Maximizing Precipitation and Flood Estimation in the American River Watershed through a Total Storm Approach of Optimizing Historical Atmospheric Rivers

Extreme precipitation and flooding pose significant risk to flood control systems and existing infrastructure. Historically, probable maximum precipitation and probable maximum flood estimates have been estimated based on 3-day peaks to assess a reservoir’s ability to contain worst-case scenario flooding. However, this methodology does not account for the risk from long-duration events that contain both intense flow peaks and sustained periods of high inflow. As such, a physically based methodology is presented to produce a maximum flood estimate for six historical atmospheric rivers in the American River Watershed, draining to Folsom Reservoir near Sacramento, California. The results are compared for the maximum precipitation, maximum flood, and historical conditions to assess the underlying mechanisms that contribute to extreme flooding. The importance of the interactions between the atmosphere and land surface are highlighted, particularly the role that temperature and snow processes contribute to amplifying flooding beyond the precipitation totals alone. By using physically based numerical models to estimate extreme flooding, the resulting estimates are not based on extrapolating a probability distribution fit to historical observations and instead directly simulate the hydro-climate interactions that control the hydrologic response to intense precipitation. The time series results can provide further insight into potential extreme flooding that may increase in frequency with climate change.