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CHAPTER 7 - ACTIVE PROTECTION METHODS


Active protection methods include activities that are done during a frost night to mitigate the effects of subzero temperatures. These methods include:

1 Heaters
2 Wind machines
3 Helicopters
4 Sprinklers
5 Surface irrigation
6 Foam insulation
7 Foggers
8 Combinations of active methods

The cost of each method varies depending on local availability and prices. For example, a range of costs for commonly used systems is given in Table 7.1. However, the benefits sometimes depend on multiple uses of the system (e.g. sprinklers can also be used for irrigation). The costs and benefits of selecting a particular system are discussed in Volume II, Chapter 2 on the "Economic evaluation of protection methods." The theory of operation, proper management and the advantages and disadvantages of each of the active protection methods are discussed in this chapter.

TABLE 7. 1
The required number of protection devices per hectare and a range of estimated costs in US dollars per hectare for the year 2000 for installation and operation in deciduous orchards and vineyards in Washington State (USA) (R.G. Evans, pers. comm.)

PROTECTION METHOD

NO. PER HA

INSTALLATION COST RANGE

OPERATIONAL COST

Return stack oil-fuel heaters - used

99

$ 988 to $ 1 112 ha-1

$ 93. 08 h-1

Return stack oil-fuel heaters - new

99

$ 2 471 to $ 2 965 ha-1

$ 93. 08 h-1

Pressurized propane-fuel heaters

153

$ 6 178 to $ 9 884 ha-1

$ 103. 98 h-1

Over-plant sprinklers


$ 2 224 to $ 2 965 ha-1

$ 4. 10 h-1

Under-plant sprinklers


$ 2 224 to $ 3 459 ha-1

$ 4. 25 h-1

Under-plant microsprinklers


$ 2 471 to $ 3 706 ha-1

$ 4. 25 h-1

Heaters

One method to replace the losses of energy from a crop, in a frost situation, is to compensate with the massive use of fuel (solid, liquid or gas) burnt in heaters of various types. Depending on orientation of the heaters relative to the plants, part of the radiation is directly intercepted by plant parts, which raises the plant temperature. In addition, air that is heated by the fire is transported by free and, if the wind is blowing or wind machines are used in combination, forced convection to the plants and air within and above the canopy. Weather conditions that favour efficiency of this method are calm conditions with little or no wind and the presence of a strong inversion.

Heaters have been used to protect crops from freezing for at least 2000 years and the effects and methodology are well known. Generally, the heaters fall into two categories. There are heaters that raise the temperature of metal objects (e.g. stack heaters) and there are those that operate as open fires. Protecting with heaters is technically dependable and growers preferred heaters until pollution problems and high costs of fuel relative to the crop value made the method too expensive for many crops. Now heaters are mainly used to supplement other methods during extreme frost events and for high-value crops. In this section, the following topics are discussed:

Theory of operation

Natural energy losses from a crop are bigger than the gains during a frost night and this causes the temperature to drop. Energy is mainly lost to net radiation and the losses are partially replaced by sensible and soil heat fluxes towards the surface (Figure 7.1). If condensation (i.e. dew or frost) occurs, then released latent heat can also replace some of the energy loss. Heaters provide supplemental energy (Q) to help replace the net loss (Figure 7.1). If sufficient heat is added to the crop volume so that all of the losses are replaced, the temperature will not fall. However, there is inefficiency in the operation of heaters and, under some conditions, it becomes cost prohibitive to introduce sufficient energy to make up for the system inefficiency. Proper design and management can improve the efficiency to the level where the crop is protected under most radiation frost conditions. However, when there is little or no inversion and the wind is blowing, the heaters may not provide adequate protection.

Heaters provide frost protection by direct radiation to the plants around them and by causing convective mixing of air within the inversion layer (Figure 7.2).

FIGURE 7.1
An orchard in an imaginary box, where the energy fluxes represented are net radiation (Rn), vertical and horizontal sensible heat flux (H), conductive heat flux from the ground (G), latent heat (LE) and energy added by heating (Q)

FIGURE 7.2
Hot air rises and cools until about the same as the ambient temperature, then it spreads out and cools until it becomes denser and descends; this creates a circulation pattern

Most of the energy from heaters is released as hot gases and heated air that mainly warms the ambient air by convection. Radiant energy from the heaters travels directly to nearby plants that are in direct line-of-sight of the heaters. However, depending on the crop canopy density and structure, only a small percentage of the radiant energy from stack heaters is intercepted.

The energy requirement to prevent damage during a radiation frost is roughly equal to the net radiation loss (e.g. between -90 W m-2 and -50 W m-2), minus downward sensible heat flux and upward soil heat flux. Both the sensible and soil heat flux densities vary depending on local conditions, but it is likely that 20 to 40 W m-2 are contributed by each source. Therefore, the energy requirement to prevent frost damage is in the range 20 to 40 W m-2. Heater energy output is typically in the range 140 to 280 W m-2; depending on the fuel, burning rate and number of heaters. Therefore, much of the energy output from heaters is lost and does not contribute to warming the air or plants and the efficiency, which is defined as the energy requirement divided by the energy output, tends to be low. However, proper management can increase efficiency of the energy supplied by heaters.

The air temperature leaving a stack heater is between 635 °C and 1000 °C, so the less dense heated air will rise rapidly after leaving a heater. As the heated air rises, because of entrainment with colder surrounding air, expansion of the heated air parcels and radiation, it cools rapidly until it reaches the height where the ambient air has about the same temperature. Then the air spreads out, mixing with other air aloft. Eventually, the mixed air will cool, becomes denser and descend, which creates a circulation pattern within the inversion layer (Figure 7.2). If the inversion is weak or if the fires are too big and hot, the heated air rises too high and a circulation pattern within the inversion is not produced. Modern heaters have more control over the temperature of emitted gases to reduce buoyancy losses and improve efficiency. The most efficient systems have little flame above the stack and no smoke. Operating the heaters at too high a temperature will also reduce the lifetime of the heaters.

When there is a strong inversion (i.e. a low ceiling), the heated air rises to a lower height and the volume influenced by the heaters is smaller. Because the heated volume is smaller, heaters are more effective at raising the air temperature under strong inversions. Heater operation is less efficient at increasing air temperature in weak inversion (i.e. high ceiling) conditions because they have a bigger volume to heat. Under weak inversion conditions, using a fuel with a higher fraction of energy output to radiation than to heating the air will improve protection. This fraction is commonly improved by having more and smaller heaters, with exhaust funnels that retain heat. Also, when fires are too big or hot, the warmed air can break through the top of the inversion, there is less circulation in the inversion layer and the heaters are less efficient at warming the air (Figure 7.3).

Because heaters warm the air, the air inside a protected crop is generally rising and cold air outside is being drawn in from the edges to replace the lifted air. Consequently, more frost damage occurs and hence more heaters are needed on the borders. Kepner (1951) reported on the importance of inversion strength and placing more heaters on borders. He studied a 6.0 ha citrus orchard that was warmed with 112 chimney heaters burning 2.8 litre h-1 with an average consumption of 315 l ha-1 h-1. The unprotected minimum air temperature was 1.7 °C, but the results are similar to what one expects on a radiation frost night. The orchard was square and the easterly wind varied from 0.7 m s-1 to 0.9 m s-1 (2.5 km h-1 to 3.2 km h-1). Figure 7.4 shows how the temperature varied in a transect across the centre of the orchard. The wind direction was from the left. The upper graph (A) shows the effects of heater operation on temperature during two nights with differing inversion strength. The lower graph (B) shows the benefits from using twice the number of heaters on the upwind border.

FIGURE 7.3
Diagram of a frost night temperature profile and the influence of heater output on heat distribution and loss from an orchard

FIGURE 7.4
Temperature effects of heater operation (A) under differing inversion conditions and (B) with different concentrations of heaters on the upwind border (Kepner, 1951)

The 6.0 ha citrus orchard had trees that were about 4.6 m tall and 4.6 m diameter planted on either 6.7 × 6.7 m or 6.1 × 7.3 m centres. Heaters were placed in the tree rows with one heater per two trees within the orchard and one heater per tree on the upwind border when the concentration was increased. The orchard was warmed with about 112 chimney heaters burning 2.8 l h-1 with an average consumption of 315 l ha-1 h-l. The wind direction was from the left.

