Rainfall Concentration Dries Western U.S. Landscapes

Rainfall across much of California and the western U.S. has become more clustered in heavier storms with longer dry spells between them. The net effect is a drying out of the landscape, according to a study published Wednesday in the journal Nature.

The research is the first to show how this concentration of rainfall into fewer, heavier events dries the land. “The more concentrated rainfall you get, the drier you become,” said Justin Mankin, an associate professor of geography at Dartmouth College who coauthored the study.

” As rainfall concentrates into heavier downpours, more water sits on the surface and evaporates rather than soaking into the soil. The trend is less clear in Southern California and more pronounced in Northern California. The western U.S. ranks among the regions where rainfall has become most concentrated.

Scientists analyzed global precipitation data from 1980 to 2022. They used satellite measurements of shifts in water across landscapes to identify areas that have grown drier or wetter. Precipitation in the Rocky Mountains has become about 20 percent more concentrated.

That change affects the Colorado River, a major water source for California that has shrunk dramatically since 2000 during a megadrought. The same pattern of concentrated rainfall appears in much of the rest of the world.

Lesk, who led the study as a researcher at Dartmouth and is now a professor of Earth and atmospheric sciences at the University of Quebec in Montreal, said the findings are consistent with expected effects of climate change. A warmer atmosphere can hold more water vapor.

Rising temperatures caused by emissions from burning fossil fuels also increase evaporation from land and lead plants to absorb more moisture. The research indicates that the amount of water available in a region depends as much on rainfall concentration as on total precipitation.

In California, warmer temperatures are shrinking the snowpack in the Sierra Nevada. As a result, more of the state's water supply comes from large downpours during atmospheric river storms. The study shows this rainfall concentration occurs regardless of whether a region has a wet or dry climate.

Current approaches for managing drought and floods in California and other western states may be insufficient, according to the researchers. “This is just another indicator ... we are not adapted to the climate we have, let alone the one that seems to be unfolding,” Mankin said.