Modifying SWAT-CS for simulating chloride dynamics in a Boreal Shield headwater catchment in south-central Ontario, Canada

Rising chloride concentrations in surface water due to applications of deicing practices is proving detrimental to aquatic systems. In this study, a new chloride module is developed for a version of the Soil and Water Assessment Tool specially designed for Canadian Shield catchments (SWAT-CS) to model long-term chloride dynamics in a headwater catchment in south-central Ontario, Canada. In this modified model (SWAT-CS-CL; extended SWAT-CS model for chloride), chloride sources, sinks, internal storages or pools, and movement between these components are depicted. Performance of SWAT-CS-CL is assessed using a two-stage evaluation process based on the generalized likelihood uncertainty analysis (GLUE) framework. SWAT-CS-CL was found to perform moderately well, with simulated monthly chloride in streams and lake outflow following overall chloride trends and capturing regular chloride dynamics. However, simulations fail to consistently reproduce some instances of large or low chloride fluxes. Limitations in simulating large chloride fluxes may be attributed to the inadequate ability for SWAT-CS-CL to closely simulate snowpack and snowmelt processes. Parameter transferability among sub-catchments does suggest that there is a potential to extend SWAT-CS-CL to other Canadian Shield catchments for chloride modelling. Further improvements are needed through more trials to other catchments in a same or different landscape, and by modifying the simulation structure, especially representation of snow hydrology and chloride inputs.