absorbance and emissivity values are about 0.995, decreasing with increasing angle
of incidence. For a wavelength of 11 lm and an incidence angle of 80°, emissivity
is about 70% (Monteith and Unsworth 1991).
For a given input of solar radiation, the differences of the radiative budget among
various crops with adequate water supply are insignificant, due to similar albedo
and emissivity, because of the water content and surface temperature homogeneity
caused by cooling from evaporation.
Soil reflectivity depends mainly on organic matter content, moisture, and particle
size, and incidence angle of solar radiation. The reflection coefficients for the solar
spectrum range from about 10% in organic soils to 30% in the sand. Small amounts
of organic matter can greatly decrease soil reflectivity. For clay soils, reflectivity is
a function of particle size. In the radiative range between 0.4 lm and 2 lm, the
reflectivity of kaolinite particles decreases with decreasing particle size. For 1600
and 22 lm particles, reflectivity is 56% and 78%, respectively (Monteith and
Unsworth 1991). In general, soil reflectivity is low in the blue region, increasing
with the wavelength in the visible and near IR, reaching a maximum value at 1–
2 lm. Particles aggregates with irregular shapes retain more radiation by multiple
internal reflections as compared to more homogeneous particles, such as fine
powder.
The radiation transmission is particularly relevant in snow and ice surfaces,
where short-wavelength radiation can reach depths of 10 m in ice and 1 m in snow,
as can be verified by Beer’s Law. The exponential variation indicates that the
attenuation of the radiation intensity is higher at the surface than in-depth (Oke
1992). The albedo of the cover is due to both reflections occurring at the surface and
to multiple reflections below the surface. Soil reflectivity decreases with increasing
moisture content, mainly because the radiation is retained by internal reflection in
the air–water interface formed in the meniscus of the soil’s capillary structure
(Monteith and Unsworth 1991). Radiation transmission through the soil influences
seed germination as well as root development (Hillel 1982).
6.3.5.3 Radiation Over Forest Canopies
Radiation geometry and qualitative principles can be used to estimate radiative
energy distribution in the forest canopy, assuming evenly distributed foliage. The
canopy leaf content can be determined by the leaf area index. If it is assumed that a
thin layer of leaves with leaf area index, dL, is exposed to direct sunlight, then the
energy content intercepted by dL is the product of the shadowed area projected by
the leaves by horizontal irradiance. By integration, we obtain an equation of the
Beer Law type representing the relationship between the global radiation flow in the
forest floor and the global radiation above the forest cover (Eq. 6.100)
S tso ¼ S t expðÀkLÞ
ð 6:100Þ
in which S tso is the density flux of the total surface radiation, S t the corresponding
flux on top of the outer atmospheric layer of the forest canopy, L the projected area
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6 Heat and Mass Transfer Processes
of incidence. For a wavelength of 11 lm and an incidence angle of 80°, emissivity
is about 70% (Monteith and Unsworth 1991).
For a given input of solar radiation, the differences of the radiative budget among
various crops with adequate water supply are insignificant, due to similar albedo
and emissivity, because of the water content and surface temperature homogeneity
caused by cooling from evaporation.
Soil reflectivity depends mainly on organic matter content, moisture, and particle
size, and incidence angle of solar radiation. The reflection coefficients for the solar
spectrum range from about 10% in organic soils to 30% in the sand. Small amounts
of organic matter can greatly decrease soil reflectivity. For clay soils, reflectivity is
a function of particle size. In the radiative range between 0.4 lm and 2 lm, the
reflectivity of kaolinite particles decreases with decreasing particle size. For 1600
and 22 lm particles, reflectivity is 56% and 78%, respectively (Monteith and
Unsworth 1991). In general, soil reflectivity is low in the blue region, increasing
with the wavelength in the visible and near IR, reaching a maximum value at 1–
2 lm. Particles aggregates with irregular shapes retain more radiation by multiple
internal reflections as compared to more homogeneous particles, such as fine
powder.
The radiation transmission is particularly relevant in snow and ice surfaces,
where short-wavelength radiation can reach depths of 10 m in ice and 1 m in snow,
as can be verified by Beer’s Law. The exponential variation indicates that the
attenuation of the radiation intensity is higher at the surface than in-depth (Oke
1992). The albedo of the cover is due to both reflections occurring at the surface and
to multiple reflections below the surface. Soil reflectivity decreases with increasing
moisture content, mainly because the radiation is retained by internal reflection in
the air–water interface formed in the meniscus of the soil’s capillary structure
(Monteith and Unsworth 1991). Radiation transmission through the soil influences
seed germination as well as root development (Hillel 1982).
6.3.5.3 Radiation Over Forest Canopies
Radiation geometry and qualitative principles can be used to estimate radiative
energy distribution in the forest canopy, assuming evenly distributed foliage. The
canopy leaf content can be determined by the leaf area index. If it is assumed that a
thin layer of leaves with leaf area index, dL, is exposed to direct sunlight, then the
energy content intercepted by dL is the product of the shadowed area projected by
the leaves by horizontal irradiance. By integration, we obtain an equation of the
Beer Law type representing the relationship between the global radiation flow in the
forest floor and the global radiation above the forest cover (Eq. 6.100)
S tso ¼ S t expðÀkLÞ
ð 6:100Þ
in which S tso is the density flux of the total surface radiation, S t the corresponding
flux on top of the outer atmospheric layer of the forest canopy, L the projected area
202
6 Heat and Mass Transfer Processes