Sprinklers and micro-sprinklers

Under tree sprinklers provide protection because heat contained in water is released into the orchard system. As water cools and freezes, it releases a great deal of latent heat. This sensible heat is radiated and/or convected into the trees, thus providing protection. Sprinklers can be safely turned off when the wet bulb temperature upwind of the protected orchard is above the critical damage temperature for the stage of the crop or when all the ice melts.

In some orchards, frost protection is limited by the amount of water or pipe available. To learn more about moveable pipe placement we ran an experiment comparing protection with sprinkler lines in every middle, every other middle or every fourth middle. Air temperatures in all sprinkled areas were 1 to 2degrees warmer than the unsprinkled control and there were no differences between these spacings. Soil surface temperatures were colder the further from the sprinklers with the dry centers between the lines in every fourth middle as cold as the unsprinkled control. Line spacing directly affects soil surface temperature but air movement evens out the benefits. Without some air movement, protection will fail between widely spaced pipelines. Limited amounts of water may provide some protection from mild frosts but protection will fail as frosts become more severe.

In our experiments with micro-sprinklers, applying 15, 25, and 40 gallons per minute per acre resulted in little difference in observed air temperatures. However, exposed temperatures were 1 to 2 degrees warmer at the higher water rates. Exposed temperature is what the buds experience as they are exposed to radiation from the orchard floor. The fact that the low water application gave a lower exposed temperature indicates that protection with under tree micro-sprinklers is coming mostly from direct radiation from the warmer wet spots under the trees rather than through convection of warmer air. We found a greater separation in exposed temperatures between the low and medium/high rates on the colder nights. Thus, micro-sprinkler application rate had little effect on air temperature but did affect the temperature of exposed buds and flowers. The low application rate gave less protection than the higher rates and the higher soil surface temperatures from higher application rates led to more radiation heating. Under windy advection freeze conditions this may be even more important since convection heating is negatively affected by wind but radiation is unaffected.

Drip irrigating in advance of a frost can help keep the orchard slightly warmer by increasing soil heat storage particularly if the soil surface is dry. Running the system during a frost may provide slight benefits due to radiation heating from the wetted area beneath the trees. Flood irrigation for frost protection works in a similar fashion but due to larger water volumes it will provide more protection.