Sprinkler Irrigation Frost Protection Systems

"Over Tree" Sprinkling Systems

The over tree system can be used for irrigation, frost protection and crop cooling. The over tree system provides the maximum potential protection and providing the system is designed properly, can provide some protection with advective freeze events. Protection can be 6°C or greater depending upon conditions. The over tree system does not rely on inversion layer strength for its protection capabilities.

Applying the water over the top of the crop provides the highest level of protection as long as the water precipitation rates are sufficient and the uniformity of application is high. There is no perfect method to protect a crop against a freeze event and at the same time the capacity of the freeze protection system exceeded. The best frost protection techniques are an important part of a complete frost protection system.

The concept of dew point, which relates to relative air humidity, is critical to over tree sprinkling systems and not very important to under tree systems. The dew point is the temperature as air cools at which condensation of water vapour in the air begins to occur and this temperature can be either above or below freezing. Approximately 7.5 times as much heat is given off when water condenses as when water freezes and approximately 7.5 times as much heat is absorbed when water evaporates as heat is given off when water freezes.

If the dew point is above freezing, significant heat is given off when water condenses, potentially eliminating the need for frost protection by sprinkling. However, if the dew point is below freezing, evaporation will occur when the over tree system is started, potentially lowering the temperature below the critical sensitive tissue temperature. Under this condition, the system must be started at a temperature such that the heat absorbed by the evaporating water does not lower the temperature below the critical sensitive plant tissue temperature. The dew point can be approximated (usually slightly lower) by the wet bulb temperature, which is measure by a standard thermometer whose bulb is covered by a muslin sleeve that has been moistened by pure water and swung through the air.

The basic principle of overhead sprinkler system is based on three factors:

1. When water freezes its latent heat is released, this latent heat keeps the temperature of a plant from dropping below the freezing point.
2. A mixture of ice and water exposed to below freezing point remains at 0°C until all the water is frozen.
3. Plants do not suffer frost damage until the temperature drops slightly below 0°C because the freezing point of the plant liquid is below that of water.

With sprinkler irrigation, if either crop (or its coating of ice) is kept wet, the temperature of the plant will not fall below 0°C so protecting it from frost damage even though the air temperature around the crop is well below freezing point. This method of frost protection is continued until the temperature of the surrounding air has risen above 0 degrees Celsius and all ice formations on the plant have melted.

Given that a sensitive crop may be destroyed by frost in a single night, protection against frost damage using sprinklers is proving to be an efficient method for an increasing number of growers. Correct design of the systems is however, critical and growers should seek specialised advice before any installation begins.

Contact us now and we will provide the most cost effective solution for your frost protection needs.