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By almost every measure, the drought in the Western U.S. is already one for the record books. Almost half the country's population is facing dry conditions. Soils are parched. Mountain snowpacks produce less water. Wildfire risk is already extreme. The nation's largest reservoir, Lake Mead, is headed to its lowest level since it was first filled in the 1930s. The past year has been the driest or second driest in most Southwestern states since record-keeping began in 1895. Farms and cities have begun imposing water restrictions, but Western states are facing a threat that goes deeper than a single bad year. The hotter climate is shrinking water supplies, no matter what the weather brings. Warming temperatures make it less likely for a raindrop or snowflake to reach a reservoir due to increased evaporation. As a result, the people who manage the West's complex water systems are realizing that with climate change, they can no longer rely on the past to predict the future. That's creating a fundamental threat to the way Western water systems operate, because they were built around the idea that the climate would remain constant. Historical climate data such as river flows and rainfall totals told engineers how big to build reservoirs and canals. The data also told them how much water was available to divide up among cities and farms. Climate change is putting that system under increasing stress, shrinking water supplies for tens of millions of people and for the farmland that produces most of the country's fruits and vegetables. Water cutbacks are reverberating through California's $50 billion agricultural industry, which employs tens of thousands of people in many small towns. Southwestern states recently negotiated a temporary agreement to use less water as reservoirs keep falling. But tough conversations remain about how the West and its complex system of water rights will adapt to a future for which it wasn't designed. "We can really no longer look at the past and say: The amount of water we've had in the last 100 years is what we can expect in the future," says Eric Kuhn, an author who worked on water policy for decades at the Colorado River Water Conservation District. "That is no longer true because of climate change."  As the West grew, states divided up water held behind the Hoover Dam, outside Las Vegas, but they allocated more water than the Colorado River typically provides on average. Ansel Adams/National Archives hide caption toggle caption Ansel Adams/National Archives "A painful reduction" The Colorado River is the main source of water for much of the Southwest. In late April, Arizona water management officials held a public meeting that everyone had hoped to avoid. The Central Arizona Project, which supplies Colorado River water to millions of people in the state, announced it expects to take 30% less water than usual. Lake Mead, the largest reservoir on the Colorado River, just outside Las Vegas, has been falling rapidly, and was just 38% full by late April. When water supplies get so low, some states are required to cut back on use. Arizona's reductions will primarily hit farmers. Some will be made up through groundwater pumping or with water previously stored in reservoirs and banked for future use. The cuts are "a painful reduction to Arizona," says Tom Buschatzke, director of the Arizona Department of Water Resources. "If the reservoir continues to decline, more aggressive actions will be taken by the lower basin users, including California, to slow the decline in the system." That decline in the Colorado River's flow is exacerbated by both a 20-year drought and climate change. But it has been a century in the making. As cities from Denver to Los Angeles began growing in the early 1900s, planners knew they'd need to secure a reliable water supply in such an arid region. Engineers drafted plans to build a massive reservoir in a steep rock canyon that is now Lake Mead. Loading... But with seven states vying for Colorado River water, officials met in 1922 to negotiate how they would share it. The first step was determining how much there was to share. The officials looked at the previous 20 years of the Colorado River's flow, which was high due to above-average precipitation. They divided up the river water anyway based on those atypical numbers despite warnings from a hydrologist. "They ended up appropriating more water than the river could actually produce," Kuhn says. "Once the commissioners negotiated the compact, which was very difficult, they just didn't want to hear that they'd overstated the amount of water supply." The promise of that water spawned a vast network of reservoirs and canals, carrying it hundreds of miles through the desert and eventually reaching millions of homes. Anyone in the U.S. who eats lettuce during winter has likely tasted Colorado River water, which irrigates the fields in Arizona and California that produce most of the season's crop. Yet since the water was divided up, the Colorado River has been shrinking. Average flows have been drier, compounded by a drought beginning in 2000. The water level in Lake Mead has fallen around 140 feet, leaving a telltale white "bathtub ring" around its perimeter. Climate amplifying bad luck Like a run-of-the-mill streak of bad luck, droughts are normal in the West. Now, climate change is exacerbating their effects. "Over the last 22 years or so, there's been quite a bit of bad luck because precipitation totals have on average been low," says Park Williams, associate professor of hydroclimatology at UCLA. "But the effect of that bad luck has been really amplified because of warmer temperatures."
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