High in the Colorado Rockies, scientists launch search for causes of western water woes | Science

Researchers will measure many atmospheric processes above Gothic Mountain that ultimately deliver water to the Colorado River.

U.S. Department of Energy Atmospheric Radiation Measurement User Facility

In a historic first, the U.S. Bureau of Reclamation earlier this month declared a water shortage on the Colorado River, triggering emergency measures that will require farmers in Arizona to cut their use of irrigation water by 20% next year. The immediate cause of the declaration is record low water levels in Lake Mead, the largest reservoir fed by the river. But scientists say the crisis has been years in the making—and could soon get worse. For reasons they don’t completely understand, but that are related to the West’s changing climate, snow that falls in the Rocky Mountains—the source of about 80% of the Colorado—has been providing the river with less and less water. “This is an existential water crisis for the Southwest,” says Jonathan Overpeck, a climate scientist at the University of Michigan, Ann Arbor.

Next week, researchers will begin an innovative campaign to better understand the fundamental processes—from the behavior of tiny particles that become snowflakes to weather patterns that influence how snow vanishes into thin air—that determine how mountain precipitation becomes surface water for 40 million people. “What gets us going in the morning is the large number of people that really rely on this resource,” says atmospheric scientist Daniel Feldman of the Department of Energy’s (DOE’s) Lawrence Berkeley National Laboratory (LBNL), who leads the effort.

For the more than $8 million project, called the Surface Atmosphere Integrated Field Laboratory (SAIL), researchers are deploying dozens of instruments that will measure wind, rain, snow, solar radiation, and atmospheric particles in a high-elevation Colorado watershed. Hydrologists have already been studying the streams and bedrock there for years. But the additional equipment will collect data intended to sharpen models that produce a variety of critical forecasts, including short-term predictions of seasonal stream flows and long-term scenarios of how climate change might alter regional water supplies. SAIL is “going to make advances in mountain precipitation and snow studies that would just be impossible without this level of instrumentation,” says Jessica Lundquist, a mountain hydrologist at the University of Washington, Seattle. “It’s really exciting.”

The southwestern United States has been in the grips of a drought for 2 decades. A drop in precipitation has been exacerbated by a decline in what’s called runoff efficiency, or the proportion of precipitation that reaches waterways. In the 1930s and ’40s, about 17% of basinwide precipitation ended up in the Colorado River, scientists say. Now, it’s roughly 14%, a decline that has contributed significantly to a recent 20% loss of river flow. An overall warming of the West seems to be driving the change, and possible mechanisms include increased water uptake by plants and a loss of snow cover, which means the ground and atmosphere become warmer because less sunlight is reflected back into space. Even though the trend in runoff efficiency is clear, computer models have had a hard time predicting exactly how much water will reach rivers, in part because the physical mechanisms involved are murky.

This rain gauge is one of many instruments that will study precipitation and other weather for 20 months.

U.S. Department of Energy Atmospheric Radiation Measurement User Facility

SAIL’s technicians have been working all summer in Colorado’s 300-square-kilometer East River watershed, installing instruments that are part of DOE’s mobile Atmospheric Radiation Measurement program. To get a better view of the rain and snow, they have installed one radar system at the top of a ski resort. It has a resolution of 100 meters, five to 10 times greater than a typical weather radar, says V. Chandrasekar of Colorado State University. It will allow researchers to identify and quantify precipitation, including various types of snow particles, that are up to 50 kilometers away.

Other instruments should enable SAIL to observe hard-to-track interactions, such as those involving the tiny aerosol particles that help create snowflakes. Researchers also hope to probe the mysteries of a process called sublimation, in which snow turns directly into water vapor. In certain conditions, sublimation claims about 20% of the peak snowpack before it melts. Sublimation has rarely been studied in combination with so many other atmospheric processes, says Jeff Lukas, a water and climate researcher and consultant based in Colorado. SAIL will also measure where, when, and how much rain and snow fall, helping hydrologists better understand how water moves through mountains and into streams, says co–principal investigator Rosemary Carroll, a hydrologist at the Desert Research Institute.

In parallel with SAIL, the National Oceanographic and Atmospheric Administration is mounting an effort to improve its weather models and mountain region forecasts. And the U.S. Geological Survey will install new stream gauges to Colorado headwater streams as part of its Next Generation Water Observing System.

The campaign’s biggest synergy, however, is with a major hydrology and biogeochemistry investigation that DOE has funded in the East River watershed since 2014, examining how it stores and releases water, carbon, nutrients, and pollution. Collaborations there now involve hundreds of scientists, notes LBNL’s Susan Hubbard, who leads the project. And that diversity thrills many SAIL participants. “It’s like a science rodeo,” says ecohydrologist Alejandro Flores of Boise State University.

One key question is how easy it will be to transfer any insights gained from SAIL to other mountain regions around the world where rivers are facing similar water supply issues. “If we don’t understand all the details, we’re going into this water management challenge with one arm tied behind our back,” Overpeck says. “Possibly more like both arms.”

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