Frost Protection - UTL Repository

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F R O S T P R O T E C T I O N : F U N D A M E N T A L S , P R A C T I C E A N D E C O N O M I C S
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Over-plant conventional sprinklers
Over-plant sprinkler irrigation is used to protect low-growing crops and
deciduous fruit trees with strong scaffold branches that do not break under the
weight of ice loading. It is rarely used on subtropical trees (e.g. citrus) except for
young lemons, which are more flexible. Even during advection frosts, over-plant
sprinkling provides excellent frost protection down to near -7 °C if the
application rates are sufficient and the application is uniform. Under windy
conditions or when the air temperature falls so low that the application rate is
inadequate to supply more heat than is lost to evaporation, the method can cause
more damage than experienced by an unprotected crop. Drawbacks of this
method are that severe damage can occur if the sprinkler system fails, the method
has large water requirements, ice loading can cause branch damage, and root
disease can be a problem in poorly drained soils.
Application rate requirements for over-plant sprinklers differ for conventional
rotating, variable rate, or low-volume targeted sprinklers. As long as there is a
liquid-ice mixture on the plants, with water dripping off the icicles, the coated
plant parts will be protected. However, if an inadequate precipitation rate is used
or if the rotation rate of the sprinklers is too slow, all of the water can freeze and
the temperature of the ice-coated plants can fall to lower temperatures than
unprotected plants.
Conventional over-plant sprinkler systems use standard impact sprinklers to
completely wet the plants and soil of a crop. Larger plants have more surface
area, so a higher application rate is needed for tall plants than for short plants.
For over-plant sprinklers to be effective, the plant parts must be coated with
water and re-wetted every 30 to 60 seconds. Longer rotation rates require higher
application rates. Also, bigger plants require more water to coat the plants. See
Table 2.1 for guidelines on application rates for various plants.
Sprinkler distribution uniformity is important to avoid inadequate coverage,
which might result in damage. If cold air is known to drift in from a specific
direction, increasing sprinkler density on the upwind edge of the crop or even in
an open field upwind from the crop can improve protection. In most cases, the
sprinkler heads should be mounted at 30 cm or higher above the top of the plant
canopy to avoid the plants blocking the spray. For frost protection, specially
designed springs are often used, which are protected by an enclosure to prevent
icing of the heads. Clean filters are needed to be sure that the system operates
properly, especially when river or lagoon water is used.
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