Agriculture Reference
In-Depth Information
Symmetrical 10°
80
75
70
65
60
E-W
N-S
55
50
21 Dec
21 Feb
21 Apr
21 Jun
Symmetrical 30°
80
75
70
65
60
E-W
N-S
55
50
21 Dec
21 Feb
21 Apr
21 Jun
Fig. 3.5.
Daily average global radiation transmissivity differences in a greenhouse with a symmetrical
roof with a pitch of 10° (up), or 30° (down), depending on their east-west (e-W) or north-south (N-S)
orientation on 21 December, 21 February, 21 April and 21 June. Data obtained by simulation, for a
latitude of 37°N, for a low-tech parral-type greenhouse, with a new three-layer Pe film cover, assuming
only direct radiation (a hypothetical situation).
reasons (e.g. to limit temperature in insuffi-
ciently ventilated greenhouses, for fruit
quality considerations, to improve the col-
our of the product, or to reduce water
stress).
The anti-dripping effect of the inner
side of a multilayer plastic film (once
located over the greenhouse) prevents the
formation of thick drops (when water
vapour condenses on the film), reducing
transmissivity (Jaffrin and Urban, 1990;
Zabeltitz, 1999) and later contributing to
water dripping on the crop, with negative
effects on plant health (see Chapter 4).
Washing the plastic film covers and
restricting greenhouse whitewashing as
much as possible, together with a good
selection of the plastic film, allow for a bet-
ter availability of radiation inside the green-
house (Montero
et al
., 1985; Morales
et al
.,
2000). Other measures, such as limiting the
shadows of the super-structure and of the
installed equipment (thermal screens, ven-
tilator's screens) and the outside wind-
breaks, are quite advisable.
The quality of radiation is affected by
the soil particles deposited on the green-
house cover, limiting the PAR even more
than the IR radiation (Takakura, 1989).
We must also consider those crop man-
agement techniques which optimize the use
of radiation (intercepting it) inside the
greenhouse: (i) north-south orientation of
the crop rows; (ii) plant density; (iii) plant
