PS 38-45 - Do wildfires always have downstream impacts? Implications from a semi-arid watershed

Abstract: In the Pacific Northwest, wildfires are increasing in frequency and intensity, leading to alterations in the quality and quantity of materials transported from terrestrial to aquatic environments. Prior research has shown impacts of wildfires to physical and chemical responses can often be variable. Therefore, it is important to understand how the timing and delivery of terrestrial materials to aquatic environments varies post-fire. The Evans Canyon Fire occurred in Washington in August 2020 and burned 76,000 acres of the semi-arid shrub-steppe landscape at low to moderate severities. This fire burned across the Wenas Creek watershed, allowing for a sampling design that included a portion of the stream within the burned area and a reference site upstream of the burn. Monthly water samples were collected from November 2020 to August 2022 and analyzed for dissolved organic carbon (DOC), total dissolved nitrogen (TDN), common dissolved ions, and total suspended solids (TSS). Additional water quality measurements including dissolved oxygen (DO), temperature, specific conductivity, and pH were taken during our sampling trips. We found no significant differences in the concentration of DOC, TDN, pH, DO, or TSS between the wildfire-impacted and reference site across all time points. The wildfire-impacted site was consistently higher in specific conductivity (p < 0.05), potassium (p < 0.05), magnesium (p < 0.1), sodium (p < 0.05), and sulfate (p < 0.05) concentrations. Monthly sampling showed a seasonality in many parameters including elevated TSS in the wildfire-impacted site during the spring with concentrations 6.5 times greater than the reference site. Although there was not a significant interaction between season and site for DOC or TDN, we found both sites were elevated during the spring. To better understand the observed DOC, nitrate, and TSS trends in Wenas Creek, we built an archetypal watershed using the Soil & Water Assessment Tool (SWAT) to identify thresholds of wildfire and watershed characteristics (burn severity, burned area, flow path, etc.) that could result in consistent changes to these variables post-fire. From our model simulations, we found that fire severity was not linearly related to increases in DOC. Moderate severity fires exhibited the highest DOC loads while low severity fires showed the highest peak values, this suggests complicated relationships between wildfire caused streamflow increases and DOC transport. Cumulatively, this work suggests that hydrological connectivity between the terrestrial and aquatic environment may be an important driver of post-fire responses in semi-arid watersheds.