Seasonality of streamflow response during the decade following wildfire in Canadian Rocky Mountain watersheds

Wildfire can lead to increases in surface runoff, commonly associated with the loss of canopy and understory interception combined with the effects of soil water repellency. Since pre-disturbance streamflow data are rarely available, studies on catchment-scale responses to wildfire are less common than studies focused on hillslope runoff. While post-wildfire runoff has been widely studied in parts of the United States, Australia and other regions there have been very few studies in northern Rocky Mountain environments.

This study makes use of multiple watersheds in a post-hoc reference-impact design and several lines of evidence to quantify changes in runoff magnitude and timing for the first 10 years following the Lost Creek wildfire in southwestern Alberta, Canada. Three burned and two unburned reference catchments (3.7 to 10.4 km²) were instrumented with a network of streamflow and precipitation gauges and monitored from 2005-2014. Hourly data were used to quantify runoff magnitude and timing at four time scales: rainfall events, and total weekly, monthly and annual runoff. Hydrograph separations were performed and quickflow magnitude, basin lag, and time-to-peak were calculated as functions of several rainfall event characteristics. Following Blume et al. (2007), a linear model was developed to predict runoff coefficients from total rainfall, quickflow, rainfall intensity (I₃₀), and other predictors. Potassium-silica ratios were calculated from bi-weekly samples over the course of the study and used to infer relative change in the amount of runoff routed to surface pathways.

Although significantly higher net precipitation was measured at this site, it did not translate into large impacts on runoff, especially during snow-free periods (mid June to late September). Analysis of rainfall-runoff events during the snow-free season show no effect of the wildfire. However, weekly and monthly yields suggest earlier delivery of snowmelt runoff in burned watersheds.