As water becomes more scarce and more sought after, managers and governments are going to get more creative in their pursuits. One creative way to change the system is to redefine the basic foundations of water law. One of those foundations is the concept of "public interest" - probably the least well-defined term in today's water law, certainly in Kansas water law.
Everyone has a notion of what the public interest is, and in Kansas water law it is very loosely defined. Therein lies the problem (opportunity). Want to eliminate inefficient ag water use so it can be used for public water supply? Define or litigate it as not being in the public interest. Done. Whatever you (or a water manager or a government) wants to do with the water, redefining "public interest" can achieve it without even changing any water law.
Another example is current S. 787 now going through Congress. The clean water act currently applies to "navigable waters of the U.S.". S. 787 redefines applicable water to simply "waters of the U.S.". Such a simple redefinition, that once done, will place all waters of the U.S. under federal regulatory control. I'm not saying this will be a good thing or bad thing - only that it will change the regulatory water focus of the federal government significantly.
The only saving grace in this approach to shifting water use is that because a redefinition of "public interest" is made, it's virtually impossible to argue that the shift is for any other reason than to satisfy the public interest. This means that regulatory (or actual) takings now becomes a real secondary issue.
But regardless of the methods used, I think the posturing for control of water is only going to get way more interesting.