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Arizona's solar-over-canal project will tackle its major drought issue
(interestingengineering.com)
This is a most excellent place for technology news and articles.
Are there not enough areas of the US that get rainfall suitable for growing the needed food?
There probably are, but Saudi Arabia owns a bunch of land in Arizona and decided it was the perfect place to grow alfalfa, a very water intensive crop. That said, some farming does make some sense even in the desert, since it is almost certainly cheaper to have local produce than to need to import everything from places that have an abundance of water, even if that means building canals to water them.
The US has lots of land that doesn't require irrigation, but also lots of land that can grow crops if irrigated. Some of that land in California is some of the best farmland in the whole country, growing things that prefer California's Mediterranean climate (similar to parts of Australia's southwest coast).
We have the technology and have had it for a while. But we don't have the laws and habits of dry countries so US water laws are a wasteful mess.
Almost everything West of the Colorado Rocky mountains is very arid and requires extensive irrigation.
Everything except for the Pacific Northwest, and only the area west of the Cascade mountain range in Oregon and Washington.