Agriculture Reference
In-Depth Information
B
C
A
D
B
E
0.5
1.0
1.5
Wavelength (
m
m)
Fig. 2.11.
Successive processes of reduction of the solar radiation flux when penetrating the earth's
atmosphere. A, extraterrestrial radiation; B, after absorption by the ozone layer; C, after molecular
diffusion; D, after the aerosol's diffusion; e, after water vapour and oxygen absorption (adapted from
Monteith and Unsworth, 1990).
elevation over the horizon is low and at
low altitudes. On the Mediterranean
coast, its role is important in the ageing of
plastic materials and for plant morpho-
genesis (Raviv, 1988). It amounts to 2-4%
of the energy of the global radiation. The
UV radiation may be subdivided into
UV-A (higher than 320 nm) which is the
one that tans the skin, UV-B (from 290 to
320 nm) responsible of the skin cancer
and UV-C (from 200 to 290 nm) poten-
tially dangerous but absorbed (Fig. 2.11)
almost completely by the ozone layer
(Monteith and Unsworth, 1990).
Visible radiation
The name PAR is used to designate the
radiation with wavelengths useful for
plant photosynthesis. It is accepted that the
PAR radiation ranges from 400 to 700 nm
(McCree, 1972), although some authors
consider the PAR from 350 to 850 nm.
The composition of the radiation
changes with time, as a function of the Sun's
elevation and the cloudiness. When the
Sun is low over the horizon, the short
wavelengths are reduced (less UV and more
red). The clouds reduce the amount of
energy, greatly decreasing the NIR. The PAR
proportion in relation to the global radi-
ation increases with scattering (diffusion).
It is lower with clear sky and in the summer
(45-48%).
•
to the human eye,
from 380 (violet-blue) to 780 nm (red).
This interval includes the PAR
radiation (photosynthetically active
or photoactive radiation; it amounts
to 45-50% of the global radiation;
Berninger, 1989).
Infrared solar radiation
2.4.3
Quantity of solar radiation
•
(IR), from 780
to 5000 nm. It amounts almost to 50%
of the energy of the global radiation
(Berninger, 1989). The fraction of
energy in the range from 2500 to 5000
nm is very low. Within the IR the NIR
(near IR) is the band between 760 and
2500 nm.
The solar constant (solar radiation inten-
sity at the outer regions of the Earth's
atmosphere) is estimated to range between
1360 and 1395 W m
−2
, as an average, meas-
ured in a perpendicular plane of the direc-
tion of the solar radiation flux (Takakura,
1989), although some authors estimate it as
slightly lower.
