PNNL

The Science

Wildfires can change the dynamics of river flows and dissolved materials, but scientists still struggle to predict when and how much rivers will be affected. In particular, they lack clear answers on how wildfire size, burn severity, and local dryness influence nitrate and dissolved organic carbon (DOC) levels in rivers. To identify thresholds where changes in material transport are outside of interannual variability, a multi-institutional team of researchers used a computer watershed model to test thousands of “what if” wildfire scenarios in two idealized basins: one humid and forested and one semi‑arid with mixed land cover. They found that DOC in rivers was very sensitive to both how much of the basin burned and how severely it burned, with increases of up to severalfold even with little change in the total water flow. By contrast, nitrate reacted mainly to the fraction of area burned, and modeled changes were modest, pointing to missing postfire soil and plant processes as key drivers of the large nitrate pulses often seen in real rivers.

The Impact

This research addressed the long-standing question of why water quality responses in rivers after wildfires vary so widely and are hard to predict. The researchers used thousands of model simulations to quantify thresholds for wildfire responses in terms of both river flow and dissolved substances across humid and semi-arid settings. By leveraging the model’s framework and process representations, they were able to isolate impacts of hydrologic changes from impacts of changes in soil nitrogen and carbon pools. This allowed them to see that transport-dominated processes affected DOC in response to both burned area and severity, while changes in nitrate were mostly in response to burned area. The work also identified clear thresholds for burned area that triggered river responses, which help explain why some wildfires lead to noticeable effects while others do not. Fields such as hydrology, ecology, biogeochemistry, and watershed modeling can build on these findings to improve understanding of river responses to landscape disturbance.

Summary

A multi-institutional team of researchers used a process-based watershed model to examine how wildfire extent, burn severity, and aridity influenced river flow and the transport of nitrate and DOC in the first year following a wildfire on lands that drain into a river. They conducted a total of 3,660 simulations across two model watersheds with similar size and topography but contrasting aridity. Each simulation varied the percentage of land burned, burn severity, and postfire precipitation to isolate how hydrologic changes affected river responses. The study defined response thresholds as the point where postfire conditions exceeded the natural interannual variability seen in unburned simulations.

The results showed that thresholds differed by watershed type, burn severity, and analyte. In the humid, forested watershed, flow increased once moderate to large portions of the basin burned, with lower thresholds at higher burn severities. In the drier watershed, flow responses remained within natural variability, even when most of the watershed burned. DOC responded strongly in both watersheds, with relatively low burned-area thresholds, reflecting increased transport along surface and shallow flow paths. Nitrate responses were smaller and less consistent, indicating that changes in water movement alone cannot explain the large nitrate increases often measured after wildfires. Together, the findings highlight that postfire river responses emerge from different physical pathways depending on the analyte, burn conditions, and watershed characteristics.

The initial draft of the text above was created using ChatGPT (version 5.5 or lower, OpenAI). The language and content were subsequently edited by the author for grammar, clarity, and accuracy, and the final document was reviewed by the author. 

Research Contact(s)

Allison Myers-Pigg

Pacific Northwest National Laboratory 

allison.myers-pigg@pnnl.gov

James Stegen, River Corridor SFA principal investigator

Pacific Northwest National Laboratory

james.stegen@pnnl.gov

Funding

This research was supported by the Department of Energy, Office of Science, Biological and Environmental Research program, Environmental System Science program. This contribution originates from the River Corridor Scientific Focus Area (SFA) project at Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle Memorial Institute for the Department of Energy.

Related Links

• https://data.ess-dive.lbl.gov/view/doi:10.15485/2529445
• River Corridor Scientific Focus Area PNNL web page