Watershed Hydrologic and Biogeochemical Responses to Wildfires in the Pacific Northwest


Zhi Li1* (Zhi.Li@pnnl.gov), Bing Li1, Peishi Jiang1, Glenn Hammond1, Pin Shuai2, Ethan Coon3, Katie Muller1, Allison Myers-Pigg1, Morgan Barnes1, Hyun-Seob Song4, Xingyuan Chen1, J. David Moulton5


1Pacific Northwest National Laboratory, Richland, WA; 2Utah State University, Logan, UT; 3Oak Ridge National Laboratory, Oak Ridge, TN; 4University of Nebraska–Lincoln, NE; 5Los Alamos National Laboratory, Los Alamos, NM



The IDEAS-Watersheds project focuses on developing general modeling capabilities and workflows that leverage a community software ecosystem to advance hydro-biogeochemical research in watersheds and river corridors, and in turn make these advances available to the broader community. The IDEAS-Watersheds partnership with the Pacific Northwest National Laboratory River Corridor science focus area aims to understand and quantify processes governing the cumulative effects of hydrologic exchange flows, dissolved organic matter (DOM) chemistry, microbial activity, and disturbances on river corridor hydro-biogeochemical function from watershed to basin scales. Researchers use the Advanced Terrestrial Simulator (ATS) and PFLOTRAN code coupled through the Alquimia interface (ATS-PFLOTRAN) to investigate watershed post-fire hydrologic and biogeochemical responses under various precipitation scenarios across a range of watersheds. In addition to using MODIS-sensor-informed phenology to capture post-fire vegetation cover changes, a new fire module within ATS-PFLOTRAN was also implemented to account for the water repellency of the surface soil burned at different severities. The top layer soil permeability and hydrophobic layer depths were modified based on mapped burn severity from the Monitoring Trends in Burn Severity database. The model results reveal that wildfires enhance surface runoff and suppress infiltration, with nonlinear relationships with post-fire precipitation events that vary under different combinations of pre-fire land cover types with pre-fire surface soil permeabilities. This modeling framework is poised to account for fire-induced changes in DOM characteristics by integrating the parallel IDEAS-Watersheds effort in developing pipelines that link organic carbon speciation with biogeochemical models and then with continuum reactive transport models in PFLOTRAN. Narrative examples were shared with the broader community in the Jupyter notebook to encourage adoption of such mechanistic watershed hydro-biogeochemical models in other watersheds and river corridors. This work is also an example of interoperable model development, where generic interfaces such as Alquimia extend the capabilities available from single codes by bringing the capabilities of other codes in the software ecosystem to bear on each specific application.