Agriculture Reference
In-Depth Information
Some authors consider more appropri-
ate the term 'irradiance' than that of radi-
ation intensity. Both designate it as the
incident energy flux per unit surface, that is,
the radiant flux density over a surface. The
irradiance or radiation intensity is usually
measured in watts per square metre (W m
−2
).
The radiation intensity on a surface
will depend on the inclination with which
the radiation impacts on such a surface. If it
impacts perpendicularly, the surface will
receive the maximum radiation per unit
area (Fig. 2.12). Lambert's cosine law dic-
tates (Jones, 1983):
as no more than 75% of it, under normal con-
ditions (Monteith and Unsworth, 1990). The
global radiation values are measured, by con-
vention, over a horizontal surface, being
admitted as a general rule that 48% of the glo-
bal radiation is PAR radiation (Hanan, 1998).
The proportions of direct and diffuse
solar energy vary with the turbidity and trans-
parency of the atmosphere, also influenced by
the Sun's elevation. Through the day, if the
sky is clear, the irradiance (radiation inten-
sity) evolves in a very regular way, but if the
sky is cloudy it varies a great deal (Fig. 2.13).
The global daily radiation varies
greatly with the latitude, with higher dif-
ferences in winter than in summer. It also
varies with time throughout the year,
depending on the irradiance (radiation
intensity) and the length of the day, being
higher during the summer months than
during the winter months (Plate 2).
The global solar radiation is quantified
by its irradiance or solar radiation intensity
(instantaneous energy flux) in watts per
square metre (W m
−2
). The quantity of global
solar radiation received or accumulated
over a period is usually expressed in mega-
joules per square metre (MJ m
−2
).
The PAR radiation (which ranges from
400 to 700 nm) can be quantified by its inten-
sity in energy units or photosynthetic irradi-
ance (W m
−2
) or in photonic units (moles of
photons). Within this range 1 W m
−2
of PAR
equals approximately 4.57 mmol m
−2
s
−1
under clear day conditions (Table 2.2).
The photometric units, adapted to
human vision, do not have a constant equiv-
alence with the energy and photonic units,
so they are now disused.
The unit for the instantaneous light
flux is the lux, which equals a lumen per
square metre; 1 W m
−2
of PAR radiation
equals 247 lux, if the source of light is solar
(Table 2.2), whereas it equals 520 lux, if the
source of light is a low-pressure sodium
vapour lamp (McCree, 1972).
The proportion of solar energy used
in the synthesis of organic matter or in
morphogenesis is minimal, and negligible
in energy balance studies (Hanan, 1998). In
greenhouse management it is worth men-
tioning that energy conversion phenomena
I
=
I
0
cos i
(2.1)
where:
I
= Radiation flux density (irradiance or
radiation
intensity)
impacting
on
the
surface
I
0
= Radiation flux density (irradiance or radi-
ation intensity) impacting on a surface per-
pendicular to the direction of the radiation
i
= Angle of incidence between the radi-
ation direction and the perpendicular to the
surface. If the surface is horizontal,
i
is the
zenith angle (
q
) (Fig. 2.8).
At the Earth's surface, due to the absorp-
tion and dispersion of the radiation through
the atmosphere, the radiation intensity is
lower than the solar constant, being estimated
I
0
i
I
Fig. 2.12.
The radiation impacting on a surface is
calculated by the Lambert's cosine law (see text).
The solar radiation values are measured, by
convention, over a horizontal surface.
