Measuring Evaporation Rate in Inches Per Hour - WAFB 9 News Baton Rouge, Louisiana News, Weather, Sports

Measuring Evaporation Rate in Inches Per Hour

Question:

Would you happen to have information on West Baton Rouge's Pan Evaporation (Ep) constant for ponds in inches/hour? 

Answer:

Pond Evap at inches per hour?  No such animal ... or at least, it is certainly not measured anywhere that I am aware.  Remember, the evap rate will change through the day, and even a MAXIMUM evap rate is unlikely to reach 1/20" per hour on most south Louisiana summer days.
 
A rough, average DAILY approximation of evap per day from a small, shallow pond (excluding "leakage" through the bottom) would be somewhere on the order of 1/4" per day.  Sunny, low-humidity, windy days will have higher evaporation rates; humid, mostly-cloudy, "calm" days will have lower evaporation rates.  Clarity of the water will also be a minor factor--and this assumes no inflow/outflow from the pond.
 
Now, there are a very few places (fewer than 6) in Louisiana where "pan" evaporation (the method the National Weather Service uses ) is recorded, including one just across the river from you at the LSU-Ben Hur Farm.  In fact, several weekends a month I am one of the folks that volunteers to go out to the farm and make the once-per-day reading, recording a 24-hour loss at about 8AM (of course, we also must adjust for any rain that falls)!
 
Truth is this: the daily values of pan evaporation are rather suspect.  Weekly and monthly sums are somewhat useful (day-to-day measurement errors get "averaged out"), but even these values must be corrected by a "fudge factor" to adjust the "pan" evap (a 4-foot metal wading pool) to a "pond" approximation (of, course, how "big" is the pond?).  In Louisiana the "pan" fudge factor is roughly 3/4" to 4/5", meaning that the loss from the pan exceeds the loss from a pond - as you would expect.
 
Another option is to use the "reference evapotranspiration" estimates calculated by the LSU AgCenter's LAIS automated weather network.  These also are only rough "guideline" numbers, derived from computer models "driven" by meteorological data collected by the LAIS weather stations (3 LAIS stations just across the river from you - 2 in EBR Parish and the third in St. Gabriel/East Iberville).  I manage the LAIS program, and can discuss details of the data and operations with you.
 
 
Jay Grymes
Chief Meteorologist
WAFB Storm Team

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