Environmental Engineering Reference
In-Depth Information
These results show that the irradiance reflectance just below the surface is pro-
portional to the backscattering coefficient and inversely proportional to the absorp-
tion coefficient in accordance with (Eq.
2.8
)
(2.8)
R
(
0
) =
C
(µ
0
)
b
b
/
a
where
C
(
μ
0
) is a function of the angular distribution of the incident light flux that
is constant for a given angular distribution. Other studies also show similar results
where the reflectance increases as solar altitude decreases, indicating that
C
(
μ
0
) is
a function of
μ
0
(Gordon et al.
1975
; Kirk
1981
). Kirk's equation has been widely
used to determine
a
and
b
in earlier studies (Belzile et al.
2002
; Weidemann and
Bannister
1986
; Shooter et al.
1998
).
3.1 Optical Variables for the Attenuation of UV
and Photosynthetically Available Radiation
The absorption and scattering coefficients of different optical variables to UV and
PAR are identified in natural waters (Smith and Baker
1981
, Kirk
1984
,
1991
,
1994
; Carder et al.
1989
; McKnight et al.
1994
; Scully and Lean
1994
; Morris
et al.
1995
; Vernet and Whitehead
1996
; Laurion et al.
1997
; Sommaruga and
Psenner
1997
; Kahru and Mitchell
1998
; Laurion et al.
2000
; Markager and
Vincent
2000
; Belzile et al.
2000
; O'Donnell et al.
2010
; Belzile et al.
2002
;
Pierson et al.
2008
; Pérez et al.
2010
; Morris
2009
). Reflectance, scattering and
absorption of light occur in any surface as a function of latitude. Once light pen-
etrates the air-water interface, it can either be scattered or absorbed by the con-
stituents present in natural waters. The various optical variables in natural waters
can be discriminated as below: (1) Content of CDOM, (2) Nature and molecular
weight fractions of DOM, (3) Absorptivity (
a
CDOM
) and fluorescence (
F
CDOM
)
of CDOM at specific wavelengths, (4) Effect of variation in incident light wave-
lengths and solar zenith angle, (5) Particulate materials, (6) Chlorophyll
a
concen-
tration, (7) Water, and (8) Ice in Arctic and Antarctic regions.
3.1.1 Contents of CDOM
CDOM is one of the key factors explaining the role of absorption and scatter-
ing to the attenuation of UV and photosynthetically available radiation in natural
waters (Vodacek et al.
1997
; Scully and Lean
1994
; Morris et al.
1995
; Laurion
et al.
1997
; Hayakawa and Sugiyama
2008
; Smith et al.
2004
; Belzile et al.
2002
;
Pierson et al.
2008
; Smith and Baker
1979
; Morris and Hargreaves
1997
; Vincent
et al.
1998
; Pienitz and Vincent
2000
; Kratzer et al.
2008
; Devlin et al.
2009
).
DOM controls the downward irradiance flux through the water column of UV-B
(280-320 nm), UV-A (320-400 nm), total UV (280-400 nm) and photosyntheti-
cally active radiation (PAR, 400-700 nm) (Markager and Vincent
2000
; Morris
