Evaluation of the TBET Model for Improving P-indices in Southern States

Radcliffe, D., Forsberg, A., Bolster, C., Mittelset, A., Storm, D., Ramirez-Avila, J. J., & Osmond, D. (2016). Evaluation of the TBET Model for Improving P-indices in Southern States. Soil and Water Conservation Society Annual Conference 2016. Louisville, KY.

Management of agricultural nonpoint source phosphorus (P) requires identification of fields susceptible to P loss. P-Indices are the most common tools used to identify critical source areas of P loss. However, the success of the P-index approach is impeded by insufficient measured P loss data. Simulated data from a quantitative P transport model may be used to modify a P-index for scenarios where P loss data is not available. The objective of this study was to compare predictions from the Texas Best Management Evaluation Tool (TBET) against measured P loss data to determine whether the model can be used to improve P-Indices in the South. Field-scale measured P loss data from study sites in AR, GA, and NC were used to assess the accuracy of TBET for predicting field-scale loss of P. Goodness of fit was measured using the Nash-Sutcliffe Efficiency (NSE) and Mean Absolute Error (MAE) statistics. We found that TBET can generate satisfactory event-based predictions (NSE = 0.3) of runoff, sediment and P loss with site-specific calibration. We also compared TBET predictions of average annual P loss against the three state P-indices for the sites in this study. Goodness-of-fit between measured average annual total P loss and the calibrated TBET model (MAE = 4.4 kg/ha/yr) was similar to the P-Indices (MAE = 6.9 kg/ha/yr). We conclude that a well-calibrated TBET model can be used to improve P-Indices for certain management scenarios. However, developing a version of TBET that can simulate all of the management, soil, and weather combinations that are likely to be encountered in a state will require extensive work.