Hydrologic responses to wildfires in western Oregon, USA
Wildfires can dramatically alter vegetation cover and soil properties across large scales, resulting in substantial shifts in runoff generation, streamflow, and water quality. In September 2020, extensive and high-severity wildfires burned more than 340,000 ha of forest land on the westside of the Cascade Mountain Range in the Pacific Northwest. Much of the area impacted by these fires is critical for the provision of water for downstream aquatic ecosystems, agriculture, hydropower, recreation, and municipal drinking water. We undertook a study to evaluate the effect of the 2020 wildfires on streamflow in nine burned catchments in western Oregon. We also included four unburned, reference catchments in our analysis to enable us to assess post-fire streamflow changes in the burned catchments. To enable us to quantify the effects of wildfire on the catchment water balance we used publicly available streamflow data and estimated precipitation (P), potential evapotranspiration (PET), and actual evapotranspiration (ET), using satellite-based meteorological data. We quantified catchment area burned and burn severity with the average differenced normalized burn ratio (dNBR). We compared hydrologic conditions for the pre-fire (2001–2020) and post-fire (2021–2022) periods by analyzing catchment runoff ratios, ET ratios (evaporative index: quotient of ET divided by P, refer to EI hereafter), and Budyko curves. We also used random forest models to explore factors influencing the variability in EI. During the post-fire period, we observed decreases in the evaporative index and increases in runoff ratio in the burned catchments. Post-fire declines in EI were positively related to burn severity (R2 = 0.76 in 2021; 0.56 in 2022) and area burned (R2 = 0.84 in 2021; 0.38 in 2022), and were primarily driven by decreases in ET. The greatest increases in runoff (16.1% in 2021 and 19.8% in 2022) occurred in the catchment with the greatest mean dNBR and burned area (94.23%). The greatest decreases in ET were also found in the same catchment (40.2% and 32.0%). These results were reinforced by the random forest analysis, which illustrated the importance of burn severity as a predictor of EI. Interestingly, the variability in changes in EI during the post-fire period was also associated with other geomorphic factors such as catchment slope, elevation, geology, aspect, and pre-fire vegetation type. Since the duration and seasonality of post-fire impacts on hydrology remain uncertain, our findings bring new insights and guide future studies into the post-fire responses on hydrology that are crucial for water and forest management.