If you 've visited Las Vegas for any purpose beyond gambling, you may have taken a side trip to Hoover Dam and marveled at Lake Mead, the man-made reservoir that backs up behind the dam for 110 miles.

The news is that Lake Mead might be running out of water. Lake Mead and its sister lake, Lake Powell, are only half-full today. They provide much of the drinking water for Las Vegas, Los Angeles and San Diego and much of the agricultural water for the region 's farmers who produce the fresh fruit and vegetables we purchase in Madison supermarkets.

Two researchers at the Scripps Institute of Oceanography warn that if present usage continues, and if present climate conditions either continue or get worse, there is a 50 percent chance Lake Mead will run out of usable water by 2021. If conditions deteriorate, there is a 10-percent chance the lake will run out of water by 2014.

Maybe it won 't happen.

Lake Mead gets its water from melting snow in the Rocky Mountains. The region has gotten lots of snow this winter (Snow? What 's that?).

Probably it won 't happen. Even the doomsayers say there 's only a one chance in 10 that one of America 's major cities will run out of water in six years. Surely, there is a Plan B.

Except there isn 't a Plan B.

There isn 't a Plan B for Lake Mead. Nor is there a Plan B for Lake Lanier, which provides much of the water for Atlanta. Heavy rainfall in the Southeast and heavy snowfall in the Rocky Mountains has raised the level of reservoirs serving our major cities a bit, but we have no idea whether the climate changes that have left many communities with water reserves of only a few months or, in the West, a few years, have actually ended or just taken a breather.

So, the Southwest and the Southeast are facing water shortages. What else do they have in common? They are where an increasing part of the American population is moving.

These are big problems and, so far as I can see, the nation is putting its head in the sand and ignoring them. I don 't hear Barack Obama or Hillary Rodham Clinton talking about them. I don 't hear John McCain or Mike Huckabee talking about them.

Why do we care? For one altruistic reason, because those folks down south are our neighbors. For one truly selfish reason, because we 've got lots of water and, sooner or later, they 're going to want it. It 's in our interest to demand our national leaders start thinking about it -- now!

Contact Bill Wineke at bwineke@madison.com or 608-252-6146. Read his blog at www.madison.com/wsj/blogs.