An analysis of 100 years of post-fire streamflow responses of British Columbia watersheds

Wildfires are becoming larger and more severe due to climate change. This trend affects the forest ecosystem and disrupts many eco-hydrologic processes in forested watersheds. Effects can include rapid runoff responses, increased surface runoff, and elevated erosion, leading to lower water quality and long-lasting effects on hydrologic ecosystem services (drinking water supply or flood regulation). However, post-fire hydrology studies often have variable and contrasting results, making cross-study comparisons difficult. Studies are typically short-term and focused on single wildfire events. Additionally, hydrologic ecosystem services are not always considered. This research has two objectives: to determine accurate indicators for post-fire flow responses; and to develop a flood risk map that considers wildfire history and the hydrologic ecosystem services. The study area includes 336 drainage basins (grouped into five ecozones) in British Columbia, Canada, known for its susceptibility to wildfires and floods. The study analyzes 110 years of wildfire data from 1910 to 2020. Of the 824 wildfires in the study period, over 400 fires were identified with five years of continuous streamflow and precipitation daily flow records. Percent changes in low, high, and peak flows were calculated using pre-fire and post-fire values. Using streamflow, precipitation, wildfire perimeters, land cover and topographic data, statistical analyses were done to determine the most influential watershed characteristic in post-fire streamflow responses. To develop the flood risk map, the same data will be combined with socio-economic and demographic data. Preliminary results suggest differing trends for low, high, and peak flows for the five ecozones in BC, demonstrating the importance of geophysical variables on streamflow response. Results will aid in understanding the effects of climate change over 110 years, specifically the wildfire effects on hydrology in forested watersheds and on the hydrologic ecosystem services provided to nearby communities. The determination of accurate post-fire streamflow indicators will also help water resource managers, urban planners, and other decision-makers allocate resources appropriately for long-term water management and reduce post-fire flood vulnerability.