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Banner Sprinklers as defence against frost

Sprinklers as defence against frost

Climate experts differ in their opinions on the future incidence of frost events. There is uncertainty around what nature will demand from us in terms of climate control in intensive crop production.

We do know that frost damage to crops in sensitive growing phases is a real risk in certain areas. It is however a risk that can be mitigated using different frost mitigation methods.

Frost mitigation methods include heaters, big fans, thermal nets, and water sprinklers. This article will focus on frost mitigation using irrigation sprinklers.

The science of combating frost with water

Before we discuss how irrigation sprinklers can be applied in practice to protect crops from frost, let’s discuss the science behind using sprinkled water in frost mitigation.

Water, as with other substances, has three phases: gas, liquid and solid. In the case of water; ice, water, and water vapour.

To understand the potential of using sprinkled water for frost mitigation, we need to understand the concept of latent heat. This is the heat or energy released or absorbed when substances change from one phase to another.

When water changes from its liquid form to ice, its temperature remains the same. However, energy is released and the surrounding temperature increases. This is latent heat.

When we move in the other direction, when water goes from its liquid form to water vapour, energy is required and therefore taken from the environment and the surrounding temperature decreases. This is evaporative cooling.

The temperature effect of water’s phase changes in both directions.

So, the essence of using water to mitigate the effect of frost, is that the surroundings temperature increases when water turns to ice and decreases when water turns to vapour. We want the former, but not the latter.

When we use latent heat as a tool in warming the plant’s surroundings in order to mitigate the risk of frost, water is sprinkled onto the plant. This water forms a layer of ice around the plant part. It is important to understand that it is the heating effect from the formation of the ice, as explained above, that increases the temperature of the plant and prevents frost damage.

If we understand this, we know that it is important to continuously sprinkle water onto the plant so that ice is continuously formed and latent heat continuously released. As long as water is continuously sprinkled onto the plant, keeping the ice wet and new ice forming, the temperature of the plant will be greater than 0°C.

We must however give a warning. Fact is that the cooling effect of water turning to vapour is much greater than the heating effect of water turning to ice. The energy involved in the cooling process is seven times more than the energy used in the heating process in this regard.

This warning underlines the importance of water being sprinkled continuously during the frost event. In fact, once you start you cannot stop. If ice is formed around the plant and new ice is not continuously formed, the ice can revert back. Ice will turn to water, and water into water vapour. Worse still, ice can even revert to water vapour, without going through a liquid phase. The resultant evaporative cooling will cause a strong temperature drop in the plant’s surroundings which we cannot afford.

Finding an efficient solution

If the ambient air temperature in the orchard does not drop below -5°C, a water application rate of 3 mm/hour will be successful in ensuring continuous water sprinkling, continuous ice formation and keeping the plant’s temperature above 0°C. So, when we design a frost mitigation system using irrigation sprinklers, we design it to deliver 3 mm/hour. As you would know, this is quite rapid water delivery.

There are generally four approaches to frost mitigation with different types of irrigation sprinklers. These are full coverage, localised coverage, high efficiency(circle) and high efficiency(strip). For all these approaches, sprinklers are placed above the canopy.

We will now continue to explain these approaches using Netafim sprinklers as examples. For the purpose of this exercise, we will use an orchard with 3-metre row spacing, 1-metre tree spacing, and a 1.2-metre canopy width.

For the full coverage approach, using a sprinkler such as MegaNet™, sprinkler spacing will be 10 metres by 9 metres. In this case, we wet 100% of the area, including between the tree rows where frost mitigation is not required. This option uses a lot of water, but not many sprinklers.

When we move on to the local coverage approach, using a sprinkler such as SuperNet™, the sprinkler spacing will be 5 metres by 3 metres. More sprinklers are used, but this is a more efficient option compared to full coverage as a lower flow rate per hectare is used for the same application rate. Only the crop itself is targeted, bmore than 30% water is saved and energy consumption is reduced in comparison to full coverage frost mitigation.

