Evaluation of Epic for Assessing Tile Flow and Nitrogen Losses for Alternative Agricultural Management Systems

The Erosion Productivity Impact Calculator (EPIC) model has been successfully applied for agricultural policyanalyses for more than a decade. EPIC has been tested and validated under a wide range of conditions; however, there is anongoing need to further test the model to improve its prediction capabilities. In this study, EPIC was calibrated and validatedusing 3 years (19901992) of data collected from a field site near Nashua, Iowa. The models performance was evaluatedby assessing its ability to replicate the effects of various tillage and crop rotation systems on subsurface tile flow,nitratenitrogen (NO3N) loss with tile flow, and crop yield. Predicted annual average tile flows and nitrate losses in the tileflows were generally within 10% of measured values; the major exceptions were errors of 19.3% and 58.3% predicted fortile flow and nitrate loss for a notilled cornsoybean rotation system. Comparisons of monthly predictions with measuredvalues resulted in r2 values between 0.67 and 0.89 for the tile flows and 0.60 to 0.83 for the associated nitrate losses in thetile flows. Paired ttests that accounted for tillage and crop rotation effects on tile flow and nitrate losses showed that EPICwas in agreement with observed values in 11 out of 14 comparisons. However, EPIC showed a limited capability to reproducetillage and crop rotation effects on crop yield, similar to results found in several previous studies. Further testing of EPICis needed to refine and improve the models performance under conditions similar to those at the Nashua site.