Projecting impacts of wildfire and climate change on streamflow, sediment, and organic carbon yields in a forested watershed

Increasing temperatures and irregular precipitation associated with climate change, along with increasing frequency and severity of wildfires, contribute to increased downstream transport of sediment and total organic carbon (TOC), with potential impacts on aquatic ecosystem structure and resilience, recreational use of water bodies, and downstream drinking water treatment. Our study aimed to investigate the effects of both climate change and wildfires on water budget, sediment, and organic carbon by simulating the response of sub-catchments and in-stream processes to changes in future climate and wildfire scenarios. To achieve this, we applied a physical process-based hydrologic model, where an in-stream Organic Carbon Simulation Module was embedded within the Soil and Water Assessment Tool (SWAT-OCSM), to the Elbow River watershed in Alberta, Canada. Post-wildfire conditions of both moderate and high burn severities were replicated in the model within two burn perimeters to assess in-stream organic carbon processes related to particulate organic carbon (POC) and dissolved organic carbon (DOC) as state variables under changing climate. Results of the climate change scenarios indicated lower streamflow relative to the baseline period (1995-2014), particularly between May-August, with 25.3-46.9% less water in the near future (2015-2034) compared to 9.9-31.8% less water in the distant future (2043-2062). Sediment concentrations generally decreased, whereas TOC concentrations increased, in both the near future and distant future scenarios reflecting uncertainty in climate effects on water quality. Wildfire simulations compounded with climate change significantly changed local hydrology, increasing surface runoff, sediment, and TOC transport by over 500% in some study sub-catchments. However, at the watershed outlet, sediment yields only increased up to 6.5% and TOC yields increased up to 13.1%. Burn severity invoked a stronger watershed response than burn area, and greater relative changes were observed for wildfires occurring with the worst-case climate change scenarios. This study provided a strong basis for analyzing watershed responses to potential future wildfires. However, recommendations are provided for further model developments to account for wildfire consequences and feedbacks with hydrological and biogeochemical processes.