FACTBOX: Key facts about water in the U.S. West

(Reuters) - The West has been one of the United States' fastest-growing regions, with its warm, dry climate a major draw. But much of its landscape is desert or semi-arid and many of its cities are facing a long-term water supply crisis.

Here are some facts and figures:

-- The Southwest was home to 14 of the 25 fastest-growing American cities with populations over 100,000 between 2006 and 2007, according to the U.S. Census Bureau.

-- It is home to seven of America's 10 largest cities. By order of ranking: Los Angeles (2), Houston (4), Phoenix (5), San Antonio (7), San Diego (8), Dallas (9), San Jose (10).

-- The six-county area of Southern California, including Los Angeles and San Diego, is home to nearly 22 million people, with population growth expected to add 6 million residents by 2030. Yet 60 percent of its overall water supply is "imported" from distant sources, such as northern California or the Colorado River. In some areas, only 10 percent of the water comes from local sources.

-- Outdoor water use, such as lawn irrigation, accounts for 40 percent of average household consumption in the city of Los Angeles, which has a population of about 4 million. The city averages close to 15 inches of rainfall annually, but that amount varies widely from year to year.

-- Las Vegas gets only 2 or 3 inches of rainfall a year, and in 1959 it went 150 straight days without measurable rain. Because the Southern Nevada climate is so much drier than Los Angeles, grass lawns there require a third more water.

-- Water use in Los Angeles is projected at about 208 billion gallons this fiscal year, roughly the same amount that the city has consumed annually for the past 25 years despite population growth of about 1 million. City water officials attribute this to a combination of greater conservation practices and more water-efficient appliances and fixtures. (Reporting by Steve Gorman and Ed Stoddard; Editing by Mary Milliken)