soils, than for mineral soils. In mineral soils, heat capacity is higher in sandy soils
compared to clay soils. The thermal diffusivity increases with up to 20% moisture
and thereafter decreases (Oke 1992; Arya 1988).
In micrometeorology, soil and ground heat fluxes variations are not taken into
account insofar that the large differences in soil physical properties in scales of 10
–
3
–10
–2 m are often not considered (Foken 2017). Indeed, e.g., soil thermal conductivity varies over time and space, being dependent on the overall conductivity of
the constituent particles, porosity, and moisture content which are determinant for
short term conductivity variability (Oke 1992). Soil moisture increases the conductivity by coating the individual particles, thereby facilitating contact among the
particles, and by replacing the air with water in porous soils. Water in the soil
surface sublayers attenuates the daytime variation in the temperature regime due to
increased surface evaporation, as well as the increased heat capacity and thermal
conductivity, as previously mentioned. In moist soil without canopy cover, a large
fraction of the radiation balance is used up in the initial evaporation, after which the
thermal capacity of the remaining water also contributes to the reduction in soil
heating due to incident radiation (Oke 1992; Arya 1988).
The on-site measurement of soil surface temperature is complicated by the
high vertical and horizontal air temperature gradients close to the ground which are
difficult to detect due to factors such as finite sensor size. Remote sensing is another
methodology for measuring surface temperature when the surface emissivity is
known. An example is a radiometer facing the ground, which records longwavelength radiation flux emitted from the surface, using the Stefan–Boltzmann
equation defined in Eq. (6.60) (Arya 1988).
Biological processes are dependent on soil temperature, examples of which are
seed germination, and the development of root systems influencing plant growth. In
practice, it is difficult to study fauna and flora in undisturbed soil or monitor the
development of an undisturbed root system and then quantify its physiological
responses to temperature gradients (Monteith and Unsworth 1991). There are,
however, empirical ways to improve soil thermal regime, including the use of straw,
peat or biochar acting as insulation and reducing heat losses in winter, or covering
soils with dark polyethylene to increase surface heat.
During the day, the ground surfaces including plants and urban areas are heated
by the incoming solar radiation. Vegetation canopy also attenuates the daytime
amplitude of surface temperatures. Some of the incident solar radiation is intercepted by the canopy, reducing the surface radiation intensity, thereby dampening
the soil temperature regime. At night, surface radiative cooling is also reduced by
downward long-wavelength radiation emitted by vegetation and the surface is also
cooler than the air and the deeper soil layers (Arya 1998; Foken 2017).
The biosphere thermal capacity above the soil is relatively low so that thermal
storage of solar radiation occurs primarily in soils, rocks, and aqueous substrates.
For example, the thermal storage rate in a 1 m layer of humid soil is about 1.7 times
higher than for forest canopy 20 m in height and with 500 m
3 ha
−1 biomass content
(Lee 1978).
6.1 Conduction
167
compared to clay soils. The thermal diffusivity increases with up to 20% moisture
and thereafter decreases (Oke 1992; Arya 1988).
In micrometeorology, soil and ground heat fluxes variations are not taken into
account insofar that the large differences in soil physical properties in scales of 10
–
3
–10
–2 m are often not considered (Foken 2017). Indeed, e.g., soil thermal conductivity varies over time and space, being dependent on the overall conductivity of
the constituent particles, porosity, and moisture content which are determinant for
short term conductivity variability (Oke 1992). Soil moisture increases the conductivity by coating the individual particles, thereby facilitating contact among the
particles, and by replacing the air with water in porous soils. Water in the soil
surface sublayers attenuates the daytime variation in the temperature regime due to
increased surface evaporation, as well as the increased heat capacity and thermal
conductivity, as previously mentioned. In moist soil without canopy cover, a large
fraction of the radiation balance is used up in the initial evaporation, after which the
thermal capacity of the remaining water also contributes to the reduction in soil
heating due to incident radiation (Oke 1992; Arya 1988).
The on-site measurement of soil surface temperature is complicated by the
high vertical and horizontal air temperature gradients close to the ground which are
difficult to detect due to factors such as finite sensor size. Remote sensing is another
methodology for measuring surface temperature when the surface emissivity is
known. An example is a radiometer facing the ground, which records longwavelength radiation flux emitted from the surface, using the Stefan–Boltzmann
equation defined in Eq. (6.60) (Arya 1988).
Biological processes are dependent on soil temperature, examples of which are
seed germination, and the development of root systems influencing plant growth. In
practice, it is difficult to study fauna and flora in undisturbed soil or monitor the
development of an undisturbed root system and then quantify its physiological
responses to temperature gradients (Monteith and Unsworth 1991). There are,
however, empirical ways to improve soil thermal regime, including the use of straw,
peat or biochar acting as insulation and reducing heat losses in winter, or covering
soils with dark polyethylene to increase surface heat.
During the day, the ground surfaces including plants and urban areas are heated
by the incoming solar radiation. Vegetation canopy also attenuates the daytime
amplitude of surface temperatures. Some of the incident solar radiation is intercepted by the canopy, reducing the surface radiation intensity, thereby dampening
the soil temperature regime. At night, surface radiative cooling is also reduced by
downward long-wavelength radiation emitted by vegetation and the surface is also
cooler than the air and the deeper soil layers (Arya 1998; Foken 2017).
The biosphere thermal capacity above the soil is relatively low so that thermal
storage of solar radiation occurs primarily in soils, rocks, and aqueous substrates.
For example, the thermal storage rate in a 1 m layer of humid soil is about 1.7 times
higher than for forest canopy 20 m in height and with 500 m
3 ha
−1 biomass content
(Lee 1978).
6.1 Conduction
167