Variability in hydrologic response to wildfire between snow zones in forested headwaters

Authors: MILLER, QUINN - Us Geological Survey (USGS); Barnard, David; SEARS, MEGAN - Colorado State University; HAMMOND, JOHN - Us Geological Survey (USGS); KAMPF, STEPHANIE - Colorado State University

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/25/2025
Publication Date: 5/8/2025
Citation: Miller, Q., Barnard, D.M., Sears, M., Hammond, J., Kampf, S. 2025. Variability in hydrologic response to wildfire between snow zones in forested headwaters. Hydrological Processes. 39(5). Article e70151. https://doi.org/10.1002/hyp.70151.
DOI: https://doi.org/10.1002/hyp.70151

Interpretive Summary: With increasing temperatures and aridity across the western United States, wildfires are burned at higher elevations in forested mountain headwater catchments, slowly encroaching into areas with seasonal and persistent snow cover. Fires at these elevations are unprecedented and have thus been understudied, leading to poor understanding of how fire severity and snow persistence will interact to affect stream flow responses to storm events after wildfire. This study looked at stream flow responses to high-intensity summer thunderstorms over three years following the 2020 Cameron Peak fire in northern Colorado. Study sites included catchments in areas with low annual snow fall (intermittent zone) and high elevations with significant seasonal snow pack accumulation. The intensity of rainfall was a strong predictor of stream flow response occurrence and magnitude, although higher elevation catchments were more responsive to lower rainfall intensity, likely due to higher soil moisture regardless of burn severity. There was an ambiguous response of burn severity and time-since-fire on stream flow response within snow zones but variable between snow zones, indicating more research is needed to improve understanding of how high elevation catchments will respond to increasing fire in the future and to contrast those responses to the existing studies completed at lower elevations.

Technical Abstract: Rising temperatures and shifting fire regimes in the western United States are pushing fires upslope into areas of deep winter snowpack, necessitating a new understanding of how hydrologic processes change following wildfire. We quantified differences in the timing and magnitude of quickflow responses to summer rainstorms between six catchments of varying levels of burn severity and seasonal snowpack cover for years 1-3 after the 2020 Cameron Peak fire. Our objectives were to (1) determine how burn severity and snow persistence influenced the magnitude, timing, and likelihood of a quickflow response, (2) quantify change in responses over time, and (3) identify the influencing factors for these responses. We identified maximum 60-min rainfall intensity (MI60) thresholds yearly for each catchment by determining which MI60 value best separated rain events that generated quickflow from those that did not. We used generalized linear models to determine which predictors were correlated to both the probability of a quickflow response and four quickflow response metrics: peak quickflow, total event quickflow, stage rise, and lag to peak time. We found that rainfall intensity thresholds were only good at predicting a quickflow response in the intermittent snow zone (ISZ), and that these were slightly higher than other reported post-fire thresholds for this region. Both threshold analysis and model results showed that a response was much more likely in the persistent snow zone (PSZ) than in the ISZ, likely due to the higher soil moisture content in that area. The effect that burn severity and year post-fire had on the quickflow response was more ambiguous, yet model results for stage rise indicate that widespread overland flow only occurred at the severely burned ISZ site. These results demonstrate that the streamflow responses to fire vary between snow zones, indicating a need to account for elevation and snow persistence in post-fire risk assessments.