We then move on to even higher efficiencies. By adding pulses to the equation, a sprinkler such as GyroNet™ can be applied for frost mitigation. Normally, the minimum flow rate for a GyroNet™ sprinkler would be 27 ℓ/hour. This is the lowest rate at which water can be sent through the sprinkler and on standard operation can still be achieved. At a lower flow rate there will not be enough velocity and the sprinkler will not be able to throw the water at the necessary trajectory. By adding a pulse device, such as Netafim’s Pulsar™, the equipment cost is increased, but the minimum required flow rate is significantly decreased. Inside the pulsar device is a bag filled with air, as pressure builds the bag is squeezed and a pulse of water is released. This pulse increases the velocity, and the required water trajectory can be achieved at a flow rate as low as 8 ℓ/hour.

If the GyroNet™ and Pulsar™ combination is used, 3 metre by 3 metre sprinkler spacing is used. Sprinkler spacing is closer and more sprinklers are therefore used, but efficiency is even higher and the flow rate per hectare is even lower for the same application rate.

Lastly, one more step can be taken. If we change the sprinkler on top of the Pulsar™, to a device such as Netafim’s SpinNet™, an oblong wetted pattern is created rather than a circular wetted pattern. The sprinkler only wets in the tree row, where frost mitigation is required.

When this method is used, the sprinkler spacing can be 5 metres by 3 metres. We now have a wider sprinkler spacing than the previous method and the highest possible efficiency and lowest possible flow rate per hectare for the same application rate.

Let’s have a look at the following table to compare all aspects of these approaches.

 

Full Coverage

Localised Coverage

High Efficiency – Circle

High Efficiency - Strip

Sprinkler flow rate

350 ℓ/hour

50 ℓ/hour

20 ℓ/hour

30 ℓ/hour

Sprinkler spacing

10 x 9 metres

5 x 3 metres

3 x 3 metres

5 x 3 metres

Sprinklers per hectare

111

667

1111

667

Flow rate per hectare

39 ℓ/hour

33 ℓ/hour

22 ℓ/hour

20 ℓ/hour

 

Table2: Compared system requirements for 10 hectares.

 

Storage

Flow rate

Pump capacity

Frost protection – Full coverage

19 500 m3

390 m3/h

110 kW

Frost protection – High Efficiency

10 000 m3

200 m3/h

55 kW

Irrigation – drip or micro-sprinklers

64 000 m3

34 m3/h

11 kW

*These figures represent an apple orchard irrigation and frost mitigation system in Bethlehem, Free State.

When designing a frost mitigation system, there are certain factors to consider when selecting the approach and necessary irrigation equipment. This includes:

  • The distance between trees and diameter of the trees.
  • The tree top diameter during the frost season.
  • The total size of the area to be protected.
  • Water and energy availability, including storage and pumping capacity.
  • Site topography, considering its micro-climate areas.
  • The meteorological properties of the site.
  • The expected duration of a typical frost event in the area.

It is not only important to choose the right equipment and approach, but also to ensure that the resulting system can be optimally operated in the given conditions when it is required, for as long as it is required.

When is it go time?

Of course, a frost mitigation system can only be effective if it is turned on at the right time. The system must be activated as soon as temperatures fall below 2.5°C. This is usually around 1:00 or 2:00 in the morning. It is very important that the frost mitigation system is not turned off too early, the system most only be turned off when all the ice on the leaves and trees has melted on its own. Generally, one can expect to turn off the frost mitigation sprinkler system towards 10:00 or 11:00 in the morning.

Frost Mitigation Dos and Don’ts

  • Complete the system installation before the frost season begins.
  • Check the system shortly before an expected frost event.
  • Don’t delegate frost mitigation to someone else.
  • Be prepared. Every second counts.
  • Open the system on time (2.5°C).
  • Ensure water is applied continuously.
  • Don’t shut down the system too early.

Important Misconceptions

The coat of ice by itself is not a jacket that protects the plant. If it begins to melt, it can even cause more damage than if it were not there at all. It is ice formation, not ice by itself that protects the plant. It needs new water continuously to maintain ice formation.

The action of pulsing of the sprinklers as discussed in the high efficiency approaches in this article, has nothing to do with frost protection. The pulsing is purely a means to traject a low flow rate further than it would without the pulse.

The benefits of using sprinklers for frost mitigation:

  • More cost-effective than other alternatives.
  • Lower energy requirements than other methods.
  • Relatively lower infrastructure costs.
  • Can work with equal efficiency over the entire area simultaneously.
  • Can also be used for cooling trees in heat waves.
  • Offers ease of operation.