This is another bit of engineering I am especially proud of. After building several rain barrels, the question isn't how to divert water into the pond, but how to divert overflow back out once the pond is full. Also, I won't want cold water entering the greenhouse in winter when I am trying to conserve heat inside.
The gutter was a bit challenging as I tried to join 15 sections at a 12 degree angle.
The downspout directs water into the greenhouse. The horizontal part is leaning only slightly inward. This is the key to allowing overflow to escape.
Here is the other side of the wall where the rain water enters the pond:
Inside, I have a shutoff valve to keep water out when I don't want it. The open T junction is part of my future sink system. It will allow sink runoff to exit and run out the back of my yard.