Atmospheric rivers are responsible for most flooding on the West Coast of the U.S., but also bring much needed moisture to the region. The size of these storms doesn't always translate to flood risk, however, as other factors on the ground play important roles. Now, a new study helps untangle the other drivers of flooding to help communities and water managers better prepare.
The research, published June 4 in the Journal of Hydrometeorology, analyzed more than 43,000 atmospheric river storms across 122 watersheds on the West Coast between 1980 and 2023. The researchers found that one of the primary driving forces of flooding is wet soils that can't absorb more water when a storm hits. They showed that flood peaks were 2-4.5 times higher, on average, when soils were already wet. These findings can help explain why some atmospheric river storms cause catastrophic flooding while others of comparable intensity do not. Even weaker storms can generate major floods if their precipitation meets a saturated Earth, while stronger storms may bring needed moisture to a parched landscape without causing flooding.
"The main finding comes down to the fact that flooding from any event, but specifically from atmospheric river storms, is a function not only of the storm size and magnitude, but also what's happening on the land surface," said Mariana Webb , lead author of the study who is completing her Ph.D. at DRI and the University of Nevada, Reno. "This work demonstrates the key role that pre-event soil moisture can have in moderating flood events. Interestingly, flood magnitudes don't increase linearly as soil moisture increases, there's this critical threshold of soil moisture wetness above which you start to see much larger flows."
The study also untangled the environmental conditions of regions where soil moisture has the largest influence on flooding. In arid places like California and southwestern Oregon, storms that hit when soils are already saturated are more likely to cause floods. This is because watersheds in these regions typically have shallow, clay-rich soils and limited water storage capacity. Due to lower precipitation and higher evaporation rates, soil moisture is also more variable in these areas. In contrast, in lush Washington and the interior Cascades and Sierra Nevada regions, watersheds tend to have deeper soils and snowpack, leading to a higher water storage capacity. Although soil saturation can still play a role in driving flooding in these areas, accounting for soil moisture is less valuable for flood management because soils are consistently wet or insulated by snow.
"We wanted to identify the watersheds where having additional information about the soil moisture could enhance our understanding of flood risk," Webb said. "It's the watersheds in more arid climates, where soil moisture is more variable due to evaporation and less consistent precipitation, where we can really see improvements in flood prediction."
Although soil moisture data is currently measured at weather monitoring stations like the USDA's SNOTEL Network , observations are relatively sparse compared to other measures like rainfall. Soil moisture can also vary widely within a single watershed, so often multiple stations are required to give experts a clear picture that can help inform flooding predictions. Increased monitoring in watersheds identified as high-risk, including real-time soil moisture observations, could significantly enhance early warning systems and flood management as atmospheric rivers become more frequent and intense.
By tailoring flood risk evaluations to a specific watershed's physical characteristics and climate, the study could improve flood-risk predictions. The research demonstrates how flood risk increases not just with storm size and magnitude, but with soil moisture, highlighting the value of integrating land surface conditions into impact assessments for atmospheric rivers. "My research really focuses on this interdisciplinary space between atmospheric science and hydrology," Webb said. "There's sometimes a disconnect where atmospheric scientists think about water up until it falls as rain, and hydrologists start their work once the water is on the ground. I wanted to explore how we can better connect these two fields."
Webb worked with DRI ecohydrologist Christine Albano to produce the research, building on Albano's extensive expertise studying atmospheric rivers, their risks, and their impacts on the landscape.
"While soil saturation is widely recognized as a key factor in determining flood risk, Mari's work helps to quantify the point at which this level of saturation leads to large increases in flood risk across different areas along the West Coast," Albano said. "Advances in weather forecasting allow us to see atmospheric rivers coming toward the coast several days before they arrive. By combining atmospheric river forecast information with knowledge of how close the soil moisture is to critical saturation levels for a given watershed, we can further improve flood early warning systems."
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More information: The full study, Wet Antecedent Soil Moisture Increases Atmospheric River Streamflow Magnitudes Nonlinearly, is available from the Journal of Hydrometeorology at https://doi.org/10.1175/JHM-D-24-0078.1
Study authors include: Mariana Webb (DRI), Christine Albano (DRI), Adrian Harpold (UNR), Daniel Wagner (USGS), and Anna Wilson (UCSD)
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