Since 1960, tens of millions of people have migrated toward the Pacific, settling in Las Vegas and Tempe and Boulder. Denver has tripled in size. Phoenix, having added some 3.6 million people, has more than quintupled. Today, one in eight Americans depends on water from the Colorado River system, and about 15 percent of the nation’s crops are grown with it. ... the demands on the river were never sustainable. In 1922, the seven states in the Colorado River watershed signed a compact dividing its water. With little historical data, they calculated the river’s capacity after a decade of unusually wet conditions. ... Since the current drought began, in 2000, that shortfall has averaged 25 percent. Instead of adjusting their allotments, states have drawn down the nation’s largest reservoirs, which are quickly draining. Even this winter’s El Niño weather pattern won’t bring enough rain to restore the region’s supply ... To determine who gets water and who doesn’t, states rely on a system that originated more than 150 years ago—when water was plentiful and people were scarce. ... “prior appropriation,” which promised rights to use a share of water based on who got there first. ... Prior appropriation became the foundation of western water law, and it established order in the West. Today, though, state water laws are largely to blame for the crippling shortages. Because water rights were divvied up at a time when few cities existed west of the Mississippi, some 80 percent of the region’s water goes to farmers, leaving insufficient supplies for growing cities and industries. And farmers must put all their water to “beneficial use” or risk losing their allotment—a rule that was originally intended to prevent hoarding but that today can encourage waste. Many farmers have not adopted modern technology that can cut water use by up to 50 percent, in part because they need to protect their water rights. ... Allowing people to buy and sell water rights is a more expedient way to redistribute the West’s water, he argues. Waste would be discouraged, water would shift to where it’s needed most, and farmers would be compensated. ... The West would have plenty of water if people used it more wisely: Most of the region’s supply goes to growing low-value, water-intensive crops such as hay and alfalfa—in many cases in the desert.