Sensitivity analysis of the Virginia phosphorus index management tool

The phosphorus index (P-Index) is a risk assessment and management tool to aid in reducing the risk to water quality due to movement of excess phosphorus (P) from fields. An allowable P application rate is specified based on the computed risk. This study focused on the Virginia P-Index. The objectives were to determine the factors to which the P-Index is most sensitive and to determine the factors to which the resulting P application rate recommendations are most sensitive. A differential analysis was used to calculate relative sensitivity for nine baseline scenarios: a low, medium, and high baseline within three different regions of Virginia. Impact of user variability in estimating factor values was evaluated using a probability distribution analysis. The P-Index was most sensitive in the low and medium baseline scenarios to P management factors, including annual application rate, method of fertilizer application, and source availability factor In high-risk baseline scenarios, the P-Index was most sensitive to transport factors (erosion, runoff, or leaching). User variability had a greater impact on the P-Index and on P application rate recommendations as P risk increased over each of the three regions. The closer afield's P-Index value is to a threshold between P recommendation rate categories, the higher the probability that alternate P application rates will be recommended due to user variability The results highlight the need for consistent estimation of factor values, particularly the factors to which the P-Index is most sensitive and particularly in the higher-risk situations.