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My response is going to come from a chemistry/engineering/molecular perspective.<p>The question as is can't really be answered very accurately. <p>Evaporation happens when molecules of H2O gather enough energy to break free from the rest of the water through molecular collisions and the heat applied to the system. The energy required to break free also depends upon the atmospheric pressure applied to the water - that's why cooking instructions vary if you live in the mountains vs at sea level - the boiling and freezing temps of water vary at dif't altitudes.<p>So, evaporation will depend on the current temperature, will also depend on the surface area of the body of water that is exposed to the air, and whether that water is dynamic/flowing.<p>I was trying to devise a simple experiment to let you extrapolate those results to your pond, but there are toomany things that are unknown.<p>It's really something that would have to be determined through empirical evidence on a pond by pond basis, rather than a formula.<p>Why do you need to know?
In my expierience with ponds, if considering evaporation as a factor for low water level, you're more likely to have a leak. Check thoroughly for any leaks and repair any that you find. Also whether or not you find one you could add an aoto fill valve and never need to worry about low water again. Oh yeah, the only time I prove evaporation being a real problem was in a pond that had extremely tall and wide waterfall, and this only seemed to be an issue in times of extreme heat and dryness, really can't explain why , but my best guess is that there was too much surface area of water speeding through the hot, dry air. anyway added an auto fill valve and problem was solved.
Guys; Thanks for the input. Our problem is twofold 1. A customer that is convinced she should never have to put extra water in the pond. 2. We know we have some small leaks on one side, but we also know that we'll need to provide as much info to this person as we can. Aquascapes' info seems to indicate anywhere from 1/2" to 1" per week depending on temperature, wind, etc. Thanks for the input.
I was losing 6 inches a week from a 5000 gallon pond with 3 falls in 18 feet of vertical drop over 40 feet laterally. Put an auto fill in and no more problem. The more falls the more evapo. Probably had a leak or two, but in that large a feature, water loss is to be expected.