You're correct that modern farming techniques are fairly efficient, but within the confines of any specific crop being grown. Efficiently watered corn, for instance, still takes less water input than efficiently watered rice, millet takes less water still. Techniques are good but crop selection is questionable. Beef/alfalfa is the thing on the top of my mind when I say this.
I think we need a discussion thread for the californian drought going on. I would like to compile information in the main post and would like help compiling it. If we really are proud to be effective altruists then this is an area we should really figure out.
Any one have any good ideas on how we can help?