Irrigation Risk for FAWN Stations

Using a Critical Temperature of 34°F (change)

StationIrrigation
Risk
Tair
(F)
WetBulb
(F)
Windmax
(mph)
AlachuaN/A*76764
ApopkaN/A*79789
ArcadiaN/A*78787
AvalonN/A*78778
BalmN/A*77775
Belle GladeN/A*77774
BronsonN/A*76768
CarrabelleN/A*80795
CitraN/A*79787
ClewistonN/A*797910
Dade CityN/A*78786
DeFuniak SpringsN/A*75745
DoverN/A*78774
Fort LauderdaleN/A*81775
FrostproofN/A*79777
HastingsN/A*80774
HomesteadN/A*79776
ImmokaleeN/A*77775
Indian RiverN/A*80785
JayN/A*75757
JoshuaN/A*76762
KenansvilleN/A*78774
Lake AlfredN/A*78773
LecantoN/A*77776
Live OakN/A*76764
MacclennyN/A*75752
MariannaN/A*74743
MayoN/A*75754
MonticelloN/A*75751
North PortN/A*78776
OcklawahaN/A*79796
OkahumpkaN/A*80799
OkeechobeeN/A*77775
OnaN/A*77773
PalmdaleN/A*78787
PiersonN/A*79773
Putnam HallN/A*76763
QuincyN/A*76766
SebringN/A*79796
St. Lucie WestN/A*79785
UmatillaN/A*79797
WellingtonN/A*78786

*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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