New Study Identifies Soil Moisture as Key Factor in Flood Risks

A groundbreaking study published in the *Journal of Hydrometeorology* reveals that soil moisture plays a critical role in the severity of flooding caused by atmospheric rivers on the U.S. West Coast. Conducted by researchers at the Desert Research Institute (DRI), the study analyzed over 43,000 atmospheric river storms spanning four decades, from 1980 to 2023. Lead author Mariana Webb, a Ph.D. candidate at DRI and the University of Nevada, Reno, emphasized that while atmospheric rivers are vital for replenishing water resources, they are also the primary cause of flooding in the region. The findings suggest that flood peaks can be 2-4.5 times higher when soils are already saturated prior to a storm event.

The research indicates that the size and intensity of storms do not solely determine flood risk; rather, pre-existing soil conditions are equally, if not more, influential. "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," Webb explained. This highlights the necessity for water managers and communities to consider soil moisture levels when assessing flood risks, particularly in arid regions like California and southwestern Oregon, where soils tend to be shallow and clay-rich, limiting their water storage capacity.

In contrast, regions such as Washington and the Sierra Nevada, with deeper soils and higher snowpack, exhibit a different response to similar storm events, making soil moisture a less significant factor in flood management there. DRI’s ecohydrologist Christine Albano, a co-author of the study, noted, "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."

The implications of this research extend beyond immediate flood management strategies. As climate change intensifies atmospheric river events, leading to more frequent and severe storms, understanding the interactions between atmospheric conditions and land surface properties will be crucial for mitigating flood risks. The researchers advocate for enhanced monitoring of soil moisture, particularly in high-risk watersheds, to improve the accuracy of flood predictions.

This study represents a significant step in bridging the gap between atmospheric science and hydrology, as it encourages interdisciplinary collaboration to develop more effective strategies for flood risk management. By integrating land surface conditions into flood assessments, communities can better prepare for and respond to the impacts of flooding, ultimately fostering resilience against future climate-induced challenges.