Planning For A Freeze
By far, the most widely used method of cold protection in Florida is the application of water. When using water for cold protection, growers must operate their irrigation systems to keep their crops from being damaged, while at the same time minimizing water use.
The Florida Automated Weather Network (FAWN) is a statewide system of 35 automated weather-observing sites — located from Homestead to Jay (near Pensacola) — that collect data every 15 minutes. The data, along with several calculated products and weather-related tools, are delivered to the public via the Internet and can also be retrieved via a telephone voice message system. FAWN has taken real-time weather information and developed the Cold Protection Toolkit — a step-by-step decision-making guide for using water for cold protection.
Inherent in every crop is a temperature at which cold air will begin to cause damage. So, the first step in the cold protection process is to determine the critical temperature for a given crop. FAWN’s online guide, Determining Critical Freezing Temperatures For Plants In Florida, provides assistance with determining the critical temperature for various crops in the state.
Next, the Fruit Frost Station Forecast tool provides a big-picture overview of whether cold protection may be needed during the upcoming week by retrieving a seven-day forecast for 180 of the prior National Weather Service (NWS) Fruit Frost Forecast locations.
On a night when critical temperatures are possible, the Forecast Tracker displays the NWS forecasted temperature and the observed temperature at any FAWN site on a 48-hour graph; a critical temperature can also be displayed as well.
The Minimum Overnight Temperature tool further evaluates the likelihood of the forecasted temperature occurring using the Brunt equation to estimate the minimum overnight temperature at each FAWN site. The sunset air and dew point temperatures can also be manually submitted for an estimate at the user’s location.
There is always a risk of further cooling due to evaporation when using irrigation systems for cold protection. The Evaporative Cooling Potential tool categorizes the evaporative cooling potential at each FAWN site.
Finally, the Wet-Bulb Based Irrigation Cutoff Temperature tool — which utilizes the basic cold-protection recommendation to discontinue irrigation when the wet bulb temperature reaches the critical temperature of the crop being protected — calculates an air temperature at which the wet bulb and your critical temperature would be equal. Air temperature, dew point temperature, and wind speed can also be manually submitted to determine the cutoff temperature at any location.
It has been estimated that use of these tools can save two hours of irrigation per cold event. During a relatively warm winter, one to three nights may require cold protection for a total savings of two to six irrigation hours. A cold winter, however, may produce four to 10 nights requiring cold protection for a savings of eight to 20 hours. Depending on the number of nights that need protection, billions of gallons of water and millions of dollars can potentially be saved by using FAWN Cold Protection tools.
The FAWN Cold Protection Toolkit provides step-by-step guidance for using water for cold protection by first helping users find their critical temperature, determining generally which upcoming nights might require cold protection, then guidance during individual cold events for when to start and stop irrigation. To use the FAWN Cold Protection Toolkit, visit http://fawn.ifas.ufl.edu, select Tools, and then Cold Protection Toolkit.
FAWN thanks the University of Florida/Institute of Food and Agricultural Sciences for financial support and infrastructure; the Florida Department of Agriculture and Consumer Services, South Florida Water Management District, Southwest Florida Water Management District, and St. Johns River Water Management District for their ongoing financial support.