We can build new power plants, and you're talking about the types of water usage that are going to be cut last. Agriculture actually takes up the vast majority of water usage, and there are lots of types of farming that use more water than others. This is also ignoring the fact that in many areas there will be increased precipitation and a larger number of growing days per year, meaning more potential food can be grown. Climate change will cause major problems, but I just don't see your scenario happening.
The climate zone is moving north. Rapidly. In as little as 10 years, there may be no crops south of Colorado. The Dust Bowl of the 1930's only took two years to get from, "Hey... this drought sucks" to a quarter million people being driven out or killed by black sand accumulation. And that was a localized drought. Not a global change in climate temperatures spanning the Equator, and hundreds of millions of people who are not in a really robust financial or food-safety position right now. As those temperate zones move away from the Equator, people living in equatorial climates will have to leave. For food. For work. And, eventually, for water. Where do you think they will go?