Identification of phosphorus index improvements through model comparisons across topographic regions in a small agricultural watershed in Vermont (USA)

Many U.S. states require farmers to evaluate the risk of P loss from farm fields using P site assessment tools, such as the Phosphorus Index (P-Index). The simplicity of the P-Index allows for users with various backgrounds to utilize the tool; however, the P-Index may be limited in its ability to reliably predict P losses across variable farm terrain, for instance where saturation-excess runoff generation dominates. The objective of this study was to identify potential improvements that could be made to the Vermont P-Index (VT P-Index) by comparing VT P-Index predictions of P loss with those of a model that considers topographic controls on P transport. Scenarios with varying P management strategies were developed and modeled for corn (Zea mays L.) silage production fields in a small agricultural watershed (360 ha) located in Vermont using both the VT P-Index and a modified version of the Soil and Water Assessment Tool (SWAT). Modeled outputs from both the VT P-Index and SWAT were aggregated based on topographic index (TI) class and compared for the directional agreement of P risk assessments and relative annual average dissolved and particulate P losses. The P-Index nutrient management recommendations gave higher P loss risk ratings than SWAT for 94% of the 120 unique scenario combinations simulated. Results from this study suggest that the VT P-Index could improve its ability to support farm nutrient management planning by incorporating topographic controls on runoff production into its estimation of P transport.

Automated summary

This study compared Vermont's P-Index with a model considering topographic controls on P transport to identify potential improvements. Results suggest that the VT P-Index could enhance its support for farm nutrient management planning by incorporating topographic controls on runoff production into its estimation of P transport.