In Figure 7.4, the increase in temperature was highest midway across the orchard and the benefits from heating were less near the upwind and downwind borders. On the night with 4.2 °C of inversion strength, the increase in temperature on the upwind edge was about 40 percent of the increase in the middle of the orchard (Figure 7.4.A). The increase in temperature on the downwind edge was about 60 percent of the increase in the middle of the orchard. On the night with 7.8 °C of inversion strength, the temperature at mid-orchard was about 1.0 °C warmer than on the night with 4.2 °C inversion strength (Figure 7.4.A). The wind speed was slightly higher during the night with 7.8 °C inversion strength, so the difference most probably resulted from more efficient use of the convective heat within the stronger inversion layer.

In Figure 7.4.B, the temperature was increased by nearly 1 °C on the upwind edge when there was one heater per tree rather than one heater per two trees along the upwind border. There was less benefit from additional heaters on the downwind border, but, because the wind direction might change, it is wise to place additional heaters on all borders. Edge effects are important and well known by growers. In fact, growers will at times extinguish some fires when heaters are lit in neighbouring orchards.

Smoke effects

Today, it is well known that the protection from heaters comes from the heat released by the fires and not from smoke production (Collomb, 1966). Smoke does cover the sky and reduces visibility, but it has negligible effect on the apparent sky temperature. The dimension of the average smoke particle is less than 1.0 mm diameter (Mee and Bartholic, 1979), which reduces radiation in the visible range (0.4-0.7 mm) but has little effect on transmission of long-wave radiation. Therefore, upward long-wave radiation from the surface mainly passes through the smoke without being absorbed. Consequently, smoke has little effect on upward or downward long-wave radiation at night and hence has little benefit for frost protection. Because smoke offers little or no benefit and it pollutes the air, it is better to minimize smoke production and maximize thermal efficiency of the combustion. Smoke at sunrise blocks solar radiation and delays heating of the crop, which can lead to higher fuel consumption and possibly more damage. There are reports that gradual thawing of frozen citrus reduces damage (Bagdonas, Georg and Gerber, 1978), but there are other reports that indicate there is no evidence for this belief (Burke et al., 1977). If true, then smoke might be beneficial, but modern pollution laws make the use of smoke illegal in most locations. Where orchards are small and close to roads, heater smoke has been known to cause automobile accidents, as in northern Italy, which led to serious legal and insurance problems. Consequently, smoke generation is not recommended for frost protection.

Heater requirements

Liquid-fuel heaters typically provide about 38 MJ of energy per litre of fuel and the output energy requirement varies between 140 and 280 W m-2 (5.0 and 10 GJ ha-1h-1) depending on the frost night conditions (Blanc et al., 1963). Dividing the energy requirement in J ha-1h-1 by the energy output J l-1, the fuel requirement varies between 133 and 265 litre ha-1h-1. The number of burners needed depends on the desired level of protection and the burning rate of the heaters. If each heater consumes about 1.0 litre h-1, then dividing the fuel requirement by the consumption rate gives a range between 133 and 265 liquid-fuel heaters per hectare (HH). For more efficient protection, it is best to keep the fuel consumption per heater low and use more heaters.

The energy output for commonly used liquid and solid fuels is provided in Table 7.2. Note that the energy output is in MJ l-1 for liquid-fuel, MJ per cubic metre for gas and MJ per kilogram for solid fuels. If the fuel consumption rate (FC) and energy requirement (ER), including additional energy required for inefficiency, are known, then the number of heaters per hectare can be determined. Use Equation 7.1 to determine the number of liquid-fuel heaters per hectare from the energy requirement (ER) in W m-2, the fuel energy output (EO) in MJ l-1 and the fuel consumption rate (FC) in litre h-1 per heater:

Eq. 7.1

The 3.6 × 107 coefficient converts ER in W m-2 to J h-1 ha-1. For solid fuels, use Equation 7.1 to determine the number of heaters per hectare (HH) from the energy requirement (ER) in W m-2, the fuel energy output (EO) in MJ kg-1 and the fuel consumption rate (FC) in kg h-1 per heater. An Excel application program "HeatReq.xls" for calculating both liquid-fuel and solid-fuel heater requirements is included on the computer application disk.

Heater placement and management

Heater distribution should be relatively uniform with more heaters in the borders, especially upwind and in low spots (Figure 7.5). If the crop is located on a slope, then more heaters should be placed on the upslope edge where cold air is draining into the crop. Under freezing conditions, when the wind speed exceeds 2.2 m s-1 (7.9 km h-1), considerable heat loss occurs due to horizontal advection and higher concentrations of heaters are needed on the upwind border. Low spots, which are colder, should also have higher concentrations of heaters. Heaters on the borders should be lit first and then light more heaters as the need increases (e.g. if the wind speed increases or the temperature drops). Heaters are expensive to operate, so they are commonly used in combination with wind machines or as border heat in combination with sprinklers.

TABLE 7.2
Energy output for a variety of common fuels

FUEL

OUTPUT PER UNIT

OUTPUT RELATIVE TO 1 LITRE OIL

SYSTEM OUTPUT

Liquid fuel

MJ l-1

litre

MJ h-1 ha-1


Oil (2.8 litre h-1 H-1 × 100 H ha-1)

37.9

1.00

10612

Kerosene (2.8 litre h-1 H-1 × 100 H ha-1)

37.3

1.02

10444

Propane (2.8 litre h-1 H-1 × 150 H ha-1)

25.9

1.46

10878

Gas Fuel

MJ m3

m3

MJ h-1 ha-1


Natural gas (1.0 m3 h-1 H-1 × 265 H ha-1)

40.1

0.95

10627

Solid fuel

MJ kg-1

kg

MJ h-1 ha-1


Wood

20.9

1.81

See note (1)

Coal (0.5 kg h-1 H-1 × 360 H ha-1)

30.2

1.25

5436

Coke bricks (0.5 kg h-1 H-1 × 365 H ha-1)

29.1

1.30

5311

NOTE: (1) Output depends on wood type, water content of fuel, and size and number of fires. Energy outputs are expressed in MJ per litre, MJ m3 or MJ kg-1 for liquid, gas and solid fuels.

FIGURE 7.5
Sample arrangement of heaters (small dots in the figure), with higher concentrations along the upwind edge and in low spots (after Ballard and Proebsting, 1978).

Liquid-fuel heaters

Liquid-fuel heaters were developed for frost protection during the early 1900s. Use of the method decreased as oil prices and concerns about air pollution increased. Although not widely used, the use of liquid-fuel heaters for frost protection is still a viable method in cases where laws do not prohibit it and the cost of fuel is not too high. Liquid-fuel heaters require considerable labour for placement, fuelling and cleaning, in addition to the capital costs for the heaters and the fuel. Typically, there are about 75 to 100 oil stack heaters or 150 to 175 propane-fuel heaters per hectare, and a well designed and operated heater system will produce about 1.23 MW ha-1 (i.e. 123 W m-2) of power. The approximate consumption rate is 2.8 litre h-1 per heater for oil- and kerosene-fuelled heaters and about 1 m3 h-1 for propane-fuel heaters. More than half of the energy output from the heaters is lost as radiation to the sky and convective heat losses on a typical radiation frost night, so the heater output is high relative to the heat gained by the crop. Note that these recommendations are for protection of large deciduous orchards that are surrounded by other orchards that are being protected. Isolated smaller orchards may require more heaters.

When lighting heaters, every second or third heater in a row should be lit first. Then go back and light the remaining heaters. This helps to reduce convective losses of heat through the top of the inversion layer. Oil-fuel heaters should be cleaned after every 20 to 30 hours of operation, and the heaters should be closed to prevent entry of rainwater that could cause leakage of oil onto the ground. The stack can be blown off or the fire extinguished if too much steam is produced. Remove oil from the heaters at the end of the season. Free-flame-type heaters will accumulate carbon and lower the fuel efficiency level. Catalytic sprays can be used to reduce carbon accumulation. They should be refilled before they run out of fuel and cleaned with a stick or simply hit to free the soot accumulation that reduces efficiency.

Various types of fuels burners are available for frost protection. A list of fuels and heaters approved for use in Florida (USA) are given in Tables 7.3 and 7.4. Because they can be improvised with cans of paint, oil, etc., the free flame type (i.e. without a chimney) is cheaper and easier to transport and fill. They are smaller, so the density of heaters can be greater, giving better mixing and less chance for the chimney effect. This sometimes results in improved protection. However, they are less fuel-efficient because there is more volatilization and they pollute more. In some locations, they are not approved for use in frost protection.

