Agriculture Reference
In-Depth Information
Fig. 10.8
Diurnal curves
of temperature difference
between shaded and un-
shaded treatments for the
three shading screens cover-
ing apple trees. Each curve
represents an average over
the 12 days DOY 232-243.
Shading screens
: dia-
mond—60 %;
square
—30 %;
triangle
—16 %. (Source:
Tanny et al.
2009b
)
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tors allowed high ventilation of the shaded plants which eliminated air temperature
differences.
Insect-proof screens impose a higher resistance to air flow than shading screens
and thus reduce the ventilation, which may cause higher temperature and humid-
ity increases. Rossel and Ferguson (
1979
) studied a relatively small screenhouse
covered with an ultraviolet-stable fine-mesh polyethylene screen which reduced
light intensity by ~ 40 % and was insect-proof and noted that with fan ventilation,
the highest inside temperature never exceeded that outside by more than 1.5 °C, but
without fan the highest temperature difference between inside and outside reached
up to 3.5 °C.
In an insect-proof screenhouse Tanny et al. (
2003
) analyzed the vertical tempera-
ture gradient in relation to the external wind speed. The diurnal variation showed
the decrease in the temperature gradient as wind speed increased just before mid-
day because the high wind speed mixed the inside air and thereby decreased the
vertical temperature gradient. The temperature gradient remained positive however
throughout the daylight hours, which means that it had stabilized the internal air.
In recent years large shading screenhouses for banana cultivation have become
increasingly popular among Israeli growers (Fig.
10.9
). Measurements showed that
inside air relative humidity was higher by 8 % than that measured by a meteoro-
logical station in an open area outside the screenhouse (Tanny, unpublished data).
Higher internal water vapor mixing ratio (ratio between mass of water vapor to
mass of dry air) within a banana screenhouse was also obtained by Siqueira et al.
(
2012
), in their one dimensional model of an infinite horizontal screen cover. They
reported an increase of about 35 % in the water vapor mixing ratio under the screen
(at 5 m height) as compared to the value at the same height above an open banana
plantation. The increased internal humidity in screenhouses is presumably one of
the reasons for the potential water saving.
Tanny et al. (
2008
) investigated the effect of roof height on inside temperature
and humidity in two adjacent 60 %-shading screenhouses with different heights of
