2. Identify all existing water rights holders. Every single farm/lot with some kind of water privileges. I'm sure there are thousands.

3. Determine how much water they used over, say, the last 5 years. This is obviously the hardest part, but it's no harder than many other large government projects. You'd probably have to use some estimation, formulas, etc - and it won't be perfect. It would take a couple of years and there would be a lot of wrangling. Similar maybe to housing assessment values. Probably include an appeal process as well, just like you can appeal a house assessment.

4. Give each user a new numerical water cap (say, 500k acre-feet), that is 125% or so of above usage. This will avoid hurting existing farms by allowing them to continue using water as they have been, with even some room for growth - but still puts a fence around the problem.

5. Require metering of all ag water, with random audits for enforcement and stiff penalties for violations.

6. Use the state's very large budget surplus to buy up those numerical water rights from the lowest bidders until the water crisis goes away.