Shutting down irrigation based on the wet bulb temperature can save a substantial amount of water and money!
The wet bulb temperature can be used to determine when it is safe to shut down irrigation systems used for cold protection. When the wet bulb temperature is 32°F or higher, the irrigation system can be stopped without danger to any part of the plant (Harrison, Gerber, and Choate, IFAS Circular 348).
There is always a risk when using irrigation systems - micro-sprinkler or conventional sprinkler - for cold/frost protection. Low humidity and wind can produce weather conditions that result in a wet bulb temperature - the lowest temperature to which air can be cooled by evaporating water into it - 5°F to 6°F lower than the air temperature. Therefore, plant surfaces that have been wetted by irrigation systems, and subsequently exposed to evaporation, can end up 5°F to 6°F cooler than the air temperature. This may result in plant damage if the wet bulb temperature is below the critical minimum temperature (CMT) for your particular crop. However, when the wet bulb temperature is equal to or above your CMT, there is no risk of cold-air damage due to evaporative cooling. Therefore, irrigation systems can be safely turned off when the wet bulb temperature reaches your CMT or higher.
The Wet Bulb-Based Irrigation Cutoff Temperature calculator calculates an air temperature at which the wet bulb and your CMT would be equal. It is based on a user-selected critical temperature (see Determining Critical Temperature for Freezing Plants in Florida for more information on critical temperature) and the difference between the observed air and wet bulb temperatures, and is calculated for each FAWN site.
Once the cutoff temperature has been reached and irrigation systems have been turned off, it is not necessary to continue irrigation until all ice has melted from the plant.
Below are links to UF Electronic Data Information Source (EDIS) searches for publications that may be useful for learning more about this topic
EDIS Search – Evaporative Cooling
EDIS Search – Wet Bulb Temperature