Hydrological impacts of wildfire and climate change on sediment and organic carbon loads at the watershed scale

  • Author / Creator
    Danielle Loiselle
  • Climate change, extreme weather events, and disturbances such as wildfires alter hydrology, which in turn influences the cycling of water quality constituents such as sediments and nutrients. Organic carbon (OC) is an important element affecting water quality, as it can transport heavy metals, contaminants, and can support bacteria and biofilms. A study was undertaken with the goal of identifying the dominant hydrological processes affecting sediment and OC load, transport, and fate at the watershed scale, and developing a framework for simulating the impacts of future climate change and wildfires on sediment and OC export, and downstream water quality. The Soil and Water Assessment Tool (SWAT) hydrological model was used to simulate hydrology, erosion, and sediment transport at the watershed scale. The Elbow River watershed of southern Alberta, Canada was chosen as a study watershed due to its diverse landscapes, heterogeneous hydro-climate conditions, and access to long-term water quality data set. An in-stream OC module was incorporated into the SWAT model in order to connect terrestrial and aquatic OC processes, and simulate in-stream transformations between OC compounds. The new module successfully captured the seasonality of monthly loads, and indicated that snowmelt and rainfall are responsible for peak total organic carbon (TOC) loads, and peak streamflow and sediment loads in late spring and early summer. The calibrated model was then used for scenario analyses, in which the downscaled climate projections from five General Climate Models (GCMs) for RCP 2.6 and RCP 8.5 emission scenarios were fed into the model. To model wildfires, land cover and soil properties were updated in the SWAT source code to replicate post-wildfire conditions. The average results from the ensemble of GCMs indicated a sharp decrease in near future (2015–2034) streamflow and sediment yields compared to baseline conditions (1995–2014), particularly between May-August. The distant future (2043–2062) scenario results indicated a slight increase in streamflow and sediment yields relative to the near future, but were still significantly lower than baseline conditions. In both near and distant future scenarios the TOC loads decreased, however, their relative concentrations increased, indicating poorer water quality compared to baseline conditions. Wildfire simulations largely influenced processes within the wildfire boundary, where surface runoff and the transport of sediments and TOC increased by more than 500% relative to climate change scenarios without wildfires. Impacts were also detectable at the watershed outlet, where annual streamflow, sediment yields, and TOC yields increased by a maximum of 9.3%, 6.5%, and 13.1%, respectively. Greater relative changes were observed for wildfires combined with the high emission climate change scenario (RCP 8.5) compared to the low emission scenario (RCP 2.6), and the watershed responded more strongly to burn severity than burn area. In summary, the development of the SWAT Organic Carbon Simulation Module for in-stream process representation and simulation of the impacts of wildfire was deemed a substantial step in modelling water quality under a changing environment. Model results indicated that climate change and wildfires would act in a synergistic fashion and negatively influence water quality in the future. However, further refinement of model parameters are necessary, such as simulating a top layer of debris and ash following wildfires. As well, this research has highlighted the importance of collecting pre-wildfire and post-wildfire data for quantifying impacts.

  • Subjects / Keywords
  • Graduation date
    Fall 2019
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
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