[amanaplanacanal]

This is the real answer. In some cases that water is already owned by someone downstream.

Here is the scenario: you are the first settler in an area. You settle by a stream and use the water for irrigation or whatever. Then someone settles upstream from you and diverts the water for another use. In many places you have property rights to the flow of that stream based on prior usage.

[denton-scratch]

> that water is already owned

Yup. If you allow people to "own" rainwater that hasn't even fallen yet, you are going to need some batshit regulations. So don't do that then.

[18pfsmt]

Water rights have long been an issue in the mountain west, and are highly coveted. They go back to the settling of areas, and directly influence property values. Even well-depth is regulated for newer properties so as to prevent them from usurping water in use by existing residents.