TABLE 7.3
Liquid-fuel and gas fuels approved by the Florida Department of Environmental Protection for frost protection

No. 2 diesel fuel

Butane

No. 2 fuel oil

Liquid petroleum gas

Propane gas

Methane

Alcohol (ethanol or methanol)


TABLE 7.4
Heaters approved by the Florida Department of Environment for frost protection

MODEL AND MANUFACTURER

MODEL AND MANUFACTURER

HY-LO Return Stack, Scheu Products

Radiant Omni-Heater, New Draulics

HY-LO Large Cone, Scheu Products

HY-LO Lazy Flame Heater, Scheu Products

Brader Heater, Brader Heaters, Inc

Sun Heater Model 2, Fleming-Troutner

Georges Heater, Georges Enterprise

Self Vaporizing Model M.B.S.-1, Burners

Agri-Heat Heater, Agri-Heat, Inc

HY-LO Auto Clean Stack, Scheu Products

A conical heater, Fultoin-Cole Seed

Mobil Tree Heat, Mobil Oil Co.

Orchard-Rite Heater, Orchard Rite Ltd.

Fireball, Sebring Frost Products

Return Stack 2000 - W.H. Clark Food


Air pollution regulations are often quite stringent and local regulations should be reviewed before purchasing or using heaters. Most regional authorities have similar regulations on burning fuels for frost protection. In addition, some authorities have other requirements for use of heaters. For example, the Florida State Environmental Agency requires that, when using heaters for frost protection, air temperature be measured using a standard louvered weather shelter or fruit frost shelter (Figures 6.1 and 6.2). All local regulations should be investigated before using heaters.

An equal mixture of fuel and gasoline [petrol] is used to light heaters. Buckets or tanks towed by a tractor, which allow filling of two lines of burners simultaneously, are used to refill the heaters after a frost. When direct heating is used, to minimize fuel consumption the protection is started just before reaching critical damage temperatures. The temperature should be measured in a Stevenson screen, fruit-frost shelter or Gill shield that shields the thermometers from the clear sky.

Propane-fuel and natural gas-fuel heaters

Labour requirements to refill liquid-fuel heaters are high, so some growers moved from using individual heaters to centralized distribution systems. The systems use tubing to transport the fuel to the heaters. The fuel can be natural gas, liquid propane or fuel oil. In more elaborate systems, ignition, the combustion rate and closure are automated in addition to fuel distribution. The capital cost to install centralized systems is high, but the operational costs are low. Propane-fuel heaters require less cleaning and the burning rates are easier to control than oil-fuel heaters. Because the burning rate is less, more heaters are needed (e.g. about 130-150 per hectare), but the protection is better. Under severe conditions, the propane supply tank can sometimes freeze up, so a vaporizer should be installed to prevent the gas line from freezing.

Solid-fuel heaters

Solid fuels were used as a method for frost protection before liquid or gas fuels. As liquid fuels dropped in price, there was a switch from solid to liquid fuels, especially in North America. When it was discovered that the ratio of radiation to total energy released was about 40 percent for burning solid fuels (e.g. wood, coal and coke) in comparison with 25 percent for burning liquid fuels (Kepner, 1951), there was a revival in the use of solid fuels. Having a higher ratio of radiation to total energy release is important as conditions become windier (e.g. during an advection frost). The main disadvantage of solid fuels is that the energy release diminishes as the fuel is used up, and energy release thus becomes limiting when needed most (Hensz, 1969a; Martsolf, 1979b). Another drawback is that solid fuels are difficult to light, so they must be started early. They are also difficult to extinguish, so fuel is often wasted if started when unnecessary.

A variety of solid fuels are used for frost protection (e.g. wood, coke, old rubber tyres, paraffin candles and coal). Some oil companies market products consisting of petroleum wax - a refinement by-product - and coke that appear in various forms, including candles and bricks.

When compared with the liquid-fuel burners, solid fuels often show better results. For example, using two oil wax candles under each grapefruit tree in an orchard gave an average increase of 1.7 °C to the fruit. Energy efficiency (i.e. the fruit temperature inside and outside of the foliage) from using the wax candles was more than double that of liquid-fuel burners (Miller, Turrell and Austin, 1966). An increase of 2.2 °C at 1.1 m height was observed from using 375 bricks of petroleum wax and coke per hectare (Parsons, Schultz and Lider, 1967). Conventional liquid-fuel burners require twice the energy output to gain the same effect on air temperature in the canopy. For example, petroleum wax heaters used only 60 percent of the energy normally needed to get the same protection (Schultz, Lider and Parsons, 1968). Modification of temperature within the inversion layer was more concentrated near the ground - where the crop is - when burning petroleum wax and coke bricks compared with feedback chimney burners (Gerber, 1969). Thus, to improve efficiency it is clearly better to have many small fires than a few big fires.

Mobile heaters

A mobile heater is commercially available as a method for frost protection; however, scientific evaluations of the machine have not yet been published. The mobile heater uses four 45-kg propane tanks to supply the fuel for the heater, which mounts on the back of a tractor (Figure 7.6). The heater uses a centrifugal fan to blow the heated air horizontally and perpendicular to the tractor direction as it moves up and down the rows. After starting the heater, the fuel supply is adjusted to give a temperature of approximately 100 °C where the air vents from the machine. When operated, the airflow extends to 50 to 75 m either side of the machine. The tractor is driven up and down rows far enough apart so that the area of influence overlaps. The manufacturer recommend that the tractor make one complete cycle through the crop about every 10 minutes, a period allowing coverage of about 5-7 ha.

In some unpublished experiments, the mobile heater showed little effect on the minimum temperatures recorded within protected orchards. Since the energy output from the machine is much less than energy losses from a crop during a radiation frost night, this was not unexpected. However, whenever the machine passed by a point within the crop, there was a short-lived increase in the temperature recorded with exposed thermocouples. It is possible that these short-lived temperature increases have a positive effect and prevent freezing of the plant tissue; however, more research is needed to verify if this is true.

FIGURE 7.6
A mobile heater for frost protection mounted on the back of a tractor

Photo: R.L. Snyder

Recently, some researchers have suggested that the mobile heater might be beneficial because it dries the plant surfaces. Since water typically freezes on the outside of plant tissue and then propagates inside the tissue to cause freezing in intercellular spaces, there may be some validity to this theory. However, more research is clearly needed to validate effectiveness of the machine.

Wind machines

Conventional wind machines

Wind machines (or fans) that blow air almost horizontally were introduced as a method for frost protection in California during the 1920s. However, they were not widely accepted until the 1940s and 1950s. Now they are commonly used in many parts of the world. Wind machines are used on a wide variety of crops including grapevines, deciduous trees and citrus. California citrus orchards are nearly all protected by wind machines.

Wind machines generally consist of a steel tower with a large rotating fan near the top. There is usually a two- or four-blade fan with a diameter typically varying from 3 to 6 m. The typical height for fans is about 10-11 m above ground level. However, lower heights are used for lower canopies. To our knowledge, the fan height is set to avoid hitting the trees and there is no aerodynamic reason for the height selection. The most effective wind machines have propeller speeds of about 590 to 600 rpm. Fans rotate around the tower with one revolution every four to five minutes. Most wind machine fans blow at a slight downward angle (e.g. about 7 °) in the tower direction, which improves their effectiveness. When the fan operates, it draws air from aloft and blows at a slightly downward angle towards the tower and the ground. Power to operate the fan usually comes from an engine mounted at the base of the tower; however, some of the older machines have engines that rotate with the fan at the top of the tower. Matching the rotation of fans around their towers so that all fans are blowing in the same direction is believed to improve mixing effectiveness.

Before investing in wind machines, be sure to investigate the local climate and local expenses. For example, if there is little or no inversion, then wind machines are not recommended. In California, wind machines are widely used in citrus orchards, which are mainly protected during December through January, but not in deciduous orchards, because inversions tend to be strong during winter months when citrus need protection, but not in the Central Valley in the spring when deciduous trees need protection. There are reports that wind machines work better after deciduous trees leaf out in the spring. Consequently, fans are not often used in almond orchards that commonly need protection before leaf-out. Conducting a temperature survey to measure temperature inversions during the frost protection period before purchasing wind machines is advisable. If there is little or no inversion, then select a different protection method. Locate machines in places where the wind drift is enhanced by the fans. In some instances, installing machines where they can push cold air out of low spots can be beneficial.

