Not sure why I need the cistern? Where is my pond water going to go?There's a video modern design aquascaping has called water in motion . I strongly suggest you watch it . Let's say you loose power water has to be stored in the pump vault area . Then it rains you have know where to capture that rain because the power being out took all the available space. The bottom pump vault should be as big as you can afford in space and dollars.. I think I spent 3k on the cistern for 3000 gallons. Thinking my god that will last all summer when in actuality it doesn't last a month.
That said I was going to put an overflow outlet off of the small pool, if the pond was to have too much water.
I also was looking at putting drain tile all around my house and pipe it to the pond but have a valve that I could reroute it off the property if the pond didnt need that rain water.