I think the problem is that desalinating water (ignoring the environmental impact of the resulting brine) will produce water that costs at a minimum many hundreds of dollars per acre-foot. This is fine for urban use, but makes many of the current agricultural uses uneconomical. Nobody will pay $5,000/acre for water so they can sell their crop for $1,000/acre. All of these discussions dance around the fact that irrigation farmers are largely turning X dollars of water into 0.25X worth of crops, and the taxpayers are making up the difference. This is true of farmers in pretty much every state that uses colorado river water - if we erased 'water rights' and allowed the water to be competitively bid on, many farmers would either leave their land fallow or switch to less water intensive crops. We're basically giving away extremely valuable water at way below market rates, we're practically guaranteed to have a 'shortage.' It doesn't make sense to add enormously expensive infrastructure just so we can turn around and keep giving the water away.