Southern California is dependent on outside water sources. Much of California is a desert, actually, 59% of it is.
That includes the Mojave Desert and the Colorado Desert. Sadly, when in a desert water comes naturally in limited quantities. Even from the sky, rainfall is much more sparse in Southern California compared to northern California.
75% of California’s water is in the upper northern third of the state, while 80% of water demands come from southern California. Such high water demand is partly due to the fact that SC is more populated than NC