Agriculture Reference
In-Depth Information
Sunny day
Cloudy day
Direct radiation
Direct radiation
Diffuse radiation
Fig. 2.13.
On a sunny day direct radiation, coming from the Sun, predominates over the diffuse radiation
which comes from the whole of the sky (dispersed by the clouds and the atmospheric gases). When the
Sun's elevation over the horizon is low, on a sunny day, the proportion of diffuse radiation increases in
relation to the direct radiation. The shadows on a sunny day are clear and well defined, whereas on a
cloudy day they are not well defined.
Table 2.2.
equivalence of radiation and illumination units within the PAR range (400-700 nm)
(adapted from McCree, 1972).
1 W m
−2
(PAR)
approximately equals:
1 W m
−2
(PAR)
approximately equals:
1
m
mol m
−2
s
−1
approximately equals:
Light source
Sun and sky
247 lux
4.57
m
mol m
−2
s
−1
54 lux
Only clear sky
220 lux
4.24
m
mol m
−2
s
−1
52 lux
Incandescent lamp
250 lux
5.00
m
mol m
−2
s
−1
50 lux
Sodium vapour lamp
(low pressure)
520 lux
4.92
m
mol m
−2
s
−1
106 lux
Sodium vapour lamp
(high pressure)
408 lux
4.98
m
mol m
−2
s
−1
82 lux
2.4.4
Measurement of solar radiation
are much related to the change from liquid
water to vapour and vice-versa.
Once the solar radiation reaches the
Earth's surface, part of the radiation is reflected.
The so called 'albedo' is the proportion of the
incident solar radiation which is reflected
over a certain surface over the whole range of
the spectrum (Monteith and Unsworth, 1990),
although some authors relate it only to visible
light. The albedo, expressed from zero (0) to
one (1), ranges from 0.15 to 0.25 for herbal
crops (Villalobos
et al
., 2002).
The effects of radiation on plants are deter-
mined by the quantity and quality (wavelength)
of the radiation impacting on them. Another
relevant factor is the direction of the radiation.
It is assumed that the leaves act as a plane
receptor, located horizontally (Langhams and
Tibbitts, 1997). The radiation measurement
sensors are constructed with filters which cor-
rect the radiation reflection when its incidence
is not perpendicular to the sensor's surface.
