Modelling Streamflow and Phosphorus Fluxes in the Lake of the Woods Watershed

This study provides an evaluation of streamflow and the spatial and temporal variability of phosphorus (P) fluxes for the transboundary Lake of the Woods (LoW) watershed using the Canadian version of the Soil and Water Assessment Tool (CanSWAT). The model calibration and validation generally indicate good performance for the simulated flow, especially for the Rainy River, the main tributary to LoW, while the sediment and nutrient calibration performance was satisfactory. Model results indicated Rainy River is the primary source of total phosphorus (TP), contributing about 88% of the external non-point source (NPS) and point source P loads to LoW, with the majority being NPS. Simulated TP loads varied seasonally with over approximately 60% occurring during the spring period and varied spatially across the LoW watershed. TP yields tended to be lower upstream of Rainy Lake in the Precambrian Shield (a.k.a., Canadian Shield) and higher downstream of Rainy Lake in the Glacial Lake Agassiz lakebed, particularly in the Lower Rainy and Little Fork sub-watersheds. Point sources along the Rainy River constituted the largest anthropogenic TP source. Tributary P loads estimated by the model were also used in a simple lake mass-balance model that suggested 32–46% of TP load to LoW was retained within the lake.