PNNL

Abstract

Wildfires can significantly impact terrestrial landscapes, affecting downstream river corridors. We investigated the impacts of burn area and burn severity on hydrologic and biogeochemical responses and identified thresholds in burn area required to observe an aquatic response in key hydrologic and biogeochemical parameters post-fire. We used the Soil and Water Assessment Tool (SWAT) model to simulate streamflow, nitrate, and dissolved organic carbon responses to wildfire in two test basins: a humid forested basin and a semi-arid mixed land use basin, allowing us to explore how responses differed with aridity and land use. We ran 1830 wildfire simulations across a range of burn areas, burn severities, and post-fire precipitation for each test-basin. Overall, our work suggests that burn area thresholds differ with burn severity and analyte. Additionally, we determined that post-fire transport of dissolved organic carbon was sensitive to both burn area and burn severity, while nitrate was primarily sensitive to burn area. Despite a muted hydrologic response in the semi-arid basin, we predicted large shifts in dissolved organic carbon, suggesting post-fire shifts in flow pathways and soil properties play a key role in the dissolved organic carbon response. The limited response of nitrate post-fire in our simulations highlight that post-fire transformations in nitrate in the terrestrial landscape may control the increases in stream nitrate often observed post-fire, not changes in hydrology. Noticeable discrepancies between our modeled results and field observations, likely due to biogeochemical processes that we were unable to include in the model, highlight the importance of continuing to develop a mechanistic understanding of post-fire biogeochemical cycling to improve post-fire watershed modeling predictions. As wildfire regimes are shifting in many parts of the world, improving our understanding of post-fire responses is critical to protect freshwater resources and aquatic ecosystems.