JoeDK, if the loss is just due to evaporation, then that's the best case; there's pretty much nothing you can do about it.
If the water loss is due to more than evaporation, then you most likely have a leak which can be a problem of varying degrees especially if you don't have a great water source for the pond.
You've got to starting measuring the loss as a first step.
JoeDK, I agree with Sunil. Stick something in the pond to measure the vertical drop in water. Keep an eye on the amount of rain you get daily/weekly. It looks like you are a little bit West of St. Louis Missouri. If so, look on the internet to find " Mean Monthly, Seasonal and Annual Pan Evaporation Rates for St. Louis Missouri" and compare that data to what you are seeing in your pond. Don't forget to compare that data to your current data including looking at the amount of rainfall in that older data vs. the current rainfall that you are seeing.
It might be nothing to worry about because until all the soil surrounding your pond is saturated with water the water level in your pond will rise and fall, and even after your pond is established with vegetation around it, the level will still rise and fall according to the amount of rainfall you see on a regular basis. The only way around that is to have a dedicated well feeding the pond to make up for evaporation and evapotranspiration losses.
ALL ponds will lose water on a daily basis. Even a pond with a liner will lose water due to evaporation. Anytime the relative humidity is less than 100% you will have some evaporation loss in the pond.
Now if the vertical water loss is considerable greater than the mean monthly pan evaporation rate, THEN I'd be worried about a leak. But until you can confirm that, I wouldn't worry.
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