Light and Temperature
17
or Kipp and Zonen, Amsterdam, The Netherlands), commonly employed at governmental or other meteorological stations.
Local cloud cover is often so variable that extrapolation of solar radiation
measurements from a standard weather station should not be made when the station is
more than about 50-km distant from the study area. The irradiance impinging on a
woodland stream can be highly variable, depending on the adjacent forest canopy, so
direct measurements of irradiance are mandatory. Portable pyrheliometers have
distinct advantages in these situations.
Pyrheliometers are sensitive in the range of 300 to 5000 nm. Slightly less than half of
this measured total irradiance is available for photosynthetic activity. Under a clear sky
and unobscured sunlight, with the sky light contributing about one-third of the total
radiation, the percentages of the total irradiance within various ranges of wavelengths
are as follows [after Strickland (1958)]:
380-720nm
380-490nm
490-560nm
560-620nm
620-720nm
50.0%
17.0%
12.0%
8.5%
12.0%
The amount and spectral composition of solar radiation impinging on the surface of
a lake or stream are influenced by an array of dynamic environmental factors. Direct
solar radiation reaching the water surface varies with the angular height of the radiation
and, therefore, with time of day, season, and latitude (Wetzel, 1983). The quantity and
quality of light also vary with the molecular transparency of the atmosphere and the
distance the light must travel through it; therefore, it varies with altitude and
meteorological conditions.
In addition, light is scattered as it passes through the atmosphere, producing indirect
solar radiation, some of which eventually reaches the surface of the water. The height of
the sun and the distance which light must pass through the atmosphere influence the
amount and spectral composition of indirect irradiance. For example, at a sun
elevation of 10° from the perpendicular, 20 to 40% of light received may be from
indirect irradiance, whereas at a sun elevation of 40° this amounts to 8 to 20%.
A significant portion of the light is reflected by the surface of the water. Such light is
unavailable to the aquatic system but may be back scattered to the lake again from
the atmosphere or surrounding topography. The extent of reflection of direct and
indirect irradiance varies widely with the angle of incidence of incoming energy, the
surface characteristics of the water, the surrounding topography, and meteorological
conditions (Wetzel, 1983). Reflection of direct solar radiation increases with increasing
angle of the sun from the perpendicular. Reflection also increases with disturbance of
the surface by wave action, when the angle of the sun is low. Ice, and especially snow,
increase reflection of light from the surfaces of fresh waters. Of the total irradiance
impinging on the surface of a lake, the average amount reflected on a clear summer day
is about 5 to 6% and increases to about 10% in winter when ice covered. At least 70%
of incident light striking fresh, dry snow is reflected (see Exercise 4).
Measurement of Underwater Irradiance
As solar radiation penetrates water, portions are absorbed both by water itself and by
dissolved and suspended materials contained in it. A significant portion of this light is
also scattered, that is deflected by the molecular components of water, its solutes, and
17
or Kipp and Zonen, Amsterdam, The Netherlands), commonly employed at governmental or other meteorological stations.
Local cloud cover is often so variable that extrapolation of solar radiation
measurements from a standard weather station should not be made when the station is
more than about 50-km distant from the study area. The irradiance impinging on a
woodland stream can be highly variable, depending on the adjacent forest canopy, so
direct measurements of irradiance are mandatory. Portable pyrheliometers have
distinct advantages in these situations.
Pyrheliometers are sensitive in the range of 300 to 5000 nm. Slightly less than half of
this measured total irradiance is available for photosynthetic activity. Under a clear sky
and unobscured sunlight, with the sky light contributing about one-third of the total
radiation, the percentages of the total irradiance within various ranges of wavelengths
are as follows [after Strickland (1958)]:
380-720nm
380-490nm
490-560nm
560-620nm
620-720nm
50.0%
17.0%
12.0%
8.5%
12.0%
The amount and spectral composition of solar radiation impinging on the surface of
a lake or stream are influenced by an array of dynamic environmental factors. Direct
solar radiation reaching the water surface varies with the angular height of the radiation
and, therefore, with time of day, season, and latitude (Wetzel, 1983). The quantity and
quality of light also vary with the molecular transparency of the atmosphere and the
distance the light must travel through it; therefore, it varies with altitude and
meteorological conditions.
In addition, light is scattered as it passes through the atmosphere, producing indirect
solar radiation, some of which eventually reaches the surface of the water. The height of
the sun and the distance which light must pass through the atmosphere influence the
amount and spectral composition of indirect irradiance. For example, at a sun
elevation of 10° from the perpendicular, 20 to 40% of light received may be from
indirect irradiance, whereas at a sun elevation of 40° this amounts to 8 to 20%.
A significant portion of the light is reflected by the surface of the water. Such light is
unavailable to the aquatic system but may be back scattered to the lake again from
the atmosphere or surrounding topography. The extent of reflection of direct and
indirect irradiance varies widely with the angle of incidence of incoming energy, the
surface characteristics of the water, the surrounding topography, and meteorological
conditions (Wetzel, 1983). Reflection of direct solar radiation increases with increasing
angle of the sun from the perpendicular. Reflection also increases with disturbance of
the surface by wave action, when the angle of the sun is low. Ice, and especially snow,
increase reflection of light from the surfaces of fresh waters. Of the total irradiance
impinging on the surface of a lake, the average amount reflected on a clear summer day
is about 5 to 6% and increases to about 10% in winter when ice covered. At least 70%
of incident light striking fresh, dry snow is reflected (see Exercise 4).
Measurement of Underwater Irradiance
As solar radiation penetrates water, portions are absorbed both by water itself and by
dissolved and suspended materials contained in it. A significant portion of this light is
also scattered, that is deflected by the molecular components of water, its solutes, and