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TECHNICAL REPORT
Ground Water Quality

EPIC Tile Flow and Nitrate Loss Predictions for Three Minnesota Cropping Systems

^a Civil Eng., Korean Water Resources Corp., Taejon, South Korea
^b Center for Agric. and Rural Dev., Dep. of Economics, Iowa State Univ., Ames, IA, 50011
^c USDA-ARS, 233 Johnson Hall, Washington State Univ., Pullman, WA 99164
^d Univ. of Minnesota, Southern Research and Outreach Station, Waseca, MN 56093

Corresponding author (pwgassma{at}iastate.edu)

Received for publication March 20, 2000. Subsurface tile drains are a key source of nitrate N (NO₃–N) losses to streams in parts of the north central USA. In this study, the Erosion Productivity Impact Calculator (EPIC) model was evaluated by comparing measured vs. predicted tile flow, tile NO₃–N loss, soil profile residual NO₃–N, crop N uptake, and yield, using 4 yr of data collected at a site near Lamberton, MN, for three crop rotations: continuous corn (Zea mays L.) or CC, corn–soybean [Glycine max (L.) Merr.] or CS, and continuous alfalfa (Medicago sativa L.) or CA. Initially, EPIC was run using standard Soil Conservation Service (SCS) runoff curve numbers (CN2) for CC and CS; monthly variations were accurately tracked for tile flow (r² = 0.86 and 0.90) and NO₃–N loss (r² = 0.69 and 0.52). However, average annual CC and CS tile flows were underpredicted by -32 and -34%, and corresponding annual NO₃–N losses were underpredicted by -11 and -52%. Predicted average annual tile flows and NO₃–N losses generally improved following calibration of the CN2; tile flow underpredictions were -9 and -12%, whereas NO₃–N losses were 0.6 and -54%. Adjusting a N parameter further improved predicted CS NO₃–N losses. Predicted monthly tile flows and NO₃–N losses for the CA simulation compared poorly with observed values (r² values of 0.27 and 0.19); the annual drainage volumes and N losses were of similar magnitude to those measured. Overall, EPIC replicated the relative impacts of the three cropping systems on N fate.

Abbreviations: EPIC, Erosion Productivity Impact Calculator • RAPS, Resource and Agricultural Policy System • CC, continuous corn • CS, corn–soybean • CA, continuous alfalfa • SCS, Soil Conservation Service • CN2, curve number • CRP, Conservation Reserve Program • EF, modeling efficiency

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