Untangling the Effects of Seasonality and Post-Fire Stream Channel
Erosion on the Hydrologic Response of a Burned Mountain Catchment
Abstract
Stream channel incision and deposition are common after wildfire, and
these geomorphic changes may impact runoff mechanisms and the
composition of pre-event and event water in runoff. To investigate this,
we monitored discharge and electrical conductivity at 6 nested sites
within a 15.5 km 2 watershed in the northern Colorado
Front Range that had recently burned, experienced large flooding, and
well-documented and significant channel erosion and deposition. Over the
study period, the watershed experienced seven precipitation events. For
each hydrograph, we separate baseflow from runoff using a new method to
characterize and account for the strong diurnal signal in the baseflow.
Electrical conductivity is used as a tracer in a two-component
end-member mixing analysis to separate the event hydrographs into event
and pre-event water. Correlation coefficients were computed between key
variables of the hydrologic response (such as runoff ratio, volumes of
event and pre-event water) to storm and basin characteristics (including
stream channel erosion/deposition, fraction of high/moderate burn
severity, precipitation intensity, and antecedent precipitation). The
strength and significance of correlations was found to vary seasonally.
In the early season, event and pre-event volumes did not vary
significantly with basin or storm characteristics. In the late season,
antecedent precipitation correlated with a decrease in event runoff (R
2 = 0.34) and total runoff (R 2 =
0.40), increased precipitation intensity correlated with an increase in
event runoff (R 2 = 0.48), and local erosion
correlated with an increase in pre-event runoff (R 2 =
0.60) and total runoff (R 2 = 0.53). These findings
indicate that seasonality and post-fire stream channel erosion influence
the makeup of runoff response, most likely through their impact on the
gradient of the near-stream groundwater table.