Paul,

I would contact your local soil service agency and quiz them about evaporation rates in your area. Also, check with other property owners, that have ponds, to see how much water loss (evaporation) they experience in an average year.
The USDA put out a handbook (I believe its number 590, search the web) that contains useful information for building ponds throughout the USA. The manual listed rainfall tables based on geographic location and suggested depths of ponds along with alot of other useful information.

Having this information will help in determing whether or not a supplimental source of water for your pond would be necessary.

Switching gears but along the same lines. I'd like to channel the runoff from my outbuilding roof into my pond and I'm curious to know if there would be any potential problems. The roofing is standard galvanized, not painted.

Russ