Location: Soil Drainage ResearchTitle: Modified APEX model for simulating macropore phosphorus contributions to tile drains Author
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2016
Publication Date: 12/1/2016
Citation: Ford III, W.I., King, K.W., Williams, M.R., Confesor, R. 2016. Modified APEX model for simulating macropore phosphorus contributions to tile drains. Journal of Environmental Quality. doi:10.2134/jeq2016.06.0218.
Interpretive Summary: Macropore flow is a major pathway for transport of soluble agricultural nutrients to tile drains. In order to test the likely usefulness of various proposed and existing technologies and management practices to reduce delivery of agricultural nutrients to streams in tile drainage water, modeling tools need to be able to adequately simulate how much macropore flow is delivered to tile drains. A newly developed macropore flow model was tested using field measured tile flow and surface runoff data and was found to significantly improve hydrology simulations at both daily and monthly scales. These results are useful for researchers, conservation planners, and policy makers.
Technical Abstract: The contribution of macropore flow to tile-drainage in agricultural landscapes remains poorly constrained at the field-scale despite the recognized deleterious impacts of contaminant transport via preferential pathways. A new sub-routine that couples existing saturation-excess and saturation-depleted macropore flow theory is developed and implemented into the Agricultural Policy/Environmental eXtender (APEX) model. The model was applied and evaluated for a case-study in a poorly drained field in Western Ohio with thirty-one months of surface and subsurface monitoring data. Results highlight that source-responsive saturation-depleted macropore theory improved edge-of-field hydrology calibration and validation for both tile and total discharge at daily to monthly time-scales. Output from the calibrated macropore simulations suggest median annual saturation-depleted macropore contributions of 25%, with the majority of loading in winter and spring months. While somewhat counterintuitive, the prominence of saturation-depleted macropore flow during seasons with less cracking reflects the importance of coupled development of macropore pathways, and adequate supply of the preferential flow source. Results of a hypothetical scenario in which tile drains were removed suggested saturation-excess macropore flow may become important for attenuating surface runoff when managing subsurface drainage via drainage water management. The innovative features of the model allow for first assessments of annual macropore contributions to tile drainage and suggest novel measurements are needed in order to reduce uncertainty in macropore parameter estimates.