UC Davis

Maximum precipitation and maximum flood estimates are a common tool for flood management and reservoir operations; to ensure a reservoir can safely operate during extreme events and provide protection to downstream communities. This is especially true for the American River watershed, draining into Folsom Reservoir, just upstream of the city of Sacramento, CA. A new methodology is presented to provide additional estimates for maximum precipitation and maximum floods by expanding the typical 72-hr estimates to maximize for both total precipitation over the entire storm period and the cumulative volume. First, atmospheric and hydrologic conditions are reconstructed for the period from 1852 to 2016 at hourly increments and a 4 km spatial resolution after the relevant physical model are calibrated and validated. The focus is on three historical atmospheric river events of importance for the American River. Each event is maximized through Atmospheric Boundary Condition Shifting and Relative Humidity Optimization, through dynamical downscaling of atmospheric reanalysis data and physical models. Given that there is no limit on the storm period examined, further insights into cumulative flow volumes and their return periods can be analyzed. For each of the three maximum flood estimates and their corresponding durations, probability distributions are fit to the annual maximum cumulative volume of the maximum flood duration for each year in the reconstruction period. Of the three historical events analyzed, the maximized 1997 flood is estimated to have a return period of 2,471 years. With a time series of the maximized precipitation and resulting inflows and cumulative volumes over the entire storm period, reservoir operators can better prepare for intensifying events that are expected due to climate change.