1/2 " per day is a fair lot through just evapouration but splashes from a waterfall can 'consume' a lot of water.
I took quite a bit of time when seting up my waterfalls to ensure that the spalshes fell in areas that directed the water back to the stream or pond. Try shifting presumably rocks or pebbles about to lessen the splashing or direct it to safer areas. When you have eliminated the water feature as a source of water loss then check the loss rate from the pond.