Hydrologic responses to the 2020 Labor Day fires in western Oregon

Effects of wildfires on vegetation cover and soil properties can dramatically change watershed hydrology and water quality in Pacific Northwest (PNW) regions. In September 2020, extensive and high-severity wildfires occurred in western Oregon due to strong winds and long periods of warm and dry conditions. This study evaluated the hydrologic responses of the fire based on observed streamflow and satellite-based meteorologic data including precipitation (P), potential evapotranspiration (PET), and actual evapotranspiration (ET). Nine burned and five unburned watersheds were selected and delineated with varying fire characteristics (e.g., burned areas, and burn severities). Burn severities of the watersheds were represented by the average Differenced Normalized Burn Ratio (dNBR). Runoff ratio, ET ratio, and Budyko Curve analyses were used to compare hydrologic conditions for the pre- (2001 - 2020) and post-fire (2021 - 2022) periods. Increases in runoff ratio and decreases in ET ratio were observed during the post-fire period, and the magnitude of differences was larger as the fire severities and burned areas increased. The Budyko Curve also showed robust differences in the evaporative index (Δd) with highly burned watersheds due to dramatic decreases in ET, while differences between less burned and unburned watersheds were insignificant. In addition, strong linear relationships were found between average dNBR and Δd, and burned areas and Δd. The R2 values were 0.76 and 0.84 in 2021, and 0.56 and 0.38 in 2022, which revealed the linear relationships were weaker in 2022 due to vegetation recoveries. Our findings suggest that increases in runoff and decreases in ET are likely to initiate and persist at least 2 years after the disturbances. Since the duration and seasonality of post-fire impacts are still debatable, our findings bring new insights into the post-fire responses and may facilitate conversations among watershed managers in the PNW region.

Impact/Purpose

EPA and OSU scientists are working together to examine the effects of the large, high severity fires in September 2020 that occurred throughout western Oregon.  This work involves examining pre and post fire data on the hydrology and biogeochemistry after fires in the area, using existing gauge data and water quality data collected by US EPA and Oregon Department of Environmental Quality.  This abstract explores the hydrologic response post-fire and shows that runoff increases for at least two years after these large fires, and that the increase is generally proportional to the fire extent.  

Citation

Kang, H., R. Cole, L. Miralha, K. Bladon, AND J. Compton. Hydrologic responses to the 2020 Labor Day fires in western Oregon. USFS Post-Fire Science and Monitoring Symposium Abstract Submission Form, Corvallis, OR, February 07 - 08, 2023.