"Prior appropriation" water rights in western US - explain? (live, farm)

[RomaniGypsy]

I don't think I'm grasping the concept of prior appropriation water rights. My wife and I are looking at establishing a self-sufficient farm or even a self-sufficient community at some point, and wherever we go, water will be a concern.

Where we live right now - Ohio - we get plenty of precipitation and nothing is ever said about harvesting rainwater or groundwater. We don't talk of water rights, nor the risk of running out of water.

In most western US states, they appear to have "prior appropriation" water rights instead of the "riparian" water rights most common in the east. I've read some confusing things and hope you can clarify what's up with water in the west, for me.

"Prior appropriation means that someone with an older water right is entitled to take his full amount of water before someone who has a junior water right. In times of drought, junior water right holders may not get their full amount of water, or any water at all." - - - O-kayyyyyy... now is this supposed to mean that we all get in line, and Grandpa fills his bucket to capacity before the rest of us get even one drop (he and/or his crops dying of thirst in the meantime), and if the supply empties before Junior gets to it, oh well, that's the breaks......... or does it mean something else?

How is water usage measured for this type of water right? And does it only pertain to groundwater?

Supposing you have a "junior water right" and can't count on whatever water source is governed by said water right. How do you get the water you need for your family and your farm?

Maybe I'll leave it at this for now. Let's rock.