Actually, the root problem I had was more of a capillary action issue.
The water level was beneath the edge, but only by an inch or so.
The roots had developed over the edge to a point that they formed a thick mat, and that mat of roots was wicking water up and over the edge. I didn't notice, because the edge was covered by thick rocks.
Yes, evaporation and evapo transpiration are real effects when it gets warmer.
Its a good idea to pay attention to these normal sources of water loss when you know things aren't leaking, since they can mask leaking.