I do not understand why we don't just plant switch grass and be done with it? It's native to a large swath of the arable land, doesn't require as much (in some areas any) irrigation and it's good for the soil.
https://cropwatch.unl.edu/benefits-switchgrass-hay-and-forage
Edit:
Also 6.24 is closer to a third of 19.25 than it is a half. Which is still a massive amount of water. Why inflate the numbers by saying nearly a half?
It's because you have to use your water to keep water rights. Flood irrigating alfalfa is an easy way to use up a lot of water to keep the rights without having to manage more complex farms. The current way that water rights works creates perverse incentives.