Frost Protection - UTL Repository

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] Once started, the sprinklers should be operated continuously without sequencing. If water supply is limited, irrigate the areas most prone to frost or areas upwind from unprotected orchards. It is better to concentrate water on areas needing more protection than to apply too little water over a larger area. Good application uniformity improves protection. Hand-move sprinkler systems should not be stopped and moved during a frost night. However, under mild frost conditions (T n > -2.0 °C), the sprinkler lines can be placed in every second row rather than every row to cover a larger area. For moderate to severe frosts, closer spacing of the sprinkler lines may be necessary. Several researchers (Perry, 1994) have recommended that having a cover crop is beneficial for protection when under-tree sprinklers are used for frost protection. This recommendation is based partially on the idea that the presence of a cover crop provides more surface area for water to freeze upon and hence more heat will be released (Perry, 1994; Evans, 2000) and partly on the idea that the height of the liquid ice mixture and hence the height where the surface temperature is maintained at 0 °C is elevated closer to the tree flowers, buds, or fruits that are being protected (Rossi et al., 2002). The difficulty in having a cover crop is that, although there might be additional protection if and when the system is used, it is also more likely that active protection will be needed if a cover crop is present. Where water and energy resources are limited and frosts are infrequent, it might be wiser to remove the cover crop and reduce the need for active protection. In climates where frosts are common and there are adequate resources to operate the under-plant sprinklers, then maintaining a cover crop may improve protection. However, energy and water usage will increase. [ 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 Microsprinklers In recent years, under-plant microsprinklers have become increasingly popular with growers for irrigation and interest in their use for frost protection has followed. Rieger, Davies and Jackson (1986) reported on the use of microsprinklers with 38, 57 and 87 litre h -1 per tree application rates and two spray patterns (90° and 360°) for frost protection of 2-year old citrus tree trunks that were also wrapped with foil backed fibreglass insulation. The trees were spaced at 4.6 × 6.2 m, so the equivalent application rates were 218, 328 and 500 litre min -1 ha -1 or 1.3, 2.0 and 3.0 mm h -1 . On a night when the temperature fell to -12 °C, the trunks of trees in the irrigated treatments were 1.0 to 5.0 °C higher than non-irrigated control temperatures. The temperature difference between the 57 and 87 litre h -1 application rates was insignificant, but the trunk temperatures were somewhat higher than for the 38 litre h -1 application rate. However, even 178
ACTIVE PROTECTION METHODS the trunk temperatures for the 38 litre h -1 treatment fell only to -2.5 °C when the air temperature fell to -12 °C, so clearly the combination of microsprinklers with trunk wraps was beneficial. The authors also reported that a 90° spray pattern gave better protection than the 360° pattern. There was no measurable difference between air temperatures or humidity in the irrigated and non-irrigated treatments, but the upward long-wave radiation was higher in the irrigated plots. More protection is afforded by covering a larger area with water; however, there is additional benefit coming from water placed under the plants where radiation and convection are more beneficial than water placed between crop rows. However, if you spread the same amount of water over a larger area, the ice is likely to cool more than if the water is concentrated into a smaller area. Again, the best practice is to supply sufficient water to cover as large of an area as possible and be sure that there is a liquid ice mixture over the surface under the worst conditions that are likely to occur. Powell and Himelrick (2000) reported successful use of under-tree sprinkling with microsprinklers in Alabama and Louisiana on Satsuma mandarin. Their goal was to find a method that would provide full protection against moderate frosts and protection to the trunk and lower branches during severe frosts. The partial protection during severe frosts helps damaged trees to recover more rapidly. They reported that two risers per tree (i.e. at 0.75 m and 1.5 m), with an output rate of 90.8 litre h -1 per sprinkler head, gave the best results. Low-volume (trickle-drip) irrigation Low-volume (trickle-drip) irrigation systems are sometimes used for frost protection with varied results. Any benefit from applying water comes mainly from freezing water on the surface, which releases latent heat. However, if evaporation rates are sufficiently high, it is possible that more energy can be lost to vaporize water than is gained by the freezing process. Because of the wide variety of system components and application rates, it is difficult to generalize about the effectiveness of low-volume systems. Again, the best approach is to test the system during the dormant season and note what happens under a range of weather conditions. If the water on the ground surface is a liquid-ice mixture at 0 °C, then the system is beneficial. However, if all the water freezes and has a milky white appearance, the system was inefficient for those conditions. One should be aware that operating a low-volume system under frost conditions might damage the irrigation system if freezing is severe. Heating the water would reduce the chances of damage and will provide more protection. However, heating may not be cost-effective. 179
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- F A O E N A N D V I MO N I T O R
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FOREWORD Agrometeorology deals with
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ACKNOWLEDGEMENTS We want to thank D
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LIST OF PRINCIPAL SYMBOLS Roman Alp
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T n °C Observed minimum temperatur
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CHAPTER 1 INTRODUCTION OVERVIEW Whe
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INTRODUCTION Freeze and frost defin
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INTRODUCTION FIGURE 1.2 Mean air an
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INTRODUCTION FIGURE 1.4 Air (T a )
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INTRODUCTION TABLE 1.2 Categories a
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INTRODUCTION example, because bloom
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INTRODUCTION south in Florida in re
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INTRODUCTION assistance might be in
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CHAPTER 2 RECOMMENDED METHODS OF FR
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RECOMMENDED METHODS OF FROST PROTEC
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CHAPTER 4 FROST DAMAGE: PHYSIOLOGY
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FROST DAMAGE: PHYSIOLOGY AND CRITIC
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CHAPTER 5 FROST FORECASTING AND MON
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CHAPTER 6 PASSIVE PROTECTION METHOD
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PASSIVE PROTECTION METHODS FIGURE 6
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PASSIVE PROTECTION METHODS of miner
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