If I only wanted a certain amount of water to enter a downspout from my roof (going to a planter) at any given time, and any extra would be diverted to a secondary downspout (going to the storm drain, or yard) how would that be possible? I was thinking that the more water flowing along a horizontal pipe, the greater its kinetic energy, the more likely some of the water would run over a lip down into the overflow pipe, with a portion of it still going down the planter pipe. If the flow rate was at or under what I needed, the water's kinetic energy wouldn't be enough to run over the lip. Am I overthinking it? Should I just have the water collect in a small cistern, have the overflow pipe set higher than the planter pipe, and call it a day?