If you can't prove that your cistern will satisfy the irrigation need every month, but the client has agreed to not have a potable backup (ie the cistern gets drained, and then the plants just live without water until it rains again), how do you explain this? It seems like LEED wants us to prove that the cistern supplies enough water, but if we aren't using potable as a backup (meaning no matter what we are reducing potable water use for irrigation by 100%) why would we need to prove that the cistern supplies enough rain water?