We chose not to allow the graph to go below zero, but you are correct below available water capacity (AWC) is a permanent wilting point (PWP).
Interesting concept. We chose to go simple and once the zone is depleted, we fill it up. This keeps things a bit simpler from a tracking perspective along with min/max values.
Great overall questions, you definitely understand these concepts 
This topic will help with some of our decisions.
If there is a domain expert out there that has a better way to implement, I’m listening.
I think there is room for improvement, but all in all the process seems to work for most customers that have chosen to use flex schedules.
Thanks for the great questions.
