In most places the rain replaces any water that evaporated. No big deal. If your in or near a large city and "acid" rain is possible. Then the pond wasn't a good idea to start with.
If its a large pond the rain will make little difference in it. A smaller pond would see a larger effect from the rain but its normally helpful.
The only problem I have with rain is heavy rain that overflows the pond. I solved that by digging a drainage ditch from the pond, so that it can only flow out that ditch, not all the way around the edges. I then lined the ditch with medium sized river rock (about 2-3" diameter), so that water can flow through, but no fish or other creatures can be washed out.
I've got an overflow in my skimmer that is conected to my gutter downspout. It then goes under my paver patio and out in the lawn past the pond to a pop-up emmitter and flows down the yard.
Think about all of the natural ponds around the world. Usually they have a water source to fill it, be it rain, an inlet stream, or an unground spring. Usually there is a overflow somewhere at a low end of that pond.
Your man made pond isn't much different, except you control the enviroment of your pond.
Rain isn't and won't be the problems you need to be concerned about.