Estimation of Maximum Precipitation and Flooding in the American River Watershed: An Analysis of Historic Storm Events to Understand Potential Future Extremes

Maximum precipitation and flood (MP and MF) have become a common tool for estimating the magnitude of potential extreme events for a given area. The American River watershed (ARW), draining into Folsom Dam, is an important watershed for the Sacramento region due to its flooding potential and subsequent protection. To further discern possible extreme precipitation and the corresponding hydrologic response, a study was done to provide new estimates of the critical atmospheric conditions and flooding for ARW by utilizing Atmospheric Boundary Condition Shifting and Relative Humidity Optimization of historical storms through dynamical downscaling of reanalysis data and physical models. The driving mechanism for extreme precipitation in the ARW region is atmospheric rivers (AR) and as such, this analysis was focused on five historic severe and important AR events. In the past, maximum precipitation estimates have generally been limited to 72-hour precipitation depths, however, this study expands the timeframe to consider the entire storm event in the estimation. Given that there is no limit on the storm times examined, further insights into flow volumes and precipitation return periods can be analyzed. By evaluating various extreme historical storm events over their entire period, this study can provide decision makers with important information about potential severe precipitation and flooding events in the future.