On hot, dry days in the States I think it is possible to reach evaporation rates as high as 0.25" per day.
One test would be for you to set out an evaporation pan in some spot near the lake. (Probably need to put a little weld-wire over it to keep the wildlife out.) Check your actual evaporation rate. If you place it in the shade, it will probably be an underestimate. If you place it in the sun, a little overestimate. (A shallow pan will be subject to more solar heating and subsequent evaporation than your pond.)
The other issue is plant roots drawing from the saturated soil around your lake.
I have two groundwater ponds and a groundwater creek on our farm. After the first hard freeze in the fall, the water level in my ponds AND the creek actually goes up with no rain once all of the plants in the area quit sucking massive amounts of water out of the aquifer.
You may have a smaller version of that process on your lake.
