In Lake Oze, the downward irradiance in the region 400700 nm
Ed was measured in 1987.
Ed was measured at depths
z=0, 0.1, 0.3, 0.5, 1 m, and then at 1-m intervals to 8 m.
Ed at
z=0 m was measured just below the lake surface. When the ratio of
Ed to
Ed0, which is the value of
Ed at
z = 0 m, is plotted with a logarithmic scale, relation between
Ed/Ed0 and z was approximately linear. The vertical attenuation coefficient of downward irradiance
Kd ranged from 0.49 to 0.85 m
-1.
In Lake Oze, absorption coefficient of the lake water was minimum at wave length λ=570 or 580 nm, so
Ed was estimated to be maximum at λ= 570 or 580 nm, and the incident direct solar radiation at λ=570 or 580 nm must reach a considerable depth. If it is assumed that the incident direct solar irradiance is mainly attenuated by absorption of the lake water,
Kd depends on solar altitude and absorption coefficient. When the values of the angle of refraction and absorption coefficient at λ=570 nm are given,
Kd for downward irradiance at λ=570 nm is calculated. The calculated values of
Kd were in approximate agreement with those obtained from the measurement of
Ed except in September.
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