Agricultural water is usually not for sale to supplement your residential usage. Your lawn or shower is competing with other residential uses, not agriculture.
But agricultural water could be entered into the market for residential usage - the current legal structure incentivizes using the water for agriculture, rather than allowing reselling for higher-value-per-gallon uses.
I was curious about the almond statistic above. Sounds like "1-3 gallons" is exaggerated, but that 1-gallon-per-almond is at least on the right order of magnitude, but farmers are working to reduce water use. https://farmtogether.com/learn/blog/dispelling-miconceptions...
In some cases water could be repurposed into the residential market, for a price. The state could buy the farmers lands and water rights, or buy out their preexisting contracts with water suppliers.
The taxpayers don't want to pay for this, so the water is effectively off limits.
Is requiring compensation for seizure the challenging legal structure you mentioned?
The gallons per nut argument is pretty arbitrary. If you look at calories per gallon, nuts are better than almost all vegetables, most meats and many fruits. The high water per mass is basically a result of nuts being one of the most energy dense foods, and photosynthesis requiring water to create calories.
If you want to go down that rabbit hole, you can start looking at the gallons per mass for different foods and comparing their caloric density.
If push comes to shove do you think Western US states will let people in population centers run out of water before they would take steps like eminent domain (no idea if this would apply as-is or if it would require new legislation, legal battles, etc) to seize (with compensation) those other water rights?
I think it really comes down to what you mean by run out of water. In the past we have seen cities crack down on lawns and pools and car washing in times of drought. I don't see similar measures in the future as far fetched. Nobody is in real threat of dying of thirst as the average household currently consumes around 300k gallons per year.
I don't know about other states, but California has already tried several times to implement legislation to take water rights without compensation, and there are tons of lawsuits on the subject, both past and ongoing. Most recent is the 2014 Sustainable Groundwater Management Act.
I don't think they will ever be willing to buy out farmers.
They will most likely succeed in taking their water without compensation through persistence. As you see in this thread, there has been several decades of messaging convincing people that that water rights are irrelevant and moral justice should trump the law. the legal system can only push back so long. Buying the farmers out is the polar opposite to this sentiment and expensive, so I think it is extremely unlikely.
If this somehow doesn't happen, I think people will eventually give ground on some environmental uses in times of drought. Environmental use is more than ag and cutting a few percent there could easily solve any of residential use constraints with the wave of a pen.
