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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 FIGURE 5.9 A sample of the Input worksheet of the FTrend.xls application program with the additional entry of the dew-point temperature (T d ) at two hours past sunset FIGURE 5.10 Sample plot resulting from data entry into the “Input” worksheet of the FTrendl.xls application using data from Figure 5.9 with a dew-point temperature of T d = -2.8 °C at two hours past sunset [ 104
FROST FORECASTING AND MONITORING Deciding whether to start sprinklers The Plot chart of the FTrend.xls application is also useful to help decide if the sprinkler should be used or not. For growers without soil waterlogging problems, shortage of water or concerns about cost, it is best to start the sprinklers when the wet-bulb temperature approaches either 0 °C or the critical damage temperature, depending on the value of the crop and concern about losses. However, for growers who are concerned about these problems, using the FTrend.xls application will help to determine when to start the sprinklers to minimize damage, waterlogging, energy usage and loss of water supply. When using under-plant microsprinklers, the starting temperature is less important than for other sprinkler systems because mainly the ground and not the plants are wetted. When first started, there may be a small short-term temperature drop as the sprayed water evaporates; however, if the application rate is sufficient, the temperature should recover quickly. With under-plant microsprinklers, one can start when the air temperature approaches 0 °C, without too much risk. The same applies to conventional under-plant sprinkler systems that do not wet the lower branches of the trees. If the under-plant sprinklers do wet the lower branches, then the same starting criteria should be used as for over-plant sprinklers. Over-plant sprinklers should be started so that they are all operating when the wet-bulb temperature approaches the critical damage temperature (T c ). However, note that the published critical damage temperatures are not always correct, so selecting a T c slightly higher (e.g. by 0.5 °C) than the published value might be advisable. The choice depends on the risk one is willing to accept. If the minimum temperature (T p ) is forecast to be more than 1.0 °C lower than T c , it is generally advisable to start the sprinklers as the wet-bulb temperature approaches T c using the FTrend.xls program, as previously described. The problem arises when T p is forecast to be near T c . Even if T p is slightly higher than T c , it is possible for the equation to be incorrect on any given night depending on the local conditions. For example, the T p forecast equation could work well for years and then it might fail completely on one night due to strange conditions on that night (e.g. often it is related to infrequent cold air drainage). This has happened to professional fruit-frost meteorologists in California and it is not so uncommon. However, in most cases, the prediction equations should work well. This is a good reason for close temperature monitoring during a frost night. When T p is forecast to be near T c , the decision whether to protect and when to protect depends on the dew-point temperature. If the dew-point temperature is low, then it is often advisable to start the sprinklers before T w falls below T c . This 105
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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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] Once started, the sprinklers shou
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CHAPTER 8 APPROPRIATE TECHNOLOGIES
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APPROPRIATE TECHNOLOGIES frost prot
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APPROPRIATE TECHNOLOGIES fashion. O
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APPROPRIATE TECHNOLOGIES CROP COUNT
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APPROPRIATE TECHNOLOGIES CROP COUNT
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APPROPRIATE TECHNOLOGIES other crop
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APPENDIX 2 CONSTANTS Specific heat
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- FAO ENVIRONMENT AND NATURAL RESOU
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