Modified APEX model for simulating macropore phosphorus contributions to tile drains

William I. Ford, Kevin W. King, Mark R. Williams, Remegio B. Confesor

Research output: Contribution to journalArticlepeer-review

24 Scopus citations


The contribution of macropore flow to phosphorus (P) loadings in tile-drained agricultural landscapes remains poorly understood at the field scale, despite the recognized deleterious impacts of contaminant transport via macropore pathways. A new subroutine that couples existing matrix-excess and matrixdesiccation macropore flow theory and a modified P routine is implemented in the Agricultural Policy Environmental eXtender (APEX) model. The original and modified formulation were applied and evaluated for a case study in a poorly drained field in Western Ohio with 31 months of surface and subsurface monitoring data. Results highlighted that a macropore subroutine in APEX improved edge-of-field discharge calibration and validation for both tile and total discharge from satisfactory and good, respectively, to very good and improved dissolved reactive P load calibration and validation statistics for tile P loads from unsatisfactory to very good. Output from the calibrated macropore simulations suggested median annual matrixdesiccation macropore flow contributions of 48% and P load contributions of 43%, with the majority of loading occurring in winter and spring. While somewhat counterintuitive, the prominence of matrix-desiccation macropore flow during seasons with less cracking reflects the importance of coupled development of macropore pathways and adequate supply of the macropore flow source. The innovative features of the model allow for assessments of annual macropore P contributions to tile drainage and has the potential to inform P site assessment tools.

Original languageEnglish
Pages (from-to)1413-1423
Number of pages11
JournalJournal of Environmental Quality
Issue number6
StatePublished - Nov 1 2017

Bibliographical note

Funding Information:
The authors would like to thank the land owner and operator of the case study site for providing access to their field; Eric Fischer for analytical expertise; and Jedediah Stinner, Katie Rumora, Marie Schrecengost, and Phil Levison for help in data collection and site maintenance. Partial funding for the data collection and analysis was provided by the 4R Research Fund, CEAP: Conservation Effects Assessment Project, EPA: DW-12-92342501-0, Ohio Corn Growers, CIG: 69-3A75-12-231(Ohio State University), CIG: 69-3A75-13-216 (Heidelberg University), and the NRCS MRBI (Mississippi River Basin Initiative). Finally, the authors would like to thank the two anonymous reviewers and Associate Editor for their detailed comments and suggestions, which have improved the quality of the manuscript.

Publisher Copyright:
© American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA.

ASJC Scopus subject areas

  • Environmental Engineering
  • Water Science and Technology
  • Waste Management and Disposal
  • Pollution
  • Management, Monitoring, Policy and Law


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