Agriculture Reference
In-Depth Information
000°
2.9.2
CO
2
content
345°
015°
330°
030°
045°
6%
5%
4%
3%
2%
1%
0%
315°
The CO
2
concentration in the air changed
considerably during the last half of the 20th
century, from 300 to 350 ppm (Allen, 1990);
today, it increases about 1 ppm year
−1
(Berninger, 1989). At the beginning of the
21st century the standard CO
2
level in the
air was already 360 ppm. Its increase influ-
ences the atmospheric greenhouse effect, at
a planetary level.
The most usual units to measure CO
2
are vpm (volumes per million) or ppm (parts
per million), although it can also be defined
as the partial pressure of CO
2
in the air, or
the percentage of CO
2
in the air (e.g. 360
ppm equals 36 kPa, or 0.036% of CO
2
; see
Appendix 1 section A.7.1).
300°
060°
075°
285°
270°
090°
105°
255°
120°
240°
135°
150°
225°
210°
165°
195°
180°
Fig. 2.16.
Compass in Almeria (Spain) showing
the predominant direction of the wind for the period
1996-2001 (experimental Station of Cajamar
Foundation-Cajamar; from Pérez-Parra, 2002).
2.9.3
Atmospheric pollution
10
Atmospheric pollution caused by the pres-
ence of harmful gases (sulfur dioxide (SO
2
),
nitrogen oxides (NO
x
) and ozone) affects the
industrial parts and periphery of cities. It
results in the corrosion of structures as well as
having harmful effects on crops. Pollution
increases the proportion of diffuse solar
radiation.
9
8
7
6
5
2.10
Rainfall
4
The rain does not affect protected cultiva-
tion, except in special cases (e.g. greenhouses
with perforated covers or screenhouses). It
influences the design of the slopes, gutters,
evacuation ducts and rainfall water storage
systems.
Snow and hail are other phenomena to
consider, due to the load they may exert
and the resultant possible damage caused
to the greenhouse structure and covering
materials.
3
2
1
0
0
5 015202530354045
Temperature (°C)
Fig. 2.17.
The saturation water vapour pressure is
dependent on temperature.
(radiation, air humidity, wind, CO
2
) and
with the water available in the soil or
substrate. The ET contributes primarily
to the water content of the greenhouse
atmosphere.
2.11
Altitude and Topography
The altitude influences the barometric pres-
sure, which decreases as altitude increases.
