After seeing several posts with users wondering how certain chemicals could build up in their water over time (e.g., salt and CH) and considering my on-going plans to eventually install a rainwater collection system from my flat roof gutters, I got to wondering about the applicability of pan evaporation rate data to pools. If the monthly average pan data for a given area is X inches of water, then what is a good correction factor to apply for that?
I won't bore anyone with the details but simply point you to a good primer on the science of Class A Evaporation Pans from our good friends at Wikipedia. There is also some good, State-by-State data of measured pan data HERE. It is possible to evaluate evaporation rates from large bodies of water theoretically using Penman's Equation (see here and here) but I don't think that is necessary when it comes to pools. For the most part, evaporation pans are typically used in agricultural sciences to measure the evaporation rates near a large crop field. One then applies a correction factor (typically 0.7 to 0.75 for moist soil) to the pan data to determine evaporation rates. There are also commercially available atmometers (see here) for measuring evapotranspiration rates, but they typically cost hundreds of dollars. Some have devised simple, DIY atmometers (see here and here), but again it would be preferable to simply use existing data especially when trying to help people who post and may not realize that sometimes evaporation and refill makes a difference.
Of course, I could simply attach a 3/4" brass water volume meter (here) onto my autofill line and measure evaporation that way. I've tried the measuring stick in the pool method but find it unsatisfactory.