Hydrologic and Water Quality Responses to Pacific Northwest Wildfires

While wildfires are known to alter streamflow and water quality, the mechanisms behind these changes are complex. Thus, uncertainty remains in the magnitude, longevity, and temporal and spatial distribution of wildfire effects, particularly for large megafires. We combine hydrologic modeling and field data collected after a large wildfire in Oregon, US in 2020 to understand the mechanisms leading to streamflow and water quality changes at the sub-catchment scale. Our hydrologic model predicted peak flow changes between -1 to 520% with variation primarily due to differences in burn area and severity, aridity, soil type, and geology. Water grab samples collected before and after the wildfire in basins burned at moderate to high severity indicate a 6% increase in dissolved organic carbon and 925% increase in total suspended solids during storm events. To better understand these increases and how wildfire alters transport mechanisms of carbon and sediment, we examined the hysteretic behavior of fDOM (a proxy for organic carbon) and turbidity in burned and unburned areas within our catchment. Analysis revealed that carbon and sediment showed stronger clockwise hysteresis in the burned basin compared to the unburned, where the concentrations peaked before the peak flow. However, this trend was only exhibited for storms in the spring and fall seasons, which are periods of drying and rewetting in our region. We hypothesize this was due to differing soil moisture conditions in the burned and unburned basins, which are altering transport mechanisms of carbon and sediment. We will further investigate the mechanisms of carbon transport after wildfire using excitation emission matrices to quantify carbon character across a range of burn severities during storm events. Understanding how wildfire alters streamflow, organic carbon, and sediment is critical to preserve aquatic ecosystem health and source water quality.