Wind machines generally have lower labour requirements and operational costs than other methods. This is especially true for electric wind machines. However, when electric wind machines are installed, the grower is required to pay the power company "standby" charges, which cover the cost of line installation and maintenance. The standby charges are paid whether the wind machines are used or not. In fact, because of increases in the cost of power, electric wind machines have become marginally cost-effective for citrus protection in some regions of California (Venner and Blank, 1995). Internal combustion wind machines are more cost-effective because they do not have the standby charges. However, they require more labour. The capital cost to install wind machines is similar to sprinkler systems, but operational costs are higher.

Generally, except for noise, wind machines are environmental friendly. Wind machine noise is a big problem for growers with crops near cities and towns. This should also be considered when selecting a frost protection method.

Theory of operation

Wind machines provide protection by increasing the downward sensible heat flux density and by breaking up microscale boundary layers over the plant surfaces. Fans do not produce heat, but redistribute sensible heat that is already present in the air. The fans mix warm air aloft with colder air near the surface (Figure 7.7). They also help by removing the coldest air close to the leaves and replace it with slightly warmer ambient air. The amount of protection afforded depends mainly on the unprotected inversion strength. The inversion strength is calculated as the difference between the 10 m and 1.5 m temperatures in an unprotected orchard. Within the region influenced by a wind machine, the average air temperature measured at 1.5 m increases by about 1/3 of the inversion strength. Near the wind machine tower, the protection achieved is better (Figure 7.8). The actual benefit depends on inversion characteristics, which cannot be generalized. However, stronger inversions clearly give better protection.

FIGURE 7.7
A schematic diagram that shows the effect of wind machines on the temperature profile during a radiation frost

FIGURE 7.8
Traces of 10 m and 2 m temperature trends without (w/o) a fan and 2 m temperature trends measured near wind machines started at 0145 and 0315 h

Temperature measurements were collected 30 m away from the wind machine

FIGURE 7.9
Temperature field response to wind machine on 26 March 2000 in northern Portugal

(A) Temperature profiles (30 m from wind machine) before and after wind machine; and (B) 1.5 m temperature response pattern produced by wind machine after 2 complete rotations around the tower (after Ribeiro et al., 2002).

Generally, a 75-kW wind machine is necessary for each 4 to 5 ha (i.e. a radius of about 120 m to 125 m). If one wind machine is used, about 18.8 kW of engine shaft power per hectare is typically needed. About 15 kW of engine shaft power is suggested per machine per hectare when several machines are used. Protection decreases with distance from the tower, so some overlap of protection areas will enhance protection. Usually, the protection area is an oval rather than a circle shape because of wind drift. For example, Figure 7.9B shows the 1.5 m height temperature response pattern to wind machine operation in an apple orchard (Ribeiro et al., 2002).

Starting and stopping

Wind machines are typically started when the air temperature reaches about 0 °C. Under stable inversion conditions, air tends to stratify near the ground and mixing is believed to become less. However, trials in California (USA) and Portugal have shown that starting fans after inversions have formed has little influence on fan effectiveness. In less than half-an-hour after starting, the 2 m temperature typically rises, sometimes approaching the 10 m temperature of an unprotected orchard (Figure 7.8). However, because the temperature of a radiating surface during a frost night is usually lower than the air temperature, it is wise to have the wind machines operating when the air temperature reaches the critical damage temperature (Tc). If the fruit is wet during the day or evening of an expected frost night, the wind machines (and heaters) should be started earlier to attempt to dry the fruit before ice can form on the fruit. Wind machines are not recommended when the wind is blowing at more than about 2.5 m s-1 (8 km h-1) or when there is supercooled fog. When the wind is more than 2.5 m s-1, it is unlikely that an inversion is present and it is possible that the fan blades could experience wind damage. A simple method to estimate the wind speed is to hang plastic ribbon from a tree branch or street sign. Police crime scene tape is an example of the type of plastic ribbon that could be used. Such ribbon can be purchased from farm or engineering supply stores. If the wind is greater than 2.5 m s-1, the bottom of the hanging ribbon should be blowing back and forth to about 30 cm from horizontal.

In a supercooled fog, water droplets can freeze on the fan and severe wind machine damage can occur if the ice cases one blade to break off but not the other.

Vertical flow wind machines

The use of vertical flow fans to pull down the warm air aloft has been investigated; however, these fans generally work poorly because mechanical turbulence mixing with the trees reduces the area affected by the ventilation. Also, the high wind speed near the base of the tower can damage horticultural and ornamental crops. Wind machines that blow vertically upwards are commercially available and there has been some testing of the machines. The idea is that the fan will pull in cold dense air near the ground and blow it upwards where it can mix with warmer air aloft. In theory, cold air is removed near the surface and the warmer air aloft drops downward hence lowering the inversion. Limited testing has shown that this method has a temporary positive effect on temperatures near the fan; however, the extent of influence and duration of the effect is still unknown.

To our knowledge, the method has only been used in small valleys where cold air ejected upwards is likely to fall back towards the surface. In a location where prevailing winds aloft might move the air horizontally away from the crop, more protection could result. However, there is no known research evidence at this time.

Helicopters

Helicopters move warm air from aloft in an inversion to the surface. If there is little or no inversion, helicopters are ineffective. Due to the large standby and operational costs, the use of helicopters for frost protection is limited to high value crops or emergencies (e.g. when the normal method breaks down).

Authors differ on their estimates of the protection area for helicopters. The area covered by a single helicopter depends on the helicopter size and weight and on the weather conditions. Estimated coverage area varies between 22 and 44 hectares (Evans, 2000; Powell and Himelrick, 2000). Passes are needed every 30 to 60 minutes, with more passes under severe conditions. Waiting too long between passes allows the plants to supercool and the agitation from a passing helicopter can cause heterogeneous nucleation and lead to severe damage.

Temperature sensors are often mounted on the outside of the helicopter and the pilots fly at a height where they observe the highest temperature reading. The optimal height is commonly between 20 and 30 m. Common flight speeds are 25 to 40 km h-1 (Powell and Himelrick, 2000) or 8 to 16 km h-1 (Evans, 2000). Higher velocities have not improved protection. Temperature increases between 3.0 °C and 4.5 °C are common for a hovering helicopter. Pilots often load helicopter spray tanks with water to increase the weight and increase thrust. Under severe frosts with a high inversion, one helicopter can fly above another to enhance the downward heat transfer.

Thermostat controlled lights at the top of the canopy are used to help pilots see where passes are needed. As the helicopter passes over the crop, the temperature rises and the lights go out. Cooling to the thermostat temperature causes the lights come on. This helps the pilot to find and fly over cold spots. Alternatively, a ground crew should monitor temperature in the crop and communicate with the pilot where flights are needed.

On the sides of hills, heat transfer propagates down-slope after reaching the surface. Therefore, flying over the upslope side of a crop usually provides more protection because the effects are felt downwind as well. Flights are stopped when the air temperature upwind from the crop has risen above the critical damage temperature.

Sprinklers

Using sprinklers for frost protection has the advantage over other methods that water application is generally less expensive. The energy consumption is considerably less than that used in frost protection with heaters (Gerber and Martsolf, 1979) and, therefore, operational costs are low compared to heaters and even wind machines. Labour is mainly needed to ensure that the system does not stop and the heads do not ice up during the night. In addition to frost protection, one can use the sprinklers for irrigation, enhancing fruit colour by over-plant evaporative cooling, reducing sun injury by over-plant irrigation, delaying bloom prior to bud break, fertilizer application and a combination these applications. Also, the method is relatively non-polluting. The main disadvantage of using sprinklers is the high installation cost and large amounts of water that are needed. In many instances, a lack of water availability limits the use of sprinklers. In other cases, excessive use can lead to soil waterlogging, which could cause root problems as well as inhibit cultivation and other management activities. Nutrient leaching (mainly nitrogen) is a problem where sprinkler use is frequent. In some instances, excessive use of sprinklers can affect bacterial activity in the soil and it can delay maturation of fruit or nuts (Blanc et al., 1963). In this section on use of sprinklers, the following topics are discussed.

