In the continental United States, the daily renewable supply of water totals about 1,400 billion gallons – 14 times what U. S. citizens consume per day. But national averages mask a central fact of American life: much of the western half of the country is arid or semiarid. Its rich greens would soon bleach to desert colors but for water pumped from aquifers or diverted from rivers by highly inventive, though sometimes extraordinarily expensive, means.

Geography may be destiny. Certainly, U.S. water riches are unevenly distributed. In the arid and semiarid West, annual water consumption averages 44 percent of renewable supplies. Everywhere else in the country, the average is 4 percent. This difference explains why rapid population and economic growth exerts particularly intense pressures in the West, why irrigation has become the lifeblood of western agriculture, and why water law and water allocation institutions are now being rattled to their foundations by the test of the times.

The American West is now living on borrowed water. Even discounting farfetched schemes to import water into the region from Canada, the West is using water faster than nature can replenish it. The borrowers are this generation, and the lenders the next.

Water-short or not, the West is still the place to go. Migration from other regions of the country and immigration from Mexico and other points south combine with rapid indigenous growth to make the western Sun Belt one of the fastest-growing regions in the United States.