Agriculture Reference
In-Depth Information
experienced in Mediterranean climates,
and the ambient humidity is notably lower,
the winds being frequently loaded with
sand and with a very low water content
(Zabeltitz, 1990, 1999; Von Elsner
et al
.,
2000a, b).
In these conditions, high ventilation
capacity and efficiency is a priority (with
the possibility of tightly closing the vents),
and there is possibly a need for humidifica-
tion systems (if the evapotranspiration of
water is insufficient) to decrease the tem-
perature and increase the RH (oasis effect).
Preventing thermal losses at night is neces-
sary (so choice of a proper cladding material
and enough sealing are important) to avoid
the need for night heating. The structural
resistance to the wind is fundamental and
the collection of rainfall water for irrigation
is normally desirable.
Under cold climate conditions, the
greenhouse effect must be enhanced and,
normally, the maximum solar energy collec-
tion (interception) should be reached with
proper roof geometry and cladding material
as well as optimized greenhouse orienta-
tion. Limiting thermal losses is always
desirable (using proper cladding material,
thermal screens and being as airtight as
possible; see Chapter 7).
Frequently, the insulation measures to
reduce thermal losses imply a decrease in
available solar radiation (the double wall
decreases the transmission, the thermal
screens generate shadows even when
folded) so it is not easy to obtain a compro-
mise solution which must be based on prof-
itability criteria in each specific case. In
these cold climates, the obvious choice
between multi-span and single-span green-
houses is clearly for the first type. Heating is
a must, not just during the winter months,
and ventilation is necessary during the sea-
son of high radiation.
In some cases, greenhouse cladding
with a screen (permeable to air and water)
aims at achieving a windbreak effect, a
shading effect, or plant protection (limiting
the access of pests), when the natural ther-
mal conditions are adequate for crop
growth and, therefore, a greenhouse effect
is not pursued.
Inside the Greenhouse
4.10.1
Introduction
In principle, except for some special cases
(such as for crops with low light require-
ments), the objective of maximizing solar
radiation inside the greenhouse must be
pursued, especially during the months in
which radiation is a limiting factor for
production, as long as the costs do not
hinder the primary goal of achieving good
profitability for the farm. The increasingly
clear and well-documented relationship
between radiation and yield makes it a pri-
ority to maximize solar radiation. Artificial
lighting is seldom used because it is of lit-
tle economic interest (although recently
this is changing in very sophisticated
greenhouses) except for crops of high
added value, or when it is used to modify
the photoperiod.
the available solar radiation
There are several factors that determine the
quantity of available solar radiation. The
sun's position in the sky in the different sea-
sons of the year, the location of the green-
house and the cloudiness influence the
amount of available solar energy.
The latitude and time of year determine
the angle of incidence of the solar rays over
the Earth's surface as well as the daily
number of sunlight hours. The angle influ-
ences the amount of radiation, reaching a
maximum at the summer's solstice (21 June
in the northern hemisphere) and a mini-
mum during the winter's solstice (21
December). When the sun is very low in
winter less energy impacts on the Earth's
surface, because the Sun's rays have to cross
a thicker atmospheric layer, and therefore,
the Earth's atmosphere absorbs a higher
proportion of energy.
Altitude and local climate conditions
also modify the amount of solar radiation
available for the plants.