1 Basic concepts
2 Over-plant sprinklers

Conventional rotating sprinklers
Starting and stopping
Application rate requirements
Variable-rate sprinklers
Low-volume targeted sprinklers

3 Sprinkling over coverings
4 Under-plant sprinklers

Conventional rotating sprinklers
Microsprinklers
Low-volume (trickle-drip) irrigation
Heated water

Basic concepts

Like air, water has sensible heat that we measure with a thermometer and the water temperature increases or decreases depending on changes in the sensible heat content. When water temperature drops, it happens because (1) sensible heat in the water is transferred to its surroundings, (2) water vaporizes, which consumes sensible heat to break the hydrogen bonds between water molecules, or (3) there is net radiation loss. As water droplets fly from a sprinkler head to the plant and soil surfaces, some sensible heat is lost to radiation, some will transfer from the warmer water to the cooler air and some will be lost to latent heat as water evaporates from the droplets. The amount of evaporation is difficult to estimate because it depends on the water temperature and quality, droplet size and path length and weather conditions.

Understanding changes in sensible heat content of water and conversions between sensible and latent heat are crucial to understand frost protection with sprinklers. Water temperature is a measure of the sensible heat content of the water and heat released to the air and plants as the water temperature falls provides some of the protection. From wells, water commonly has a temperature near the mean annual air temperature at the location. For the water temperature to fall from 20 °C to 0 °C, each kg (or litre) must lose 83.7 kJ of sensible heat. This heat can be lost by radiation; transferred to sensible heat in the air, plants or ground; or it can contribute to evaporation. When 1.0 kg of water freezes at 0 °C, the phase change converts 334.5 kJ of latent heat to sensible heat. The total amount of energy released in cooling water from 20 °C and freezing it is 418.3 kJ k-1. If the initial water temperature were 30 °C rather than 20 °C, then cooling to 0 °C would provide an additional 41.9 kJ kg-1 for a total of 460.1 kJ kg-1. However, cooling 1.1 kg of 20 °C water and freezing it provides 460.9 kJ k-1, so applying 10 percent more water provides the same energy as heating the water by 10 °C.

The cooling and freezing of water replaces energy lost during a radiation frost. However, evaporation from the surface removes sensible heat and causes the air temperature to drop. Although evaporation rates are low, the energy losses can be high. For a phase change from liquid to water vapour (i.e. evaporation), the loss is 2501 kJ kg-1 at a temperature of 0 °C. For a phase change from ice to water vapour (i.e. sublimation), the loss is 2825.5 kJ kg-1 at 0 °C. Therefore, the energy released by cooling 1.0 kg of 20 °C water to 0 °C and freezing it is 418.3 kJ kg-1, and the amount of water cooled and frozen must be more than 6.0 times the amount evaporated (or 6.8 times the amount sublimated) just to break even. Additional energy from the cooling and freezing process is needed to compensate for the net energy losses that would occur without protection.

When water droplets strike a flower, bud or small fruit, the water will freeze and release latent heat, which temporarily raises the plant temperature. However, energy is lost as latent heat when water vaporizes from the ice-coated plant tissue. This, in conjunction with radiation losses, causes the temperature to drop until the sprinklers rotate and hit the plant with another pulse of water. The secret to protection with conventional over-plant sprinklers is to re-apply water frequently at a sufficient application rate to prevent the plant tissue temperature from falling too low between pulses of water. For non-rotating, low-volume, over-plant, targeted sprinklers, the idea is to continuously apply water at a lower application rate, but targeted to a smaller surface area.

For conventional under-plant sprinklers, the idea is apply water at a frequency and application rate that maintains the ground surface temperature near 0 °C. This increases long-wave radiation and sensible heat transfer to the plants 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 over-plant sprinklers to be effective, the plant parts must be coated with water and re-wetted 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 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 temperature will fall to as low as the wet-bulb temperature. If the dew-point temperature (humidity) is low, then the wet-bulb temperature can be considerably lower than air temperature, so the temperature of wet plant tissue can fall well below air temperature and cause damage if insufficient water is applied. Also, if the rotation rate is too slow or if the sprinklers are stopped too early, temperatures can drop below the critical damage temperature and cause damage.

In older literature on the use of over-plant sprinklers, it was common to warn against a sharp drop in temperature when sprinklers are started during low-dew-point conditions. Under windy low-dew-point temperature conditions when the air temperature is relatively high (e.g. 10 to 15 °C), evaporation from the droplets causes the water droplet temperature and hence the air temperature to drop rapidly when the sprinklers are started. However, the water droplet temperature commonly falls to near 0 °C as they pass from the sprinkler heads to the plant surfaces. Consequently, there is no reason why the air temperature would drop below 0 °C when the sprinklers are started. As shown in Figure 7.10, the plant surface temperatures will drop below 0 °C as water sublimates from the plant surfaces. But the temperature rises again when hit with a new droplet of liquid water.

FIGURE 7.10
Leaf-edge temperature changes when wetted by a sprinkler system applying water at 2.8 mm h-1 with rotation rates of 120, 60 and 30 s, air temperature near 0 °C, a wet-bulb temperature near -2 °C and a wind speed near 5.5 km h-1 (after Wheaton and Kidder, 1964)

Because the critical damage (Tc) temperatures are somewhat questionable and because they are based on temperatures of dry rather than wet plants, it may be wise to start the sprinklers when the wet-bulb temperature is a bit higher than Tc. Starting sprinklers when the wet-bulb temperature reaches 0 °C is less risky and it may be prudent if there is no water shortage and waterlogging and ice loading are not a problem.

Even if the sun is shining on the plants and the air temperature is above 0 °C, sprinklers should not be turned off unless the wet-bulb temperature upwind from the crop is above Tc. If soil waterlogging or shortage are not problems, permitting the wet-bulb temperature to exceed 0 °C before turning off the sprinklers adds an extra measure of safety.

The wet-bulb temperature can be measured directly with a psychrometer or it can be derived from the dew-point and the air temperature. For direct measurements, the cotton wick on the wet-bulb thermometer is wetted with distilled or de-ionized water and it is ventilated until the temperature of the wet-bulb thermometer stabilizes. Ventilation is accomplished by swinging a sling psychrometer or by blowing air with an electric fan using an aspirated psychrometer (Figure 3.9). If the recorded temperature is below 0 °C, the water on the wick might be frozen. Then the observed temperature is called the "frost-bulb" rather than wet-bulb temperature. However, there is little difference between the frost-bulb and wet-bulb temperature in the range important for frost protection.

Rather than using a sling or aspirated psychrometer with a ventilated wet-bulb thermometer, a simple thermometer with a wetted cotton wick and no ventilation can be employed to approximate the wet-bulb temperature. However, if possible, it is better to ventilate the cotton wick with a fan. One can use for the wick a cotton shoestring that fits snugly on the thermometer bulb.

If you decide to not measure the wet-bulb upwind from the crop, an alternative is to use the dew-point temperature from a weather service or from a measurement. Dew-point sensors are expensive, but a simple method is to use a shiny can, water, salt and ice (Figure 7.11). First pour some salted water into the shiny can. Then start adding ice cubes to the can while stirring the mixture with the thermometer. Watch the outside of the can to see when dew condenses (or ice deposits) on the surface. If there is no condensation or deposition, add more ice and salt to further lower the temperature. When you see ice deposit, immediately read the thermometer temperature. The thermometer recording is the "ice point" temperature, which is a bit higher than, but close to, the dew-point temperature. Shining a flashlight (torch) onto the can surface will help you to see the ice form and to read the thermometer. This method is less accurate than using a dew-point hygrometer, but it is often sufficiently accurate for determining start and stop temperature for sprinklers.

FIGURE 7.11
A simple method to estimate the dew-point temperature

Slowly add ice cubes to the water in a shiny can to lower the can temperature. Stir the water with a thermometer while adding the ice cubes to ensure the same can and water temperature.

When condensation occurs on the outside of the can, note the dew point temperature.

In most of the literature on using sprinklers for frost protection, the start and stop air temperatures are determined relative to the dew-point and wet-bulb temperatures. In reality, they should be based on the ice point and frost-bulb temperatures since ice covered plants are more common than water covered plants at subzero temperatures. However, a table of air temperatures corresponding to subzero dew-point and wet-bulb temperatures is nearly identical to a table of air temperatures corresponding to the ice point and frost-bulb temperatures. Therefore, only the dew-point and wet-bulb temperatures are used in Table 7.5, to avoid confusion with common practice.

