> In many instances this water was already paid for, or the state decided that farmers must continue to pay while the state doesn't hold up their side of the contract.

Do you have citations for this?

>The closest analogy for the groundwater situation is if you owned a farm in a migratory bird path and the state passed a law for minimum roosting land. Then, instead of buying your farm and land, they didn't compensate you but said you aren't allowed to continue using it as you have been.

isn't this how literally all laws prohibiting something work? The government generally doesn't prohibit something if no one is doing it. Why should the government buy their land?

For example, the government built the interstate highway network of which all of those central valley farmers need to ship their products by using the law to seize land and then changing the character of millions of acres of homes from quiet and peaceful to loud and toxic. They did not generally buyout homeowners who were not necessary to build the road despite fundamentally changing how they were using their property.