I assume most of these farmers are also getting massive government subsidies, so you possibly wouldn't have to get into the nasty legal tangle of the water rights, just remove subsidies from anyone doing anything they shouldn't be doing, that can include: spraying the wrong pesticides, wasting too much water, not using techniques that store carbon vs release carbon/methane, avoiding monocultures etc.

There is an argument for subsidizing farmers, but it also effectively makes them part of the government and somehow those two things are kept very politically seperate.