To use Table 7.5, locate the wet-bulb (Tw) temperature in the top row that is greater than or equal to the critical damage (Tc) temperature for the crop. Then locate the dew-point (Td) temperature in the left-hand column and find the air temperature in the table that corresponds. Make sure that the sprinklers are operating before the air temperature measured upwind from the crop falls to the selected air temperature. The values in Table 7.5 are for sea level, but they are reasonably accurate up to about 500 m elevation. For more accuracy at higher elevations, the SST.xls application program, which is included with this book, does these calculations and it can be used to determine exact starting and stopping temperatures for any input elevation.

TABLE 7.5
Minimum starting and stopping air temperatures (°C) for frost protection with sprinklers as a function of wet-bulb and dew-point temperature (°C) at mean sea level

DEW-POINT TEMPERATURE

WET-BULB TEMPERATURE

°C

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.0







0.0

-0.5






-0.5

0.3

-1.0





-1.0

-0.2

0.6

-1.5




-1.5

-0.7

0.1

1.0

-2.0



-2.0

-1.2

-0.4

0.4

1.2

-2.5


-2.5

-1.7

-0.9

-0.1

0.7

1.5

-3.0

-3.0

-2.2

-1.4

-0.6

0.2

1.0

1.8

-3.5

-2.7

-2.0

-1.2

-0.4

0.4

1.3

2.1

-4.0

-2.5

-1.7

-0.9

-0.1

0.7

1.5

2.3

-4.5

-2.2

-1.4

-0.7

0.1

1.0

1.8

2.6

-5.0

-2.0

-1.2

-0.4

0.4

1.2

2.0

2.8

-5.5

-1.7

-1.0

-0.2

0.6

1.4

2.2

3.1

-6.0

-1.5

-0.7

0.1

0.9

1.7

2.5

3.3

-6.5

-1.3

-0.5

0.3

1.1

1.9

2.7

3.5

-7.0

-1.1

-0.3

0.5

1.3

2.1

2.9

3.7

-7.5

-0.9

-0.1

0.7

1.5

2.3

3.1

3.9

-8.0

-0.7

0.1

0.9

1.7

2.5

3.3

4.1

-8.5

-0.5

0.3

1.1

1.9

2.7

3.5

4.3

-9.0

-0.3

0.5

1.3

2.1

2.9

3.7

4.5

-9.5

-0.1

0.7

1.5

2.2

3.1

3.9

4.7

-10.0

0.1

0.8

1.6

2.4

3.2

4.0

4.9

NOTE: Select a wet-bulb temperature that is above the critical damage temperature for your crop and locate the appropriate column. Then choose the row with the correct dew-point temperature and read the corresponding air temperature from the table to turn your sprinklers on or off. This table is for mean sea level, which should be reasonably accurate up to about 500 m elevation.

TABLE 7.6
Dew-point temperature (°C) corresponding to air temperature and relative humidity*

RELATIVE HUMIDITY

AIR TEMPERATURE

%

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

100

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

90

-3.4

-1.4

0.5

2.5

4.5

6.5

8.4

10.4

80

-5.0

-3.0

-1.1

0.9

2.8

4.8

6.7

8.7

70

-6.7

-4.8

-2.9

-1.0

1.0

2.9

4.8

6.7

60

-8.7

-6.8

-4.9

-3.0

-1.2

0.7

2.6

4.5

50

-11.0

-9.2

-7.3

-5.5

-3.6

-1.8

0.1

1.9

40

-13.8

-12.0

-10.2

-8.4

-6.6

-4.8

-3.0

-1.2

30

-17.2

-15.5

-13.7

-12.0

-10.2

-8.5

-6.8

-5.0

20

-21.9

-20.2

-18.6

-16.9

-15.2

-13.6

-11.9

-10.2

10

-29.5

-27.9

-26.4

-24.8

-23.3

-21.7

-20.2

-18.6

NOTE: Select a relative humidity in the left column and an air temperature from the top row. Then find the corresponding dew-point temperature in the table.

When using a frost alarm, set the alarm about 1 °C higher than the starting air temperature identified in Table 7.5 to ensure sufficient time to start the sprinklers. If the sprinkler starting is automated with a thermostat, the starting temperature should be set 1 °C or 2 °C higher than the starting air temperature from Table 7.5, depending on thermostat accuracy.

If only the relative humidity and air temperature are available, then use Table 7.6 to estimate the dew-point temperature. Then use the dew-point temperature and the selected wet-bulb temperature corresponding to the critical damage temperature to decide the air temperature to start and stop sprinklers.

For those who prefer to use equations to estimate the start and stop air temperatures, the vapour pressure (ed in kPa) at the dew-point temperature (Td in °C) is estimated from the wet bulb temperature (Tw in °C) as:

Eq. 7.2

where the saturation vapour pressure at the wet-bulb temperature is:

Eq. 7.3

and the barometric pressure (Pb) as a function of elevation (EL) in metres is:

Eq. 7.4

Therefore, the corresponding air temperature (Ta) can be calculated as:

Eq. 7.5

where the saturation vapour pressure at the dew-point temperature is:

Eq. 7.6

Application rate requirements

Application rate requirements for over-plant sprinkling with conventional sprinklers depend on the rotation rate, wind speed and unprotected minimum 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 (RA) is given by:

Eq. 7.7

where u (m s-1) is the wind speed and Tl (°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:

Eq. 7.8

for 0.1 £ u £ 5 m s-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 (Ta) in °C is given by:

Eq. 7.9

For practical purposes, the wind speed entered into equation 7.9 should fall between 0.5 and 5 m s-1. An additional application amount should be added to the result from Equation 7.10 to ensure good wetting of the leaves. The additional amount varies from approximately 0 mm h-1 for sprinkler systems with uniform coverage over a sparse crop canopy to 2.0 mm h-1 for canopies with dense foliage and/or with less uniform sprinkler coverage.

The application rates generated by Equation 7.9, with the corrections to ensure adequate wetting, fall in the range of application rates recommended for tall crops in Figure 7.12. The values from Tables 7.7 and 7.8 for tall grapevines also fall within the range of application rates in Figure 7.12. Application rates are less for short crops because there is less surface area to cover, there tends to be less evaporation and it is easier to obtain uniform wetting of the vegetation when it is shorter (Figure 7.13). The rates in Figure 7.13 are typical for strawberries and slightly higher rates are applied to potatoes and tomatoes. Other intermediate sized crops have rates between those shown in Figures 7.12 and 7.13.

TABLE 7.7
Application rates for overhead sprinklers protection of grapevines based on minimum temperature and rotation rate for wind speeds of 0.0 to 0.5 m s-1 (after Schultz and Lider, 1968)

TEMPERATURE

30 s ROTATION

60 s ROTATION

30 s ROTATION

60 s ROTATION

°C

mm h-1

mm h-1

litre min-1 ha-1

litre min-1 ha-1

-1.7

2.0

2.5

333

417

-3.3

2.8

3.3

467

550

-5.0

3.8

4.3

633

717

TABLE 7.8
Application rates for overhead sprinklers protection of grapevines based on minimum temperature and rotation rate for wind speeds of 0.9 to 1.4 m s-1 (after Schultz and Lider, 1968)

TEMPERATURE

30 s ROTATION

60 s ROTATION

30 s ROTATION

60 s ROTATION

°C

mm h-1

mm h-1

litre min-1 ha-1

litre min-1 ha-1

-1.7

2.5

3.0

417

500

-3.3

3.3

3.8

550

633

-5.0

4.6

5.1

767

850

FIGURE 7.12
Tall Crops

Over-plant conventional sprinkler application rate requirements for frost protection of tall crops with head rotation rates of 30 s (horizontal hatching) and 60 s (vertical hatching). Wind speed ranges from 0.0 m s-1 at the bottom to 2.5 m s-1 at the top.

FIGURE 7.13
Short Crops

Over-plant conventional sprinkler application rate requirements for frost protection of short crops with head rotation rates of 30 s (horizontal hatching) and 60 s (vertical hatching). Wind speed ranges from 0.0 m s-1 at the bottom to 2.5 m s-1 at the top.

The effectiveness of sprinklers depends mainly on the evaporation rate, which is strongly influenced by wind speed. However, the minimum temperature is an indication of a deficit of sensible heat in the air, so a higher application rate is also needed if the minimum temperature is low. If there is a clear liquid-ice mixture coating the plants and water is dripping off the ice, then the application rate is sufficient to prevent damage. If all of the water freezes and it has a milky white appearance like rime ice, then the application rate is too low for the weather conditions. If the application rate is insufficient to adequately cover all of the foliage, then damage can occur on plant parts that are not adequately wetted. For example, trees could suffer little damage on upper branches while damage occurs on lower branches where the buds, blossoms, fruit or nuts were not adequately wetted. As conditions worsen, more damage will occur. Under windy, high evaporation conditions, inadequate application rates can cause more damage than if the sprinklers are not used.

