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."

"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.

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."

A hotter atmosphere is thirstier, drawing water out of plants and soils and into the air. Snowpacks melt earlier, which in turn boosts that evaporation, because without the reflective surface that snow creates, soils heat up faster. And when soils are dry, they act like a sponge. They need to soak up more moisture before they're saturated enough for the water to run off into rivers and streams.

Studies show that since 2000, about half the reduction in the Colorado River's flow has been due to warmer temperatures. For every degree Celsius of warming, the river's flow is expected to shrink by 9%, according to another study.

The same climate impacts are being felt elsewhere in California, where a massive network of reservoirs and canals connects the water-rich Northern California mountains to the Bay Area and Southern California, where most people live. That water also flows to the Central Valley and other agricultural areas, which grow two-thirds of the country's fruits and nuts.

The state's snowpack in the Sierra Nevada up north is a vital source of water. But in a hotter climate, it's shrinking and melting earlier in the season. And as in the Colorado River basin, less of that snowpack is turning into water supply.

"We're getting a lot less streamflow coming off the same amount of snowpack," says Andrew Schwarz, climate action coordinator at California's Department of Water Resources. "We're not seeing as much water showing up in our rivers as we would have expected with the same amount of rain or snow that we had gotten historically."

According to state research, California's water supply, which reaches 25 million residents through two state and federal water projects, will shrink with every degree of warming, even if the state gets more rain than average.

Bumpy road ahead

With long-term decline on the horizon, water managers across the West are grappling with a system that wasn't built with flexibility in mind.

Water rights follow a pecking order of "first in time, first in right," with the oldest water rights taking priority over others. For many, those rights are considered untouchable, and any effort to curtail them spawns numerous lawsuits.

Seeing the writing on the wall, states on the Colorado River negotiated a hard-fought plan in 2019 to deal with water shortages. As Lake Mead drops, Arizona and Nevada face cutbacks first. If water levels continue to fall, other states such as California will also face restrictions.

The solution is temporary, however. States will need to negotiate another plan that would start in 2026, and reservoirs are projected to keep dropping over the next two years. Reexamining the water allotments in the original agreement in 1922 is a far thornier prospect. It was ratified by both states and Congress, making it federal law.

"It's difficult," Kuhn says. "It creates a lot of legal conflicts. It's a bumpy road, but we'll get there because we have to."

California has its own system of rights where water users have claimed more water than is available on average. During the state's last drought, some water users challenged the state's authority to regulate them. California officials released a report this year identifying a need to have more adaptable water rights in a changing climate as well as better water use data.

"California's water system was designed for a climate we don't have anymore," says Alvar Escriva-Bou, a research fellow at the Public Policy Institute of California. "What we are seeing, especially in some parts of California, is that we have been using more water than we have. And that's causing problems. So the reality here is that we have to make a reduction of water use over the long term."

Conservation will be critical in a hotter climate. The vast majority of water used in the West goes to agriculture, and some regions have conserved by investing in more efficient irrigation. Other regions, with older water rights less at risk, have had less incentive to do so.

The good news is that some drought measures seem to stick. Since California's last drought from 2012 to 2016, residential water use has remained lower than it was before the drought hit. It's more than just shorter showers. Residents made investments such as water-efficient fixtures and low-water landscaping that lock in water savings for years to come.