PNNL

Abstract

Historical records in the Delaware River Basin (DRB) reveal complex and spatially diverse flood-generating mechanisms influenced by the region’s mountains-to-plains gradients. This study focuses on future flood hazards and understanding the underlying mechanisms driving these changes across the region. By analyzing the simulated hydrometeorological states for each flood event using a process-based hydrological model, we attribute floods at the subbasin scale to mechanisms including snowmelt, rain-on-snow (ROS), and short-duration and long-duration rain. These rain mechanisms are further categorized based on the condition of the soil before the event (AMC), whether dry, normal, or wet. Historical analysis suggests that ROS is the dominant flood mechanism at high elevations, while floods at low elevations are dominated by short rain under normal AMC. ROS floods are more frequent at high elevations but exhibit lower magnitudes compared to floods caused by short rain under normal AMC, which have the highest mean flood magnitude in much of the DRB. To account for climate projection uncertainties, we use an ensemble of general circulation model (GCM) scenarios in future flood simulations. Despite disagreements in flood projections among GCM scenarios, a coherent perspective emerges from the majority of scenarios. Under a warmer and wetter future climate, most GCM scenarios predict a shift in flood mechanisms at high-elevation basins, marked by a significant decrease in ROS floods and a substantial increase in short rain floods with wet AMC. Overall, extreme floods are projected to increase across the DRB, with the largest increase caused by short rain with wet AMC in high elevations and short rain with normal AMC in low elevations.