Influence of Climate Change on 21st Century Fire Regime in the City of Portland Oregon's Source Watershed

Abstract:

Increasing fire disturbance in the U.S. Pacific Northwest due to climate change is linked to significant changes in forest watershed hydrology and water quality, including increases in runoff, erosion, suspended sediment, and debris flow. Studies have projected that, by the end of the 21st century, annual area burned by wildfire will increase 200-400% in forests west of the Cascade Mountains relative to the natural fire regime. This could cause complications for municipal water utilities that depend on surface water sources from forested watersheds, particularly in places with historically infrequent fire regimes, where empirical data and post-fire planning tools do not currently exist. In this study, we apply a process-based ecohydrologic model (RHESSys) which is coupled to a stochastic fire spread model (WMFire), to explore the influence that climate change could have on the fire regime of a municipal watershed throughout the 21st century (with analysis periods focused on the 2040s and the 2070s). The Bull Run Watershed was used as the target watershed; it is located west of the Cascades in northern Oregon, and is the primary drinking water source for the city of Portland. Results indicate that climate change will increase the frequency of fires events but decrease their spatial extent because of future fuel limitations. We demonstrate that this historically flammability-limited watershed is becoming less limited by climate conditions as temperatures warm and the watershed becomes more arid. This research improves our understanding of how climate change influences fire regime and may have implications for drinking water quantity and quality. Our model and results can also be used by source water managers for risk assessment and mitigation planning.