Effects of climate change on Olympic National Park flood risks

A warmer future climate is expected to alter flood risks in the Pacific Northwest, posing considerable challenges to natural resource managers in the Olympic National Forest (ONF) and Olympic National Park (ONP). A key part of management in the ONF and ONP involves maintaining the network of forest roads, and particularly those located near creeks and rivers. The sizing of replacement culverts and related infrastructure is already a problematic aspect of maintaining the road system, and climate change impacts introduce additional challenges. Current practice uses estimates of Q100 (or the peak flow with an estimated 100 year return frequency) as the standard metric for stream crossing design. Simple regression models relating annual precipitation and basin area to Q100 are used in the design process. Under the projections of a changing climate, these methods for estimating flood risk are ill equipped to capture the complex and spatially varying effects of seasonal changes in temperature, precipitation, and snowpack on flood risk. As an alternative approach, this study applies a physically-based hydrologic model to estimate historical and future flood risk at 1/16th degree (latitude/longitude) resolution (about 32 km2). We downscaled climate data derived from 10 global climate models to the 1/16th degree spatial resolution over the region of the Olympic Peninsula. The downscaled meteorological data are then used as input for the Variable Infiltration Capacity (VIC) model, a macro-scale hydrologic model, which simulates various hydrologic variables at a daily time step. At each 1/16th degree grid cell baseflow and run-off were used to estimate Q100 for the historical period and under two emission scenarios, A1B and B1, at three future time intervals: the 2020s, the 2040s and the 2080s. These ensemble estimates of changing flood risk will serve as a tool for evaluating needed changes in culvert design in the Olympic Peninsula by providing high-resolution maps of changing flood risk over the ONF and ONP.