Frost Protection - UTL Repository

ACTIVE PROTECTION METHODS
temperature. When the wind speed is higher, there is more evaporation, higher
sensible heat losses from the plant surfaces and more water must be frozen to
compensate for these losses. When the unprotected minimum temperature is
lower, then more energy from the freezing process is needed to make up for the
sensible heat deficit. Sprinkler rotation rates are important because the
temperature of wet plant parts rises when the water freezes, but it falls as water
vaporizes and radiative losses continue between pulses of water striking the
plants (Figure 7.10).
Frequent wetting of the crop is needed to reduce the time interval when the
plant temperature falls below 0 °C (Figure 7.10). Generally, the rotation rate
should not be longer than 60 seconds and 30 seconds is better. For example, the
widely used sprinkler application rate recommendations for grapevines for wind
speeds of 0.0 to 0.5 m s -1 (Table 7.7) and wind speeds of 0.9 to 1.4 m s -1 (Table 7.8)
depend on the sprinkler rotation rate and minimum temperature as well as the
wind speed. Gerber and Martsolf (1979) presented a theoretical model for
overhead sprinkler application rate for protection of a 20 mm diameter tree leaf.
Using that model a simple empirical equation giving nearly the same sprinkler
application rate (R A ) is given by:
R A = ( 0.0538 u 2 − 0.5404 u − 0.4732 ) T l mm h -1 Eq. 7.7
where u (m s -1 ) is the wind speed and T l (°C) is the temperature of a dry
unprotected leaf.
Using the approach outlined by Campbell and Norman (1998), the difference
between air and leaf temperature of a 0.02 m diameter leaf on a typical frost
night, with high stomatal resistance, can be estimated as:
T a − T l = 1.4458 u -0.4568 °C Eq. 7.8
for 0.1 ≤ u ≤ 5ms -1 . Combining the two equations, a simple equation for the
sprinkler application rate in terms of wind speed (u) in m s -1 and air temperature
(T a ) in °C is given by:
R A = ( T a − 1.4458 u -0.4568 ) ( 0.0538 u 2 − 0.5404 u - 0.4732 ) mm h -1 Eq. 7.9
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