Irrigation Risk for FAWN Stations

Using a Critical Temperature of 34°F (change)

StationIrrigation
Risk
Tair
(F)
WetBulb
(F)
Windmax
(mph)
AlachuaN/A*80794
ApopkaN/A*82798
ArcadiaN/A*83796
AvalonN/A*83799
BalmN/A*84809
Belle GladeN/A*83814
BronsonN/A*80784
CarrabelleN/A*75752
CitraN/A*837812
ClewistonN/A*83805
Dade CityN/A*84815
DeFuniak SpringsN/A*79776
DoverN/A*83805
Fort LauderdaleN/A*87827
FrostproofN/A*80767
HastingsN/A*82797
HomesteadN/A*83825
ImmokaleeN/A*85806
Indian RiverN/A*83806
JayN/A*81784
JoshuaN/A*83794
KenansvilleN/A*82796
Lake AlfredN/A*84796
LecantoN/A*90819
Live OakN/A*82798
MacclennyN/A*84796
MariannaN/A*81778
MayoN/A*78774
MonticelloN/A*78784
North PortN/A*868010
OcklawahaN/A*82813
OkahumpkaN/A*80788
OkeechobeeN/A*81798
OnaN/A*83805
PalmdaleN/A*83806
PiersonN/A*84794
Putnam HallN/A*837812
QuincyN/A*757410
SebringN/A*82797
St. Lucie WestN/A*83809
UmatillaN/A*83797
WellingtonN/A*85807

*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.

back