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Evaporation (Pan)
2

Evaporation (Pan)

Evaporation (Pan)

(OP)
I am trying to determine a rough volume for the daily evaporation from a reservoir using evaporation pan data.  I have been looking through several textbooks which only discuss using pan data in relation to reservoirs by referring to the correction factor Cp for the rate.  The data I have for the pan is the evaporation rate in in/day.  There is no discussion on the area over which this rate occurs.  I am trying to determine how to calculate the volume per time that is lost from a reservoir of a certain size.  Just for argument sake the (factored) evaporation rate is 0.24 in/day and the reservoir is 2 acres. Is the rate from the pan data over the area of the pan? is it over a square inch? a square foot?

Thanks
Curt

RE: Evaporation (Pan)

Just as rainfall is considered as a "loading rate", evaporation is considered an "unloading rate"....in inches/time.  The pan rate is over the whole pan...the area is irrelevant if the same conditions apply to the whole area in question.

RE: Evaporation (Pan)

(OP)
what would be the volume of evaporation over the 2 acre reservoir per day in ac-ft?  The purpose of this evaluation is to determine how much make-up water should be introduced to maintain a required working pond volume.

RE: Evaporation (Pan)

2
(0.24 in/day) x (1/12 ft/in) x 2 acres = 0.04 acre-ft.  Is that all you are asking?  

RE: Evaporation (Pan)

correction factor is to relate the small pan to a larger reservoir. obviously the water in the pan will evaporate at a different rate (faster) due to temperature differences. commonly used coefficient is 0.75.

RE: Evaporation (Pan)

He did say the rate was factored.  These things are all just estimates, varying a lot with atmospheric conditions, and then it rains...

RE: Evaporation (Pan)

(OP)
Thanks hokie66.  That is what I was doing, it just felt too simple.   

RE: Evaporation (Pan)

You are welcome.  Simple is best.

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