Irrigation Risk for FAWN Stations

Using a Critical Temperature of 34°F (change)

StationIrrigation
Risk
Tair
(F)
WetBulb
(F)
Windmax
(mph)
AlachuaN/A*74735
ApopkaN/A*77762
ArcadiaN/A*75745
AvalonN/A*77753
BalmN/A*74736
Belle GladeN/A*77774
BronsonN/A*73737
CarrabelleN/A*76751
CitraN/A*757411
ClewistonN/A*78754
Dade CityN/A*75733
DeFuniak SpringsN/A*75742
DoverN/A*75743
Fort LauderdaleN/A*85798
FrostproofN/A*77755
HastingsN/A*75745
HomesteadN/A*817416
ImmokaleeN/A*78763
Indian RiverN/A*77761
JayN/A*77762
JoshuaN/A*74740
KenansvilleN/A*74730
Lake AlfredN/A*78764
LecantoN/A*76752
Live OakN/A*74735
MacclennyN/A*76733
MariannaN/A*75743
MayoN/A*777510
MonticelloN/A*75751
North PortN/A*907613
OcklawahaN/A*75741
OkahumpkaN/A*76752
OkeechobeeN/A*74742
OnaN/A*74732
PalmdaleN/A*78772
PiersonN/A*76761
Putnam HallN/A*75737
QuincyN/A*74732
SebringN/A*75742
St. Lucie WestN/A*76743
UmatillaN/A*77761
WellingtonN/A*80783

*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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