475 - Maximum Flooding in the American River Watershed: Can Atmospheric Rivers be Optimized to Intensify Precipitation and Flooding

Within the American River watershed, atmospheric rivers have long been a driving mechanism in producing record breaking flooding. Given the expected intensification of atmospheric rivers due to climate change, analysis on how these storms will change the flooding regime is necessary to manage the potential increased risk to reservoir safety and downstream flooding. In the study to be presented, analysis was done by a new methodology to compute a maximum precipitation estimate for the American River Watershed utilizing a complete storm approach. The standard 72-hr estimate was expanded to maximize precipitation over the entire storm period through Atmospheric Boundary Condition Shifting (ABCS) and Relative Humidity Optimization (RHM) in the WRF atmospheric model. Five historical storms were analyzed and for each of these, 174 model runs are produced with different ABCS and RHM combinations. To simulate the corresponding hydrologic response for each of the WRF model runs, the outputs are routed through the Watershed Environmental Hydrology-Hydroclimate Model (WEHY-HCM). By modeling each simulation, a maximum flood estimate can be produced independently of the maximum precipitation estimate. In terms of reservoir management for long duration events, the interactions between precipitation, snow cover, temperature, and watershed saturation can provide further insights into how the watershed responds under different conditions by analyzing the full storm period cumulative flow volumes, as well as the peak flows. As the stationarity of the climate system changes, physically based models, such as WRF and WEHY-HCM, are vital tools in the estimation of potential future extreme events.