You don't have to defend Central Valley farmers from me, Steph. Their practices must provide half the food many Americans have to eat. BTW - 100 miles west of here is also desert. At under 20" of yearly precipitation (& most of that snow), right here isn't far above desert status, either.
Both the idea that 1/2 inch of water can make any difference and the idea that 4" of water at a time is needed, are foreign to my gardening, however. Flooding my rocky soil would just feed the aquifer, the plants would get little of the water before it quickly moved beyond the reach of their roots. I'm afraid that I'd just have to get water back on it in just a few days, anyway! And, I'm afraid that 1/2 inch of water would nearly all evaporate
!
Heat is one thing, humidity (or the lack of it) is another. Soil is yet a third . . . Then there are the needs of specific plants.
I am not sure what the water needs are to grow rice. The fact that the Central Valley
can grow rice I think is wonderful. They are remarkably productive plants! Californians must decide what is the best use of the water they have and we all should hope that food production continues to get a good part of it.
Steve
♪ ♫ 90F ♪ ♫ here yesterday & predicted again today!
The Central Valley was mostly desert before the agriculture got started. The soil is very fertile, but it takes a lot of water to keep anything alive here.
It was enough to lay me out for a couple days. I couldn't go outside because the sun was SO bright all I wanted to do was hide under a blanket and 
(which I did some... LOL)
