Agriculture Reference
In-Depth Information
can be maintained at any level with a well-designed heating system and
overhead or perforated tube ventilation. Good control is possible in the
spring and fall for temperatures equal to or higher than ambient levels.
In the summer, however, night temperatures will be higher than outdoor
temperatures until the radiant energy absorbed by plants, benches, and
walks is dissipated. Heat transfer can be accelerated with exhaust fans.
Evaporative cooling, however, is not very effective at night and it raises
the relative humidity to levels that favor disease development, particularly
Botrytis. Summer night temperatures above ambient levels can be main-
tained easily if the heating system is in operation.
Day temperature control is an entirely different situation. Although
greenhouses are constructed to regulate temperatures, they actually inter-
fere with temperature control. The problem is mainly one of keeping the
greenhouse cool. When outdoor temperature are low, it is relatively easy
to maintain day temperatures within desired limits. Heat can be added
through the heating system or it can be removed by overhead or perfo-
rated tube ventilation. As seasonal temperatures increase, however, pre-
cise control of day temperatures becomes more difficult. It generally re-
quires forced ventilation or evaporative cooling. In the summer months,
acceptable control of day temperatures requires ventilation with roof and
side ventilators, use of curtains for roof shading, ventilation with exhaust
fans, fan and pad for evaporative cooling, misting and the operation of
an evaporative cooling system. Several methods for cooling greenhouses
are available. They depend on convection or forced air movement with or
without the evaporation of water.
1.12 CONCLUSIONS
Predictions on future climatic conditions indicate possibilities of increases
in temperature from 1 to 3 °C by 2050 combined with some complex spa-
tially explicit changes in rainfall. However, there remains high uncertainty
in predictions of extreme events, especially hurricanes. Consequently, cli-
mate change is likely to invoke substantial changes to production of hor-
ticultural crop in a region and the severity with which biotic and abiotic
stresses will affect the productivity of these crops. Since climate change
can be expected to have varying effects in different areas on the expres-
sion of drought, salinity, water logging and pest infestation, the mitigation
