I've read that trees near the waters edge can consume quite a bit of water. You could try something that I did after repairing a leak. I marked the inside and outside of a 2.5 gallon bucket (empty detergent bucket) in 1/8 inch intervals starting about 2 inches from the top and did my best to make sure the lines on the outside were inline with the lines on the inside. I put a few rocks in the bucket and set it in the pond with the water level at the topmost mark. Then I filled the inside of the bucket up with water until it reached the topmost mark. After 24 hours I found that the water inside the bucket was about 1/8 of an inch lower than the outside of the bucket and the water level on the outside of the bucket had dropped about 1/4 inch. I ran the same test for a few days and pretty much saw the same results. I gathered that the water loss I was seeing after the leak repair was mostly due to evaporation because I have no aquatic growth or trees anywhere near the pond. Now I could be wrong but I would think this method may help you determine if your loss is mostly evaporation. After having a leak I'm still very paranoid and have several measuring sticks in my pond which I check 2 times a day and track in a spreadsheet. I see on average about 1/16 - 1/8 inch loss overnight and 1/4 - 5/16 inch loss in 24 hours.