Bakersfield, CA averages 6.36" of precipitation per year, Los Banos is 9.48". The Central Valley where much of this is grown is very dry. (Merced + Fresno do a little better being closer to the mountains, but still under 12").

Anyway, the problem isn't so much in an average or good water year but in a bad one. CA is prone to lengthy droughts.

(Interactive map of these crops, not recommended for mobile: https://databasin.org/datasets/57a5c9c0ddf54e75861b0621a7e41...)

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This is in some senses, not that big a problem for growing annual crops - just plant less/don't plant, and if you have to give up on the crop, you at least are only losing that year's crop.

However, this doesn't work for tree nuts.

If the tree isn't watered in the year(s) of drought - and watered significantly more heavily than usual, (because of the drought), the tree dies, and it will take a decade or more for a replacement to start yielding significant amounts of that crop again. That's a much larger loss than a single year's crop.

This makes the almond and pistachio farming a much less flexible form of agriculture in an environment where water availability varies drastically.