shows how this transport can be incorporated by increasing
the thermal conductivity of the air phase (it may increase by a factor of 4). Additional
modes of heat transport in soils are discussed in Farouki ( 1986 ).

Question 2.17: Note down the name and physical interpretation of the following soil
thermal properties: λ s , κ s , c s and C s .

Question 2.18: The increase of the soil temperature (e.g., Figure 2.18b ) is an expres-
sion of the storage of heat in the soil. Assume that the temperature curve for a depth of
5 cm is representative of the entire layer of 0-10 cm depth.
a) For Figure 2.18b , how much heat is stored in the upper 10 cm in the period between
4 and 15 UTC (assume a volumetric heat capacity of 3.0 10 6 J K -1 m -3 ).
b) What is - over the same time period - the mean difference between the soil heat lux
at the top of the soil ( z d = 0 ) and at 10 cm depth?