Frost Protection - UTL Repository

ACTIVE PROTECTION METHODS
temperature in a citrus grove irrigated with water at 23 °C. In this method, water
is applied to a field and heat from the water is released to the air as it cools. The
temperature of the water is important because warmer water will release more
heat as it cools. Protection is best on the first night after flooding and it becomes
less efficient as the soil becomes saturated. Water can be applied until there is
partial or total submersion of tolerant plants; however, fungal disease and root
asphyxiation are sometimes a problem. Generally, the method works best for
low-growing tree and vine crops during radiation frosts. In an experiment on
tomatoes, unprotected plants showed complete damage (Rosenberg, Blad and
Verma, 1983). Using over-plant sprinkler irrigation gave better protection than
with furrow irrigation, but the damage was minor for both methods.
Flooding
Direct flooding is commonly used for frost protection in many countries. For
example, in Portugal and Spain, growers apply a continuous flow of water to a
field that partially or totally submerges the plants (Cunha, 1952; Díaz-Queralto,
1971). In Portugal, it has mostly been used to protect pastures of ryegrass and
Castilian grass (Cunha, 1952), but it has been successfully used on a variety of
crops in California and other locations in the USA. Because of the relatively low
cost of flood irrigation, the economic benefits resulting from its use for frost
protection are high. The volume of water to apply depends on the severity of the
frost and the water temperature. Businger (1965) indicates that 4 °C of
protection can be achieved with this method if irrigation is done prior to a frost
event; whereas Georg (1979) reports that direct flooding has given temperature
rises near 3 °C in a pimento pepper crop on a frost night.
Liquid water is denser when the temperature is about 4 °C than at lower
temperatures, so water at temperatures less than 4 °C will rise to the surface and
hence water freezes from the top down. Once the ice forms on the surface, an air
space develops between the liquid water below and the ice above that insulates
against the transfer of heat from below. Then the ice-covered surface temperature
can fall below 0 °C and lead to colder surface and air temperatures.
Furrow irrigation
Furrow irrigation is commonly used for frost protection and the basic concepts are
similar to flood irrigation. Both free convection of air warmed by the water and
upward radiation are enhanced by flow of warmer water down the furrows. The
main direction of the radiation and sensible heat flux is vertical, so the best results
are achieved when the furrows are directly under the plant parts being protected.
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