Agriculture Reference
In-Depth Information
17
17.1
Introduction
From the point of view of greenhouse
management, we may consider three man-
agement levels: (i)
strategic
; (ii)
tactical
; and
(iii)
operational
(Van Straten and Challa,
1995). At the strategic level, decisions on
the investment in structure and equipment
determine the future possibilities for green-
house climate control. At a tactical level,
before the start of the growing season the
grower chooses, depending on the different
predictions (weather, commercial, etc.), the
species and growing calendar, defining
expected patterns of crop development and
production. This tactical plan determines
the conditions that need to be controlled at
the operational level.
The control system is the instrument
for the grower to follow his or her tactical
plan and, if necessary, modify it to cor-
rect any deviations that may arise with
respect to the original predictions
(weather, commercial). The control sys-
tem is an operational management tool.
The time span for the strategic decisions
is many years, because the investments
in the structure and equipment will have
an influence throughout the whole
lifespan of the enterprise. The tactical
plan spans the duration of the growing
season and the operational decisions have
a brief operational influence (from several
minutes to a week).
The increase in the greenhouse area
experienced in several parts of the world
during recent decades (in the Mediterran-
ean area, China, Korea, Mexico) is based on
the use of plastic greenhouses and mainly
oriented towards vegetable production.
The selection of the production sites has
been determined by production costs and
the integral cost prices in the market, which
depend mainly on the local climate condi-
tions and the transport distance to the mar-
ket (see Chapter 14) among other factors.
The conventional concept of the green-
house as a collector of solar energy, based on
the greenhouse effect, in which the inside
temperature is increased with respect to that
of the open field and in which crops are pro-
tected from inclement weather, is today con-
sidered obsolete. In many areas the
'windbreak effect' or 'shelter effect' are more
important than the 'greenhouse effect' itself,
at least during certain periods of the year
(Castilla, 1994). In very arid regions, the
insulation of the protected enclosure from
the external environment, which is very hot
and dry, involves generating an 'oasis effect'
as the main function (Sirjacobs, 1988), as
well as the 'shading effect'. In tropical areas
protection from rain, the 'umbrella effect'
prevails (Wittwer and Castilla, 1995).
