Modeling conservation practices in APEX: from the field to the watershed

Evaluation of USDA conservation programs are required as part of the Conservation Effects Assessment Project (CEAP). The Agricultural Policy/Environmental eXtender (APEX) model was applied to the St. Joseph River watershed, one of CEAP's benchmark watersheds. Using a previously calibrated and validated APEX model, the simulation of various conservation practices (single and combined) was conducted at the field scale. Seven variables [runoff, sediment, total phosphorus (TP), dissolved reactive phosphorus (DRP), soluble nitrogen (SN), tile flow, and soluble nitrogen in tile (SN-Tile)], were compared between the simulated practices. The field-scale outputs were extrapolated to the areas encompassed by the different conservation practices at the watershed scale. The speculative estimations are presented as percentage reductions compared to the baseline scenario. When single conservation practices were implemented, reductions were 39% for sediment, 7% for TP, and 24% for SN-Tile. In contrast, losses of DRP and SN increased by 5% and 57%, respectively. When the conservation practices were combined, percentage reductions increased for all variables. The total reductions for combined two and three practices were 68% and 91% for sediments, 35% and 74% for TP, 1% and 48% for DRP, − 43% and 28% for SN, and 50% and 85% for SN-Tile. Negative reductions were due to the slightly higher DRP and SN loads in no-till, mulch-till, and conservation crop rotation practices, and their greater extent of incorporation at the watershed scale. Overall, the cumulative and combined effects of field conservation practices can help address the watershed's excess nutrient and sediment concerns and improve water quality.