What is "Priority Doctrine"?

One of the humbling aspects about running for public office is you discover how many aspects of public policy about which you know nothing. I received a questionnaire from the Idaho Farm Bureau today, and one of the questions that they asked was my opinion about "priority doctrine." What the heck is that?

It turns out that Idaho, unsurprisingly, has long had a policy that the first user of a water source has priority over later arrivals when it comes to arguing who gets how much. It appears that the Idaho Department of Water Resources had other plans.

My first reaction is to giving priority to the first user of a water supply is that this seems fair. I can imagine that there are circumstances where this might not be appropriate, and I'm sure that environmentalists would insist that the first user of some of these water supplies have scales.