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 [ Heated water Davies et al. (1988) reported that water droplet cooling as they fly through the air is the main mechanism of heat supply to orchards during under-plant sprinkling. They hypothesized that freezing water on the surface to release the latent heat of fusion provides little sensible heat to air (i.e. it does not raise the air temperature). Because of the low trajectory of the under-plant spray, evaporation is reduced relative to over-plant systems and preheating water might provide some benefit for the under-plant sprinklers. Martsolf (1989) applied water heated to 70 °C through a microsprinkler system to a Florida citrus orchard and found little effect on the temperature of leaves that were 3 m from the sprinkler heads. However, he found as much as 4 °C rise in temperature of leaves in dense tree canopy directly above the heads. On average, temperatures rises varied between 1 °C and 2 °C depending on proximity to the sprinkler heads. However, the efficiency resulting from use of a heat exchanger to heat water and the resulting uniform distribution of energy within the orchard was much improved over using point-source orchard heaters. Also, because the water temperature is low relative to heater temperatures, inversion strength is less important. Where inexpensive energy is available and/or water is limited, they recommend using an economical heating system to warm water to about 50 °C. This will lower the required application rate for growers with inadequate water supplies. When water is heated to 50 °C, the energy released by cooling to 0 °C and freezing is 544 kJ kg -1 . However, a 2.0 mm h -1 application rate of 50 °C water gives the same amount of energy as a 2.6 mm h -1 application rate of 20 °C if all of the water is cooled and frozen. Because of enhanced sensible heat transfer from warmer water droplets to the air, heating the water will raise air temperature in the crop regardless of the frost conditions. However, for growers with adequate water supply and mild to moderate frost conditions, it is probably more cost-effective to design the sprinkler system with the higher application rate than to pay the additional costs for a heating system, fuel and labour. However, the use of heated water might be a useful alternative for growers with severe frost problems, a source of low cost energy or a limited water supply. Evans (2000) estimates a cost range from $6180 to $8650 ha -1 for a heat exchanger to heat water for under-plant sprinklers, which is roughly equivalent to twice the cost of wind machines. SURFACE IRRIGATION One of the most common methods of frost protection is to directly apply water to the soil using furrow, graded border, or flood irrigation. Jones (1924), the earliest known research on using surface water, found a 1 °C increase in air 180
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. <strong>Protection</strong> 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. 181
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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 enhance
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PASSIVE PROTECTION METHODS use an
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PASSIVE PROTECTION METHODS FIGURE 6
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PASSIVE PROTECTION METHODS After st
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PASSIVE PROTECTION METHODS of miner
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PASSIVE PROTECTION METHODS FIGURE 6
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PASSIVE PROTECTION METHODS CANOPY T
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