Integrated snow and hydrology modeling for climate change impact assessment in Oregon Cascades

Abstract:

In the Pacific Northwest (PNW), increasing temperatures are expected to alter the hydrologic regimes of streams by shifting precipitation from snow to rain and forcing earlier snowmelt. How are such changes likely to affect peak flows across the region? Shifts in peak flows have obvious implications for changing flood risk, but are also likely to affect channel morphology, sediment transport, aquatic habitat, and water quality, issues with potentially high economic and environmental cost. Our goal, then, is to rigorously evaluate sensitivity to potential peak flow changes across the PNW. We address this by developing a detailed representation of snowpack and streamflow evolution under varying climate scenarios using a cascade-modeling approach. We have identified paired watersheds located on the east (Metolius River) and west (McKenzie River) sides of the Cascades, representing dry and wet climatic regimes, respectively. The tributaries of these two rivers are comprised of contrasting hydrologic regimes: surface-runoff dominated western cascades and deep-groundwater dominated high-cascades systems. We use a detailed hydro-ecological model (RHESSys) in conjunction with a spatially distributed snowpack evolution model (SnowModel) to characterize the peak flow behavior under present and future climate. We first calibrated and validated the SnowModel using observed temperature, precipitation, snow water equivalent, and manual snow survey data sets. We then employed a multi-objective calibration strategy for RHESSys using the simulated snow accumulation and melt from SnowModel and observed streamflow. The Nash–Sutcliffe Efficiency between observed and simulated streamflow varies between 0.5 in groundwater and 0.71 in surface-runoff dominated systems. The initial results indicate enhanced peak flow under future climate across all basins, but the magnitude of increase varies by the level of snowpack and deep-groundwater contribution in the watershed. Our continuing effort is to link these hydrologic changes with the channel morphology and fluvial hydraulics of streams in order to evaluate the geomorphic implications of these hydrologic changes.