Agriculture Reference
In-Depth Information
of PAR by sheets of cladding material, subject to normal incidence of a parallel
beam, was 88-90% for 3-4 mm thick horticultural glass, 85% for twin walled
acrylic and about 90 % for 180 μm horticultural polyethylene (Critten and Bai-
ley
2002
). However, these values may decay with time due to dust accumulation
(Möller et al.
2010
) and water droplet condensation. Pollet and Pieters (
2000
) inves-
tigated PAR transmission through dry and wet glass. For glass covered structures
with condensed water droplets, the transmission loss reached up to 13-15%, at
50-65° incidence angles. Pollet et al. (
2000
) also showed that on glass, water drop-
lets increased scattering significantly, from 4 to 81 %, whereas on polyethylene the
increase was much lower, from 71 to 82 %.
The parameter of most interest for the grower is the overall transmittance of
the structure, which determines the amount of light that would reach the plant and
its uniformity in time and space. The overall transmittance may be significantly
different from that of the cover material itself, mainly due to structural infrastruc-
ture. Transmittance of global radiation for single-cover Mediterranean greenhouses
is usually between 55 and 70 % (von Zabeltitz
2011
) and for double-cover green-
houses the range is between 50 and 60 %. Measurements (Teitel et al.
2012
) and
numerical modeling (Critten
1983
) have been employed to estimate the spatial and
temporal distribution of radiation intensity in multi-span greenhouses. Results in
naturally ventilated greenhouses with roof openings showed a significant effect of
the openings on radiation distribution. The mean daily PAR level directly below
the cover of the greenhouses was 58-66 % of the external PAR; above the crop, the
daily mean PAR level along a 10-m transect was 39-51 % of the outside level (Teitel
et al.
2012
). This reduction in light transmission was mainly caused by structural
elements, gutters and roof openings. Teitel et al. (
2012
) further showed that the larg-
est drop in radiation (15-28 %) was measured at midday, and in the region below
the roof openings, it was dependant on the greenhouse type, and was larger than the
drop measured at the centerline of the greenhouse span.
Giacomelli et al. (
1988
) studied the availability of global solar radiation (GSR)
and PAR inside a greenhouse by placing sensors at fixed positions: above the crop,
at truss level, and outside the greenhouse which showed that the transmittance
through a polyethylene film was equal for both GSR and PAR, and its value was
about 67 %. In recent years many growers use porous screens to protect their crops.
Cohen and Fuchs (
1999
) measured radiometric properties of screens composed of
highly reflective aluminized materials. For short and long wave lengths, screen
transmittance varied between 0.18 and 0.5, based on their measurements and data
from other sources, which demonstrated that screen radiation properties, can be de-
termined with standard meteorological equipment, i.e. pyranometers, pyrgeometers
and net radiometers. Möller et al. (
2010
) extended the study to show that transmis-
sion of direct radiation declined with solar elevation angle and became zero below
a cutoff angle depending on screen texture. In a banana screenhouse, Möller et al.
(
2010
) showed that transmission decreased linearly with time by about 0.1 % day
−1
,
during the rainless summer due to dust accumulation on the screen but recovered
after rain (Fig.
10.2
).
