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Why Do We Irrigate When there is Potential for Frost?


Freeze Protection in Almond Orchard

By Ben Smith
Hortau Director of Grower Support

If only Mother Nature were predictable. One thing that is predictable is that as soon as bloom emerges and cold hits, there will be a lot of tired farmers out all night tending irrigation to protect their tender crop blooms from icy temperatures.

From a strictly need standpoint these irrigations can easily cause excessively wet, potentially harmful soils.  So, why do we always turn on the water as soon as the needle drops?

In much of the U.S., sensitive crops are set up with irrigation systems placed over the top of the crop that rain water down on top of the sensitive blooms.  The icy weather and steady application of water results in frighteningly beautiful trees and bushes encapsulated in ice.  It seems contrary to logic that covering the blooms in ice would save them, however, in order for water to turn into ice, it must release the heat that keeps it liquid.  The continued application of new water that continually freezes and releases heat keeps the surrounding area at a relatively safe 32 degrees.  Typically, damage does not occur to crops until the temperature drops below 30 degrees.  Thus, the blooms are kept safe.

Frost Protection AlmondsThis is very effective as long as the ambient temperature doesn’t drop too low, and as long as there is an overhead irrigation system in place that can continually sprinkler water on the crop to supply the constant phase conversion.  However, frost events are uncommon enough in California to warrant the cost of such a system, so why do we turn the sprinklers on for frost here?  It really comes down to two reasons: 1) Increased transmission of heat in darker materials and 2) The thermal energy storage potential of water.

  1. Black trucks, black shirts, asphalt roads. What do all of these things have in common?  They are all miserable to have in the summer.  Black loves heat.  The darker a material, the more effective it is at taking in heat.  Dark materials don’t just like heat, they love it, they absorb it and store it.  Black is the most effective color at capturing thermal radiation.  Naturally, a wet soil, is a darker soil, being darker, it is much more effective at capturing heat than is a light-colored soil.  As such it can capture the heat during the day and release it at night when it is needed.
  2. Water has a very high latent heat of vaporization. In other words, it takes a lot of energy to convert water from a liquid to a gas.  Because of this, water can hold a large amount of heat.  Think about this, whenever water, in liquid form, is sprinkled onto the field, it contains heat.  Even if it is water that would make you turn blue in the shower, it still isn’t frozen and so it holds heat.  This is the first reason we irrigate fields when there is potential frost, the water we put onto the field has some heat to it, and as there is a gradient (a difference) between the water and the cold air, that heat will pass from the water into the air and will slightly warm the air.

It doesn’t stop there however, when we irrigate (especially in cold dry air) some water will inevitably evaporate and increase the humidity in the surrounding area.  Even water in gaseous form has a tremendous ability to hold heat, as such it gives more “bulk” to the air, requiring a larger exchange of heat energy to cause a change in temperature.  In other words, humidity in the air acts as a buffer and slows the speed at which the temperature can change.

Just as humid, you could even call it, wet, air holds more heat, wet soil also holds more heat.  Not just because a dark soil is more effective at trapping heat, as mentioned above, but also because the water increases its ability to store heat.  A wet soil can store much more heat than a dry soil.  As soon as the sun goes down and the temperature follows, a soil wet all day in the sun can then release the heat and help to warm the air in the field.

Irrigating During FreezeThere are many intricacies involved in frost protection that alter the effectiveness of every frost protection method and alone none of these methods do much to make a large change.  However, the potential for damage to a crop is measured by individual degrees and individual minutes, so even a small change can prevent or lessen the total damage.

At the very least the next time you are out in the wee hours of morning driving from field to field watching the temperature gauge, you can consider the incredible properties of water and know the difference you are making.

In a cruel mashup between Star Wars and the Magic School Bus: “May the Latent Heat of Vaporization be with you.”

ABOUT THE AUTHOR

Ben Smith provides grower support in California’s Central Valley for Hortau. He grew up on a small dairy farm in northern Utah, where he spent most of his summers in the alfalfa and corn fields shoveling dirt, desperately trying to get the water to flood the fields evenly. He knew that there had to be a better way.

Ben went to Utah State University and received a B.S. degree in Crop Science with a minor in Soil Science. He has an extensive background working for irrigation equipment manufacturers and in large-scale production agriculture  installing irrigation systems, maintaining existing systems, and scheduling irrigations over large acreages.

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