Often, the problem is not a lack of available water...
...but that it's simply not captured. In Atlanta, which almost ran
out of water 3 years ago, it rained almost 40 inches in a drought
year. The problem was the lack of adequate storage capacity.
(and the gross mismanagement of existing capacity, but that's
another story)
The region, which has seen drastic population growth over the
last 20 years has done little to add to its storage capacity, mostly
being reliant upon the water that just comes down the
Chattahoochee river, and is controlled by the Federal
government. And when you don't control your water, you are at
the mercy of someone else for your very survival.
The Seattle region has the same problem; massive population
growth, but they rely almost entirely upon what is melting up in the
Cascades. Most of that water simply flows into the sea. Until
relatively recently, they've done little to address this problem.
California, on the other hand, has spent over 100 years investing
in transport and storage infrastructure, which is why the southern
half of the state gets to exist at all. They literally have years of
capacity on hand should it not rain or snow for a season or two.
They own or control most of their water supply, so they don't have
the political crisis that many other regions do during droughts.
