Connecting pond to gutter downspout?

I'm thinking of building a relatively small pond (~100 gallons) in my yard this spring. Would it be a good/bad idea to connect it to a downspout to top off and refresh the water? Half an inch of rain would be ~80 gallons of water, and summers tend to be pretty dry but spring is rainy. I have 2 of my downspouts already connected to 150 gallons of water barrel collection and by August those have run out of water.

My plan would be to have turns/bumps/rocks/small pools between the downspout and the main pond. The yard is sloped away from the house so any overflow from the pond would drain into the yard and I would surround it with native water loving plants.

Thanks for any help/advice!