Atmospheric rivers are known for causing most of the flooding on the U.S. West Coast, while also providing essential moisture to the region. However, a new study reveals that the size of these storms is not always indicative of flood risk, as other factors play significant roles. The research aims to assist communities and water managers in better preparation by identifying additional drivers of flooding.
Published on June 4 in the Journal of Hydrometeorology, the study examined over 43,000 atmospheric river storms across 122 watersheds from 1980 to 2023. It found that wet soils unable to absorb more water during a storm are a primary cause of flooding. Flood peaks were observed to be 2-4.5 times higher when soils were already saturated.
“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 and Ph.D. candidate at DRI and University of Nevada, Reno.
The study further explored environmental conditions where soil moisture significantly impacts flooding. In arid regions like California and southwestern Oregon with shallow clay-rich soils and limited water storage capacity, saturated soils during storms increase flood likelihood. Conversely, in areas like Washington with deeper soils and snowpack, soil saturation plays a lesser role due to consistent wetness or insulation by snow.
“We wanted to identify the watersheds where having additional information about the soil moisture could enhance our understanding of flood risk,” Webb explained.
Although soil moisture data is collected at weather stations such as USDA’s SNOTEL Network, observations remain sparse compared to rainfall measurements. Variability within watersheds often necessitates multiple stations for accurate predictions. Enhanced monitoring in high-risk watersheds could improve early warning systems as atmospheric rivers become more frequent.
The research highlights how integrating land surface conditions into impact assessments can refine flood-risk predictions beyond storm size alone. “My research really focuses on this interdisciplinary space between atmospheric science and hydrology,” Webb stated.
Webb collaborated with DRI ecohydrologist Christine Albano on this research project. Albano emphasized its significance: “Advances in weather forecasting allow us to see atmospheric rivers coming toward the coast several days before they arrive.”
For further details: The full study titled "Wet Antecedent Soil Moisture Increases Atmospheric River Streamflow Magnitudes Nonlinearly" is accessible through 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).