How about taxing them? Is that allowed by the constitution?
What does confiscatory and excessive mean in this case? People do have to sell off properties to pay off tax debt (some properties are exempt I guess) so I would hope water rights could be taxable even if the result is that the owners would be forced to sell the rights. Am I incorrect?
I think the gov't paying to buy out all existing rights, is better than allowing arbituary taxation like this.