Variable rate sprinklers

For most growers, the selection of a sprinkler precipitation rate is made once and it cannot be easily changed after the system is installed. Most systems are designed to apply the amount needed for the worst conditions in the region. This leads to over-application on nights when conditions are less severe. To overcome this problem some growers design systems with changeable riser heads to permit higher or lower application rates. In addition, variable rate sprinklers, which are switched off and on, have been studied extensively (Gerber and Martsolf, 1979; Proebsting, 1975; Hamer, 1980) as a method to reduce application rates. For example, by using an automated variable rate sprinkler system, Hamer (1980) obtained efficient protection during a frost night using only half of the normal amount of water. Water was applied whenever the temperature of an electronic sensor placed in the orchard to mimic a plant bud fell to -1°C. However, he noted that, due to non-uniform application, placement of the temperature sensor was critical. Also, at the end of long periods of frost protection, ice accumulation slowed the temperature response of the sensor and led to excessive water applications. Kalma et al. (1992) noted that, rather than measuring the temperature of an ice-coated sensor, NZAEI (1987) pulsed sprinklers for one minute on and one minute off whenever the minimum temperature measured by an exposed sensor at 1.0 m in an unprotected area was above -2.0 °C and operated the sprinklers continuously for lower temperatures. This resulted in 18 percent water savings during one season. A model to predict the application requirements for variable rate (i.e. pulsed) sprinkler systems is reported in Kalma et al. (1992).

A recent paper by Koc et al. (2000) reported that up to 75 percent water savings was achieved by cycling water off and on with solenoids during over-tree sprinkling for frost protection of an apple orchard. Environmental parameters and bud temperatures were used to model the on and off periods.

Low-volume (targeted) sprinklers

Use of one over-plant micro-sprinkler per tree was reported to provide good protection with less water use in the southeastern USA (Powell and Himelrick, 2000). However, they noted that the installation costs are high and the method has not been widely accepted by growers. Evans (2000) reported that one over-plant microsprinkler per tree will reduce the application rate requirement from between 3.8 and 4.6 mm h-1 for conventional sprinklers to between 2.8 and 3.1 mm h-1 for the surface area covered by trees. However, under windy conditions, rates as high as 5.6 mm h-1 might be needed to protect orchards.

Jorgensen et al. (1996) investigated the use of targeted over-plant microsprinklers for frost protection of grape vineyards. They evaluated a pulsing action that produces large diameter droplet sizes while maintaining lower application rates, compared with those found in conventional microsprinkler designs. The microsprinkler applies a band of water approximately 0.6 m wide targeted over the cordon of the vine. Microsprinklers were installed in every vine row about half a metre above the cordon on every second trellis stake, with approximately 3.6 m between heads. The targeted system was compared to a conventional impact sprinkler system with 15.6 m by 12.8 m spacing using 2.78 mm diameter nozzles. The targeted system had an 80 percent water savings; however, there were no severe frost events during the two-year experiment.

Targeted sprinklers were used to protect grapevines at a higher elevation site (820 m) in northern California (USA) and the results were promising. In that location, there was a shortage of water, which forced the grower to look for an alternative to conventional over-plant sprinklers. The low-volume system applied approximately 140 litre min-1 ha-1 as compared with the grower's conventional system application of 515 to 560 litre min-1 ha-1. In the first year of the trial, the lowest temperature observed was -3.9 °C, but no difference in crop loads or pruning weights between the low-volume and the conventional protection blocks were observed. In the second year, air temperatures fell as low as -5.8 °C on one night, which was low enough for some of the impact sprinkler heads to freeze up and stop turning. Although there was considerable ice loading, the grower observed that the frost damage losses were similar in both the conventional and low-volume sprinkler blocks. The low-volume sprinkler system was designed to spray water directly onto the vine rows and little was applied to the ground between rows. The grower pointed out that it was important to orient the non-rotating sprinkler heads to obtain a uniform coverage of the vine rows. It was also important to start the sprinklers when the wet-bulb temperature was above 0 °C and not to stop until the wet-bulb temperature rose above 0 °C again.

Sprinkling over coverings

Sprinkling over covered crops in greenhouses and frames provides considerable protection. Like over-plant sprinkling, continuous application of sufficient water to plant covers keeps the covers at near 0 °C. The thin layer of water intercepts upward terrestrial radiation and radiates downward at a temperature near 0 °C, which is considerably higher than the apparent sky temperature. As a result the net radiation on the plant canopy is considerably higher than a canopy exposed to the clear sky. Hogg (1964), in a two-year trial, reported average protection of 2.4 °C using sprinkling irrigation over a Dutch frame (i.e. with a glass cover). During colder nights, the protection was closer to 4.5 °C. However, the precipitation rate of 7.3 mm h-1 was high. The use of sprinklers on greenhouses with 0.2 mm thick plastic covers maintained temperatures inside up to 7.1 °C higher than the subzero temperatures that were registered outside (Pergola, Ranieri and Grassotti, 1983). Relative to an identical plastic greenhouse that was heated to the same temperature difference, the sprinklers saved up to 80 percent in energy costs. The sprinklers operated intermittently and the average precipitation rates on the colder nights were near 10 mm h-1, which is a high rate. However, more research is needed to determine if lower precipitation rates are possible and to study the effects of water quality on the plastic. Because there is less surface area to cover, the precipitation rate should be similar or possibly less than that used over tall crop canopies. However, this needs further study. The use of sprinklers over plastic greenhouses has also been used in southern Portugal with positive results (Abreu, 1985).

Under-plant sprinklers

Under-tree sprinklers are commonly used for frost protection of deciduous tree crops in regions where the minimum temperatures are not too low and only a few degrees of protection are needed. In addition to the low operational cost, one can also use the system for irrigation, with fewer disease problems and lower cost, so it has several advantages relative to over-plant sprinklers. Also, limb breakage due to ice loading and sprinkler system failure are not a problem with under-plant sprinkler systems. Lower application rates (2.0 to 3.0 mm h-1) are needed for under-plant sprinkler systems. The protection afforded depends on severity of the frost night and the application rate. For example, Anconelli et al. (2002) found little benefit difference between application rates and sprinkler head types for minimum temperatures above -3 °C. Below -3 °C, an outflow of 65 litre h-1 per tree gave better performance than 45 litre h-1.

When under-plant sprinklers are used, the main goal is to maintain the wetted surface temperature at near 0 °C. Protection derives partly from increased radiation from the liquid-ice covered surface, which is warmer than an unprotected surface. In an unprotected orchard, the air temperature is generally coldest (i.e. often well below 0 °C) near the surface and increases with height. Because sprinkler operation increases the surface temperature to near 0 °C, the air near the surface is also warmer than in an unprotected crop. The warmed air near the surface creates atmospheric instability near the ground and causes upward sensible heat flux to warm the air and plants. In addition, the water vapour content of air in the orchard is increased by the sprinkler operation and condensation or deposition of ice on the cold plant surfaces will release some latent heat and provide protection.

The effectiveness of the sprinklers again depends on the evaporation rate, which increases with wind speed. The best way to test your system is to operate the sprinklers during various freezing conditions during dormancy to identify conditions when all of the water freezes. If the soil is covered with a liquid-ice mixture and the surface temperature is at 0 °C, the application rate is adequate. If all the water freezes and the surface temperature falls below 0 °C, then the application rate is too low for those conditions. Care should be taken to avoid wetting the lower branches of the trees.

Conventional rotating sprinklers

Perry (1994) suggested that temperature rises of between 0.5 °C and 1.7 °C up to a height of about 3.6 m are expected during a typical radiation frost when using rotating under-plant sprinklers. Evans (2000) indicates that temperature increases up to about 1.7 °C are possible at 2.0 m height in an orchard protected with cold water. Connell and Snyder (1988) reported an increase of about 2 °C at 2.0 m height in an almond orchard protected with a gear-driven rotating sprinkler head system rather than impact sprinklers. Water temperature from the sprinkler heads was about 20 °C and the application rate was 2.0 mm h-1. Typical under-plant sprinkler systems use 2.0 to 2.4 mm diameter, low-trajectory sprinkler heads with 276 to 345 kPa of pressure and application rates between 2.0 and 3.0 mm h-1.

