Storms coming from atmospheric rivers bring vital moisture to the US West Coast. However, they also cause most of the region's floods. Storm size alone does not always predict flood risk, ground conditions also play a critical role.
New research explores the drivers of floods, by analyzing decades of atmospheric river storms. It examined numerous West Coast watersheds from 1980 to 2023 and identified that a primary driver for flooding was soils that were already saturated. These soils cannot absorb more water during storms.
These findings explain why some intense storms cause catastrophic floods, while others do not. Even weaker storms can generate major floods if rain hits saturated ground. Conversely, stronger storms might bring needed moisture without flooding dry landscapes. This highlights the soil's crucial role in flood moderation.
In arid regions, storms hitting saturated soils more often resulted in floods. Watersheds here typically have shallow, clay-rich soils and have limited water storage capacity. In contrast, lush regions with deeper soils and snowpack have higher water storage capacity. While soil saturation can still contribute to flooding, accounting for soil moisture is less critical for management in these areas. Their soils remain consistently wet or insulated by snow.
Improving flood prediction could depend on better soil moisture data. Current measurements exist but are sparse. Soil moisture can also vary greatly within a watershed meaning multiple stations are necessary for a clear picture. Increased monitoring in high-risk watersheds may enhance early warning systems and real-time soil moisture observations could become vital as atmospheric river events intensify.
