Estimation of achievable performance standards for nutrients and sediments in streams draining agricultural watersheds through scenario-based modeling

For Canada’s National Agri-Environmental Standards Initiative (NAESI), SWAT model was used to develop achievable performance standards (APS) for nutrients and suspended sediments (TSS) in streams of two agricultural watersheds. For the Black Brook watershed (BBW) in New Brunswick, simulations of BMP implementation (including diversion terraces, crop rotation, tillage practices, and fertilizer application rates) forecast reductions in average concentrations of TSS by 92%.  The provisional average daily APS value for BBW was 56.7 mg/L for TSS.  For nutrients, provisional APS for average daily concentrations of nitrate-N and soluble P were 2.13 and 0.010 mg/L, respectively.  Compared to current conditions, the impacts of these BMPs on water quality are greater for sediments and soluble P than nitrate-N.  For the Raisin River watershed (RRW) in Ontario, simulations of “ideal” and realistic biodiversity conservation scenarios (including changes to extent of forest and wetland coverage, and buffer zone widths) forecast that TSS would be reduced by 12.8% and 2.6%, respectively, in comparison with current conditions, and that APS levels of 1.78 mg/L for TN and 0.031 mg/L for TP could be achieved.  APS for both watersheds are likely to change as additional BMPs are incorporated into the modeling framework (i.e., grassed waterways and riparian management for BBW, and tile drainage management and restriction of livestock access for RRW).