Irrigation Risk for FAWN Stations

Using a Critical Temperature of 34°F (change)

StationIrrigation
Risk
Tair
(F)
WetBulb
(F)
Windmax
(mph)
AlachuaN/A*71702
ApopkaN/A*76754
ArcadiaN/A*74735
AvalonN/A*73722
BalmN/A*74734
Belle GladeN/A*74743
BronsonN/A*72723
CarrabelleN/A*78762
CitraN/A*72723
ClewistonN/A*77765
Dade CityN/A*73734
DeFuniak SpringsN/A*73732
DoverN/A*73721
Fort LauderdaleN/A*77752
FrostproofN/A*74735
HastingsN/A*73721
HomesteadN/A*79777
ImmokaleeN/A*75754
Indian RiverN/A*73731
JayN/A*75752
JoshuaN/A*72723
KenansvilleN/A*73731
Lake AlfredN/A*74734
LecantoN/A*73724
Live OakN/A*877918
MacclennyN/A*71711
MariannaN/A*73732
MayoN/A*72722
MonticelloN/A*75733
North PortN/A*74733
OcklawahaN/A*73731
OkahumpkaN/A*76764
OkeechobeeN/A*74743
OnaN/A*73733
PalmdaleN/A*75754
PiersonN/A*72721
Putnam HallN/A*72721
QuincyN/A*73724
SebringN/A*73733
St. Lucie WestN/A*73732
UmatillaN/A*73732
WellingtonN/A*78764

*Note: 'N/A' for Irrigation Risk indicates that Tair was at least 10 degrees F above your critical temperature, or was at least 6 degrees above Twetbulb with little wind. In either case you should always use your best judgement to determine whether to irrigate.

Evaporative Cooling Potential

John Jackson, Phil Cross, & Nick Faryna

There is always a risk when using water systems, micro-sprinkler or conventional sprinkler, for cold and/or frost protection. Low humidity and wind can produce evaporative cooling which can chill plant surfaces to the wet bulb temperature. Dry and windy conditions can result in wet bulb temperatures 5F to 6F degrees lower than air temperature. Therefore, wetted plant surfaces that experience evaporation would be 5F to 6F degrees cooler than air temperature. Evaporative cooling may result in plant damage when water is used for cold protection during dry windy conditions. Evaporative cooling should always be taken into consideration.

It is possible that, on nights when temperatures are close to critical levels, introduction of water could produce more damage than would result if no action was taken!

The table below provides a guideline to help evaluate the risk of using water on a given night. The values and corresponding risk level are based on experience and observations over forty winters in Florida. The purpose of this chart is to encourage those using water to protect plants to take into consideration the possibility of causing more harm to the targeted plants by running the water than would occur if the irrigation system is not utilized. It should be noted that during severe freeze events, when the forecast is 3 to 10 degrees F below the critical temperature for the targeted plant, most growers are not concerned with the Evaporative Cooling Potential for there is little chance of additional damage occuring.

To obtain the predicted wind, air temperature and wet bulb temperature use the National Weather Service pin point forecast for your location. FAWN will develop an automated method to extract this data for each site and provide the level of risk for any given night. We hope to have this program in place before the 2005/2006 winter is over.

It appears that if the risk level is LOW or MODERATE, the operation of irrigation systems will produce little damage to the plants. On the other hand if the level is HIGH or EXTREME, growers should carefully evaluate their confidence in the forecast for considerable damage is possible. It is probably a toss up if the risk is STRONG, once again confidence in the forecast is a major consideration. The most difficult decisions to operate an irrigation system on cold nights is when the forecast is within one or two degrees of the critical temperature for the targeted plant. One does not want to incur damage to the plants, whether from cold temperatures or evaporation. These are the long nights that create grey hair and heart burn!

Risk level

Considerations

Low

Maximum temperature difference between air temp and wet bulb is <1 °F.
Maximum wind speed is < 5 mph.

Moderate

Maximum temperature difference between air temp and wet bulb of 1° to 2° F.
Maximum wind speed is 5 to 8 mph.
If wind <5 mph, move to Low Risk.
If wind >8 mph, move to Strong Risk.

Strong

Maximum temperature difference between air temp and wet bulb of 3° to 4° F.
Maximum wind speed is 9 to 12 mph.
If wind <9 mph, move to Moderate Risk.
If wind > 12 mph, move to High Risk.

High

Maximum temperature difference between air temp and wet bulb of 5° to 6° F.
Maximum wind speed is 12 to 15 mph.
If wind <12 mph, move to Strong Risk.
If wind >15 mph, move to Extreme Risk.

Extreme

Maximum temperature difference between air temp and wet bulb is >6° F.
Maximum wind speed is >15 mph.

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