Evaporation and rain aren't a water change, all that is changing is the concentrations of whatever is in the water. No different than topping off a fish tank. I periodically siphon several hundred gallons of water out of my pond and replace it with fresh water and the proper conditioners.
With a great filter system, of course, you may be fine without water changes in regards to ammonia, etc. but it needs to be a really great filter system to deal with things like the buildup of hormone that results in stunted fish growth.