The Influence of Wildfires on Hydro-Biogeochemical Processes: A ModEx Perspective
Xingyuan Chen1* (Xingyuan.Chen@pnnl.gov), Allison Myers-Pigg2,3, Morgan Barnes1, Kevin Bladon4, Glenn Hammond1, Peishi Jiang1, Hyunwoo Kang4, Zhi Li1, Tim Scheibe1, Katherine Wampler4
1Pacific Northwest National Laboratory, Richland, WA; 2Marine and Coastal Research Laboratory, Pacific Northwest National Laboratory, Sequim, WA; 3The University of Toledo, Toledo, OH; 4Oregon State University, Corvallis, OR
The iterative ModEx approach underpins hypothesis-driven research that advances predictive understanding of complex environmental systems. Such an approach often consists of one technical ModEx cycle that includes using experimental or field data to parameterize and calibrate models and using models to guide data collection in the field and/or in the lab. This study further defines a knowledge cycle that builds upon both modeling and experiments to generate new hypotheses that advance the state of predictive understanding.
Successful ModEx practices rely on coordinated team efforts to enable information exchange at both the technical and knowledge levels. This project utilizes this approach to understand the influence of wildfires on river corridor hydro-biogeochemistry. The Wenas Creek watershed within the Yakima River Basin was selected as a testbed, with 76,000 acres of semi-arid shrub-steppe landscape burned at low to moderate severities during the 2020 Evans Canyon Fire. Because fires directly alter landcover and surface soil properties, the research team hypothesized that wildfires would increase surface runoff and reduce infiltration, thus leading to higher and earlier peak flow in responding to precipitation events. The team used the integrated watershed model Advanced Terrestrial Simulator to study post-fire watershed hydrologic responses by explicitly representing changes in surface soil permeability and vegetation properties. By comparing simulated responses with and without fire impacts, researchers found no significant differences in hydrology. Field observations showed no statistical differences in the concentration of measured dissolved organic carbon (DOC), total dissolved nitrogen, pH, dissolved oxygen, or total suspended solids between burned and reference sites across monthly sampling for the first 2 years following the fire.
These observations fueled a new hypothesis that stream responses to fires are dependent on the relationship among fire size, severity, and catchment characteristic. Soil and Water Assessment Tool model simulations—where wildfire and catchment characteristics were manipulated—indicated that increases in DOC were not linearly related to fire severity. Rather, the highest DOC loads occurred when the catchment was burnt at moderate severity, while the highest peak concentrations occurred at low severity. 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, a new hypothesis that will be tested by further modeling and observations.