Untangling the effects of seasonality and stream channel erosion on the runoff composition in a previously burned mountain catchment

Abstract

AbstractStream channel incision and deposition are common after wildfire, and after these geomorphic changes occur, they 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 six nested sites within a 15.5 km2 watershed in the northern Colorado Front Range that had burned several years prior, and then experienced large flooding and well‐documented and significant channel erosion and deposition in the following years. Over the study period, which occurs 3 years after the fire, and 2 years after the beginning of significant geomorphic changes, 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 (R2 = 0.34) and total runoff (R2 = 0.40), increased precipitation intensity correlated with an increase in event runoff (R2 = 0.48) and local erosion correlated with an increase in pre‐event runoff (R2 = 0.60) and total runoff (R2 = 0.53). We hypothesize that the difference in correlations between early season and late season is due seasonal variations in the groundwater table gradient. These findings indicate that seasonality and postfire stream channel erosion influence the makeup of runoff response.