My understanding is that the overall move towards almonds is actually a move towards crops that are efficient in terms of dollars of income for a fixed amount of water.

Wheat exports for $250 per ton of wheat. This month almonds export at $3,783 per ton. (This is actually down quite a bit from before and is resulting in fewer almonds being grown this year.)

In other words, given that farmers have a roughly fixed allotment of water to work from, if you completely banned all almond growing from CA, farmers would just use that same water on other plants and water usage would not change at all.