Radiation Fluxes in Natural Environments
diffuse) irradiance of a horizontal surface S, need to be known. St is
sometimes reffered to as the global irradiance. These last two quantities
are related to the fist by
Sb = Sp cos $
(1 1.8)
and
St = Sb + Sd
(1 1.9)
where $ is the solar zenith angle.
Reflected radiation is the product of the average surface reflectance
for the solar waveband and the total shortwave irradiance of the surface:
The shortwave surface reflectance is called albedo. Typical albedos for
several surfaces are given in Table 11.2. These values are influenced by
amount of cover, color of soil or vegetation, and sun elevation angle, so
the values in the table should be regarded as approximate. Tall canopies
and water surfaces have reflectances that depend strongly on solar zenith
angle. The values in Table 11.2 are for small (midday) zenith angles.
Though a number of models are available for estimating clear sky Sp
and Sd with considerable accuracy (McCullough and Porter, 197 l), they
require data that are not generally available to the ecologist without special
measurements, and are quite complicated to use. We use a simpler model
based on Liu and Jordan (1960). We expect Sp to be a function of the
distance the solar beam travels through the atmosphere, the transmittance
of the atmosphere, and the incident flux density. A simple expression
combining these factors is:
where Sp, is the extraterrestrial flux density in the waveband of interest,
normal to the solar beam. The term t is the atmospheric transmittance
and m is the optical air mass number, or the ratio of slant path length
TABLE 11.2. Shortwave reflectivity (albedo) of soils and
vegetation canopies.
Surface
Grass
Wheat
Maize
Beets
Potato
Deciduous forest
Coniferous forest
Tundra
Steppe
Reflectivity
0.24-0.26
0.16-0.26
0.18-0.22
0.18
0.19
0.10-0.20
O.OSO.15
0.15-0.20
0.20
Surface
Snow, fresh
Snow, old
Soil, wet dark
Soil, dry dark
Soil, wet light
Soil, dry light
Sand, dry white
Road, blacktop
Urban area (average)
Reflectivity
0.75-0.95
0.40-0.70
0.08
0.13
0.10
0.18
0.35
0.14
0.15
diffuse) irradiance of a horizontal surface S, need to be known. St is
sometimes reffered to as the global irradiance. These last two quantities
are related to the fist by
Sb = Sp cos $
(1 1.8)
and
St = Sb + Sd
(1 1.9)
where $ is the solar zenith angle.
Reflected radiation is the product of the average surface reflectance
for the solar waveband and the total shortwave irradiance of the surface:
The shortwave surface reflectance is called albedo. Typical albedos for
several surfaces are given in Table 11.2. These values are influenced by
amount of cover, color of soil or vegetation, and sun elevation angle, so
the values in the table should be regarded as approximate. Tall canopies
and water surfaces have reflectances that depend strongly on solar zenith
angle. The values in Table 11.2 are for small (midday) zenith angles.
Though a number of models are available for estimating clear sky Sp
and Sd with considerable accuracy (McCullough and Porter, 197 l), they
require data that are not generally available to the ecologist without special
measurements, and are quite complicated to use. We use a simpler model
based on Liu and Jordan (1960). We expect Sp to be a function of the
distance the solar beam travels through the atmosphere, the transmittance
of the atmosphere, and the incident flux density. A simple expression
combining these factors is:
where Sp, is the extraterrestrial flux density in the waveband of interest,
normal to the solar beam. The term t is the atmospheric transmittance
and m is the optical air mass number, or the ratio of slant path length
TABLE 11.2. Shortwave reflectivity (albedo) of soils and
vegetation canopies.
Surface
Grass
Wheat
Maize
Beets
Potato
Deciduous forest
Coniferous forest
Tundra
Steppe
Reflectivity
0.24-0.26
0.16-0.26
0.18-0.22
0.18
0.19
0.10-0.20
O.OSO.15
0.15-0.20
0.20
Surface
Snow, fresh
Snow, old
Soil, wet dark
Soil, dry dark
Soil, wet light
Soil, dry light
Sand, dry white
Road, blacktop
Urban area (average)
Reflectivity
0.75-0.95
0.40-0.70
0.08
0.13
0.10
0.18
0.35
0.14
0.15