The water level will be 12", the height of the overflow.
We've had somewhat of a change in plans. Due to concerns about the Ash branches rotting out too quickly, and honestly mostly due to my desire to have mosses growing in the tree, I am scrapping the Ash branches.
We will build the tree with an internal irrigation system. I will try to make it as similar in appearance to a mangrove as I can. I will make it out of PVC, 1/4" copper tubing (it's for refrigerator ice makers, so water safe), floral foam, Great Stuff, and two to three shades of tinted concrete painted on.
This tree will be quite a project, but could be pretty darn cool if we can figure it out.