Variability in Hydrologic Response to Wildfire between Snow Zones in
Forested Headwaters
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 (MI 60) thresholds yearly for each catchment
by determining which MI 60 value best separated
rainstorms 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 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 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.