How Dust Might Make Drought Worse (or a Bit Better) in California

A love-hate relationship between dust and snow may affect the state's water supply.

Dust rises from fallow farm fields in California's San Joaquin Valley. As California gets drier, it gets dustier, and that dust may impact snowmelt.

Most of California's water comes from the snow stored in the Sierra Nevada each winter. In the spring, melting snow helps fill the state's reservoirs for the dry summer. (Read"When the Snows Fail" in National Geographic magazine.) As the state's historic drought drags on, scientists are watching the Sierra snow with intense interest—and they're worrying that even tiny airborne particles of dust may have a big effect on water supplies.

Here's how: As California gets drier, it's getting dustier, and at least some of that dust is landing in the Sierra. Dusty snow, with its darker surface, absorbs more solar radiation than clean snow does, meaning it heats up faster and melts more quickly.

That earlier spring snowmelt could mean that spring runoff will happen when the reservoirs are still full from winter rains, sentencing the state to a longer, drier—and dustier—summer. Rising global temperatures are already speeding snowmelt, and dust can create a positive feedback loop that makes the problem worse.

It's Happening in the Rockies

Thomas Painter, a snow hydrologist with NASA's Jet Propulsion Laboratory in Pasadena, California, has already documented the dramatic effects of dust on snow in the Rocky Mountains. He and his colleagues have found that the increasingly severe spring dust storms from the Colorado Plateau are causing snow in the southern Rockies to melt as much as 50 days earlier than clean snow would have.

Dust has an even bigger effect than warming on melt rates: Raising the temperature by 4°C (7.2°F) accelerates the melt by only 18 days.

Painter estimates that the earlier snowmelt and longer summers have reduced average annual runoff in the Colorado River by more than 5 percent—no small matter for the seven states, including California, that use the river's already overtapped flow.

He and his colleagues are now studying the effects of dust on snow in the Sierra Nevada through the Airborne Snow Observatory, which uses remote-sensing technology to take detailed measurements of snowpack size and reflectivity. Dust isn't expected to have as powerful an effect in the Sierra as it does in the Rockies, partly because some of the dust in the Sierra comes from the light-colored soils of Central Valley farm fields, not the red-rock deserts of the Southwest.

But Painter and his team are already seeing more dust in the Sierra than they anticipated. "Even if the acceleration of melt is only half of what we've seen in the Rockies, that's a profound effect," he says.