Should == ‘I wish’.
If it comes down to it, something has to break the tie somehow right? Because while in most years, there is plenty of water, in the years there isn’t it can literally be life and death. And timelines for figuring out who does and does not get access need to be as short as days sometimes.
Different courts have decided different rules, but senior rights holder having priority is basically ‘first person to be using it (historically) gets to keep using it if there is a conflict’.
Which stops someone from moving in and screwing over the existing people (as hard!). Think of the alternative - you build a house in a remote area, dependent on a spring. You have plenty of water.
Housing development gets built 20 years later, also dependent on the spring. In good years, everyone has plenty. In bad years…. You die of thirst? Or what? At least the subdivision knew what they were getting into, when you built your place there was no subdivision!
Same with your farm example. First guy to setup the farm did so with expectations of a certain situation. New guy moves in, and should he now get the rights to half of the ‘the first guys water’ in lean years? Maybe meaning neither have enough water to farm? Or should the first guy have the right to the water he was guaranteed when he first moved in, and the second guy might not get enough in a lean year - but at least knew that was what he deal before he moved in?
Others are king of Solomon like ‘split it down the middle’ (everyone hates this idea, since no one might end up with enough water), or ‘fixed allocations first, then proportional later’, or ‘life critical uses first, then lower priority later’ (easy to game/screw with existing rights holders - add enough new mouths, and none of the ‘less critical’ rights holders get any).
Courts move too slow to resolve these kinds of issues when they become critical - which could be on as short a term as a week or a few days, with no notice.
And not having a general rule resolve these
kinds of things leaves a massive amount of uncertainly that due to the severe consequences of these kinds of situations can easily lead to people getting killed or even wide scale wars starting.
Or as they say in the US West - whiskey is for drinking, water is for fighting over.