Honors College Thesis
 

A preliminary assessment of post-fire dissolved organic carbon concentrations in the McKenzie River basin, Oregon

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https://ir.library.oregonstate.edu/concern/honors_college_theses/2n49t9397

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  • In recent decades, the occurrence of large, high severity wildfires has increased substantially in many parts of the world, including the western United States. These fires have the potential to impact water quality, aquatic ecosystems, and downstream communities for many decades. As such, it is increasingly important to understand the magnitude of post-fire water quality impacts. In particular, there is increasing concern about the effects of wildfires on dissolved organic carbon (DOC) due to its influences on global carbon cycles, nutrient and heavy metal transport, stream ecosystem processes, algal productivity, and drinking water treatment. Recent studies across different geographic regions have shown contradictory results on this topic, which has required additional research. In 2020, the Holiday Farm Fire burned over 70,000 hectares in western Oregon’s McKenzie River basin. We analyzed data from 27 stream sites (11 of which were burned and 16 of which were unburned) in the basin over 13 pre-fire years and 3 post-fire years to assess the effects of the fire on DOC concentrations. We also explored the effects of the fire on the roles of precipitation and landscape drivers (i.e., burn severity and catchment topography) on DOC concentrations. During the pre-fire period, mean DOC concentrations in the burned catchments of the McKenzie River basin during the fall were on average ~2,006 µg/L (95% CI: -177–4,189 µg/L) greater than during the summer. Comparatively, in the same period mean DOC concentrations in the unburned catchments during the fall were on average ~981 µg/L (95% CI: 316–1,647 µg/L) greater than during the summer. In the post-fire period, mean DOC concentrations in the burned catchments during the fall were on average ~1,442 µg/L (95% CI: 655–2,229 µg/L) greater than during the summer. Comparatively, in the same period mean DOC concentrations in the unburned catchments during the fall were on average ~622 µg/L (95% CI: -251–1,496 µg/L) greater than during the summer. We found strong evidence that DOC concentrations in the post-fire period in both burned and unburned catchments were positively related to precipitation with concentrations increasing by 3.6 ± 0.9 µg/L per millimeter of precipitation. We also found strong evidence that DOC concentrations in the post-fire period in both burned and unburned catchments were negatively related to elevation with concentrations decreasing by –1.6 ± 0.2 µg/L per meter of elevation. Lastly, the linear relationships between DOC concentrations and precipitation depths were found to differ between burn severity classes, as the rates of change in DOC concentrations per millimeter of precipitation were greater in unburned and low severity catchments than in moderate and high severity catchments. Our study provides valuable insights into the post-fire DOC response in the streams of the western Cascades. However, our data do not show definitive trends from which we can confidently draw conclusions; instead, there is substantial variability and uncertainty in the responses of DOC across our study sites. Therefore, it remains critical to continue to quantify DOC across different subregions and post-fire conditions.
  • Key Words: wildfire, dissolved organic carbon, precipitation, elevation, burn severity
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