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 (Tn > -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.

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 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.

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 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. Protection 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.

For example, furrows are needed on the edges of citrus tree rows so that air warmed by the furrow water transfers upwards along the tree edges rather than under the trees where the air is already warmer or in the middles between rows where the air rises without intercepting the trees. For deciduous trees, the water should run under the canopy, where the warmed air will transfer upwards to warm buds, flowers, fruit or nuts. The furrows should be wide so that they present a greater surface area of water. The energy emitted is in W m-2, so increasing the width of the furrows provides a larger surface area to radiate energy and to heat the air.

Furrow irrigation should be started early enough so that the water reaches the end of the field before the air temperature falls to the critical damage temperature. Water at 20 °C will radiate 419 W m-2 of energy, whereas water or ice at 0 °C radiates 316 W m-2 of energy. Also, the warmer water will transfer more heat to the nearby air, which will transfer vertically into the plant canopy. Ice formation on the water surface insulates the transfer of heat from the water and reduces protection. With a higher flow rate, the ice formation will occur further down the row (Figure 7.14), so high application rates afford better protection. Cold runoff water should not be recirculated. Heating the water is definitely beneficial for protection; however, heating may or may not be cost-effective. It depends on capital, energy and labour costs compared with the potential crop value.

FIGURE 7.14
Upward long-wave radiation (W m-2) from furrow-irrigation water while it cools and freezes as it flows down a field during a radiation frost night. In the upper figure, the water cools more rapidly and ice forms closer to the inlet

Foam insulation

Application of foam insulation to low growing crops for frost protection was widely studied mostly in North America and it has been shown to increase the minimum temperature by as much as 12 °C (Braud, Chesness and Hawthorne, 1968). However, the method has not been widely adopted by growers because of the cost of materials and labour, as well as problems with covering large areas in short times due to inaccuracy of frost forecasts (Bartholic, 1979). Foam is made from a variety of materials, but it is mostly air, which provides the insulation properties. When applied, the foam prevents radiation losses from the plants and traps energy conducted upwards from the soil. Protection is best on the first night and it decreases with time because the foam also blocks energy from warming the plants and soil during the day and it breaks down over time. Mixing air and liquid materials in the right proportion to create many small bubbles is the secret to generating foam with low thermal conductivity. Several methods to produce foam and apply it are reported in Bartholic (1979). However, Bartholic (1979) notes that growers show an interest after experiencing frost damage, but they rarely adopt the use of foam in the long term. Recently, Krasovitski et al. (1999) have reported on thermal properties of foams and methods of application.

Foggers

Natural fog is known to provide protection against freezing, so artificial fogs have also been studied as possible methods against frost damage. Fog lines (Figure 7.15) that use high-pressure lines and special nozzles to make small (i.e. 10 to 20 mm diameter) fog droplets have been reported to provide good protection under calm wind conditions (Mee and Bartholic, 1979). Protection comes mainly from the water droplets absorbing long-wave radiation from the surface and re-emitting downward long-wave radiation at the water droplet temperature, which is considerably higher than apparent clear sky temperature. The water droplets should have diameters about 8 mm to optimize the absorption of radiation and to prevent the water droplets from dropping to the ground. A fairly dense cloud of thick fog that completely covers the crop is necessary for protection. This depends on the presence of light wind and relatively high humidity. For example, Brewer, Burns and Opitz (1974) and Itier, Huber and Brun (1987) found difficulties with the production of sufficient water droplets and with wind drift. Mee and Bartholic (1979) reported that the Mee foggers have energy use requirements that are less than 1 percent of heaters, about 10 percent of wind machines and about 20 percent of sprinklers. They also reported better protection under some conditions than from use of heaters.

The capital cost for line fogger systems is high, but the operational costs are low. However, based on personal communications with growers and researchers who have tested line foggers in locations with moderate to severe frosts, the fog prevented trees from being killed but it did not save the crop. Therefore, line foggers should only be used for protection against mild frost events. In addition, fog drift can be hazardous, so foggers should not be used in locations where car traffic is present.

FIGURE 7.15
An artificial line fogger system operating in a California almond orchard

Natural fogs that were created by vaporizing water with jet engines have been observed to provide protection. Fog created by the Gill saturated vapour (SV) gun (Figure 7.16) is considered a natural rather than an artificial fog. The SV gun adds water vapour to the air until it becomes saturated and causes fog to form. The jet engine approach has the advantage that it can be moved to the upwind side of the crop to be protected. Therefore, the capital cost of a SV gun is considerably less than for a line fogger system. However, because it has a jet engine, noise is a serious problem. Also, the same problem with fog drift exists, so the SV gun should not be used where there is car traffic. Operation of the machine is somewhat complicated and results from field trials have been mixed.

FIGURE 7.16
A Gill saturation vapour gun for natural fog generation

Combination methods

Wind machines and under-plant sprinklers

Under-plant sprinklers with low trajectory angles can be used in conjunction with wind machines for frost protection. In addition to heat supplied by the water droplets as they fly from the sprinkler heads to the ground, freezing water on the ground releases latent heat and warms air near the surface. While this warmed air will naturally transfer throughout the crop, operating wind machines with the sprinklers will enhance heat and water vapour transfer within the mixed layer to the air and plants. Typically, growers start the lower cost sprinklers first and then turn on the wind machines if more protection is needed. Unlike using heaters with wind machines, the sprinkler heads near the wind machine can be left operating. Evans (2000) reports that the combined use of wind machines and water can double the benefit of using either method alone. Also, he notes that the combination of method reduces the water requirement. Because operating wind machines artificially increases the wind speed, evaporation rates are higher. Consequently, the combination of wind machines and over-plant sprinklers is likely to be detrimental for frost protection and should not be used.

Wind machines and surface irrigation

The combination of wind machines and surface irrigation is widely practiced in California and other locations in North America, especially in citrus orchards. Growers typically start with the surface water and turn on the wind machines later to supplement protection when needed. As with under-plant sprinklers, the wind machines facilitate the transfer to the air and trees of heat and water vapour released from the water within the mixed layer. It is well known by growers that the combination of wind machines and surface water improves frost protection. However, the additional amount of protection afforded is unknown.

Wind machines and heaters

The combination of wind machines and heaters is known to improve frost protection over either of the methods alone (Martsolf, 1979a). In fact, Brooks (1960) reported that a wind machine and 50 heaters per hectare was roughly equal to 133 heaters per hectare alone. In California, the combination of methods was found to be 53 percent, 39 percent and 0 percent cheaper in years with 100, 50 and 10 hours of protection, respectively. In California, the combination has protected citrus orchards to temperatures as low as -5 °C and only half as many heaters are needed when the two methods are combined. A typical system has a 74.5 kW wind machine with about 37 evenly spaced stack heaters per hectare, with no heaters within 30 m of the wind machine (Angus, 1962). Many efforts to use wind machines to distribute supplemental heat through or nearby the fans have failed. Fossil fuel heaters placed too close to the fans cause buoyant lifting and decrease wind machine effectiveness. Because the fan operation tends to draw in cold air near the ground on the outside edge of the protected area, placing heaters on the outside edge warms the influx of cold air. Placing about half as many heaters (25 to 50 ha-1) with each burning oil at a rate of 2.8 litre h-1 on the periphery of the area protected by a wind machine saves as much 90 percent of the heater fuel over the season and improves frost protection because the heaters are not used on many mild frost nights (Evans, 2000). The heaters can be spaced between every second tree on the outside edge of the orchard and widely spaced within the area affected by each wind machine. The concentration should be a little higher on the upwind side of the orchard. No heaters are needed within about 50 m of the wind machine and the wind machines are started first. If the temperature continues to fall, the heaters are then lit.

Sprinklers and heaters

Although no research literature was found on the use of sprinklers and heaters in combination, Martsolf (1979b) reported successful use of the combination by a grower in Pennsylvania, USA. The grower had designed a cover (i.e. a round metal snow sled mounted horizontally on a pole at about 1.5 m above the heater) to prevent water from extinguishing the heater. The grower would start the heaters first and would only start the sprinklers if the air temperature fell too low. This combination reduced ice accumulation on the plants and sometimes the sprinklers were not needed. Whether water hitting the heater caused a reduction in heat generation or if it enhanced vaporization and beneficial fog formation was unknown.


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