New Modeling Techniques Provide Fresh Insight to Red River Phosphorus Sources

A newly published study has used a modeling approach developed by the IJC for transboundary watersheds to better pinpoint the sources of nutrient pollution and their contributions in the Red River basin, spanning parts of Manitoba, North Dakota, South Dakota and Minnesota.

The SPARROW modeling approach had previously been applied to the Red-Assiniboine River basin in the 2010s. It was developed through IJC partnerships with several federal and provincial agencies, notably the US Geological Survey, Environment and Climate Change Canada (ECCC) and the National Research Council Canada. A study published in 2016 broke the system down into smaller, sub-watershed categories and used harmonized data from both countries to identify which areas saw the largest amount of nutrients (specifically phosphorus and nitrogen) entering the broader system. 

To calibrate this model, the researchers adjusted their model to best match observed loads, identifying statistically significant sources for nutrient runoff. This approach had its limits, however, as it was unable to quantify specific results for wastewater treatment plants and how they are influencing nutrient levels in the Red River and Lake Winnipeg. Instead, the researchers had to make an educated assumption about the amount of nutrients entering the waterways from treatment plant discharges. The IJC published a technical report on this effort in 2015, which showed how the transboundary, multijurisdictional datasets are assembled and harmonized.

This new study, published in the npj Clean Water journal, continued to use the SPARROW model approach, but adjusted how these models were initially calibrated. They used what’s known as a “Bayesian” approach, where unknown factors are treated as probabilities that are updated as more information becomes available. This process takes more computational time but can be helpful in situations such as the Red River’s nutrient runoff issues. This is the first time a Bayesian approach had been used with SPARROW in a binational watershed, and as part of this process, the new team of researchers working with members of the original team ran the SPARROW model through multiple scenarios and compared them against observations and the original study’s results.

Across the four key scenarios, the researchers found that approximately 54-62 percent of total phosphorus going into Lake Winnipeg from the Red River basin came from US sources, slightly lower than the 62 percent contribution the original study estimated. Agriculture was the biggest source of phosphorus, contributing between 62-72 percent of total loads; this was broken down further between fertilizer (49-65 percent) and manure (8-16 percent), with slightly more coming from the United States. Wastewater treatment plants were estimated at contributing 12-15 percent, with the bulk of that coming from Canada and the city of Winnipeg. The remainder of the modeled phosphorus loads came from stream channels.

Due to the SPARROW approach, the researchers were also able to pin down the areas with the highest amounts of phosphorus entering the basin. These included the Lower Red River and Rat River sub-sub-drainage areas in Canada and the Upper Red, Western Wild Rice and Buffalo areas in the United States. These are all largely agricultural hotspots, and this information can help with targeted nutrient reduction efforts.

This project was funded by ECCC through its Freshwater Action Plan for Lake Winnipeg.