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 [ relative to an unprotected crop. For under-plant microsprinklers, which apply less water than conventional sprinklers, the goal is to keep only the ground under the plants near 0 °C, to concentrate and enhance radiation and sensible heat transfer upwards into the plants. Over-plant sprinklers Over-plant sprinkler irrigation is used to protect low-growing crops and some deciduous fruit trees, but not for crops with weak scaffold branches (e.g. almond trees), where excessive weight of ice on plants could snap branches. 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 would be experienced by an unprotected crop. Drawbacks to this method are that severe damage can occur if the sprinkler system fails, the method has large water requirements, ice loading can cause 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 and low-volume targeted sprinklers. In addition, the precipitation rate depends on (1) wind speed, (2) unprotected minimum temperature, (3) the surface area of the crop to be covered and (4) distribution uniformity of the sprinkler system. As long as there is a liquid-ice mixture on the plants, with water dripping off the icicles, the coated plant parts will maintain their temperature at about 0 °C. However, if an inadequate precipitation rate is used or if the rotation interval of the sprinklers is too long, all of the water can freeze and the temperature of the coated plants will again start to fall. Conventional rotating sprinklers 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 than for short plants. For overplant sprinklers to be effective, the plant parts must be coated with water and rewetted every 30 to 60 seconds. Longer rotation intervals require higher application rates. Sprinkler distribution uniformity is important to avoid inadequate coverage, which might result in damage. A sprinkler system evaluation (i.e. a catch-can test) should be performed prior to frost season to be sure that the application 164
ACTIVE PROTECTION METHODS uniformity is good. Information on how to perform a sprinkler can test is usually discussed in most textbooks on irrigation management, and guidelines are often available from local extension advisors. 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. Do not include the higher density area in the system evaluation area. Any over-plant irrigation system that delivers an appropriate application rate can be used for frost protection, but systems specifically designed for frost protection are best (Rogers and Modlibowska, 1961; Raposo, 1979). The system needs to be in place during the entire frost season. Once in place and operating during a frost night, a system cannot be moved. Generally, the distribution uniformity is improved by using an equilateral triangle rather than rectangular head spacing. Systems designed for irrigation rather than frost protection can be used providing uniformity is good and the precipitation rate is adequate. In most cases, the sprinkler heads should be mounted at 0.3 m or higher above the top of the plant canopy to prevent the plants blocking the spray. For frost protection, specially designed springs, which are protected by an enclosure to prevent icing of the heads, are typically used. Clean filters are needed to be sure that the system operates properly, especially when river or lagoon water is used. Portable hand-move sprinkler systems with the heads rising just above the canopy top can be used for low growing crops like strawberries. For deciduous trees and vines, use permanent sprinkler systems with either galvanized or polyvinyl chloride (PVC) pipe risers that place the heads just above the canopy top. Wooden posts can support the risers. Typical sprinkler head pressures are 380 to 420 kPa with less than 10 percent variation. Starting and stopping Starting and stopping sprinklers for frost protection depends on the temperature and humidity in the orchard. When a sprinkler system is first started, the air temperature will drop; however, the air temperature will not drop below the temperature of the water droplets and it will normally rise again once water begins to freeze and release latent heat. The effect of over-plant rotating sprinkler application is illustrated in Figure 7.10, which shows the response of leaf-edge temperature to wetting by sprinklers every 120, 60 or 30 seconds (based on Wheaton and Kidder, 1964). Between wettings, evaporation (or sublimation) occurs and the phase change from liquid or ice to water vapour converts sensible to latent heat. The removal of sensible heat causes temperature of wet plant tissue to fall. Because the plant tissue is wet, the 165
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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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APPENDIX 2 CONSTANTS Specific heat
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- FAO ENVIRONMENT AND NATURAL RESOU
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