I don’t know if you’re in the US, but if so, it’s probably not that anyone purchased those rights, but rather that under, as I recall, Western US water right conventions, where land is more arid, if someone is a pre-existing user of water, you can’t go upstream from them and block off water from flowing to them (which people would probably tend to do, otherwise).

kagis

Yeah. Apparently the legal term is “prior-appropriation water rights”.

https://en.wikipedia.org/wiki/Prior-appropriation_water_rights

In the American legal system, prior appropriation water rights is the doctrine that the first person to take a quantity of water from a water source for “beneficial use” (agricultural, industrial or household) has the right to continue to use that quantity of water for that purpose.[1][2] Subsequent users can take the remaining water for their own use if they do not impinge on the rights of previous users. The doctrine is sometimes summarized, “first in time, first in right”.

Prior appropriation rights do not constitute a full ownership right in the water, merely the right to withdraw it, and can be abrogated if not used for an extended period of time.

Origin

Water is very scarce in the West and so must be allocated sparingly, based on the productivity of its use. The prior appropriation doctrine developed in the Western United States from Spanish (and later Mexican) civil law and differs from the riparian water rights that apply in the rest of the United States. The appropriation doctrine originated in Gold-Rush–era California, when miners sought to acquire water for mining operations. In the 1855 case of Irwin v. Phillips, Matthew Irwin diverted a stream for his mining operation. Shortly afterward, Robert Phillips started a mining operation downstream and eventually tried to divert the water back to its original streambed. The case was taken to the California Supreme Court, which ruled for Irwin.[3]

EDIT: Oh, though they do say that the rights can be sold, so maybe someone did purchase them.