Agricultural extensification and forest cover loss can significantly impact aquatic ecosystems. This study considered the conversion of forests to agriculture (and vice versa) in an agriculturally dominated watershed in Eastern Ontario, Canada. A series of de- and reforestation scenarios were developed, and water quantity/quality simulations were executed using the Soil and Water Assessment Tool (SWAT) using 30 years of real-world weather observations. Results indicated that streamflow and sediment loads were not sensitive to forest loss, while continuing the recent rate of deforestation of 0.8% (0.2% of the watershed area) per year would, by 2032, increase annual loads of nitrate by 5.6%, total nitrogen by 1.5%, and total phosphorus by 6.8%. Additionally, the same land-use scenarios were simulated with the inclusion of vegetated filter strips (VFS) and grassed waterways. Some reforestation scenarios were sufficient to reduce total nitrogen concentrations below water quality guidelines, particularly under the combined effect of VFSs along all river reaches. However, meeting water quality guidelines for total phosphorus concentrations requires additional improvements to management practices beyond those simulated here.
Forest cover change scenarios were developed for the South Nation watershed, Ontario.
Streamflow/nutrient loads inversely vary with forest cover in SWAT simulations.
Significant reforestation is required to reduce nitrogen below federal standards.
Vegetated filters and grassed waterways can reduce sediment and nutrient exports.
Total phosphorus levels stay above provincial standards in all tested scenarios.